Python Tutorial PDF

Financial Accounting Tutorial Python About the Tutorial Python is a general-purpose interpreted, interactive, object-oriented, and high-level programming language. It was created by Guido van Rossum during 1985- 1990. Like Perl, Python source code is also available under the GNU General Public License (GPL). This tutorial gives enough understanding on Python programming language. Audience This tutorial is designed for software programmers who need to learn Python programming language from scratch. Prerequisites You should have a basic understanding of Computer Programming terminologies. A basic understanding of any of the programming languages is a plus. Disclaimer & Copyright  Copyright 2017 by Tutorials Point (I) Pvt. Ltd. All the content and graphics published in this e-book are the property of Tutorials Point (I) Pvt. Ltd. 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If you discover any errors on our website or in this tutorial, please notify us at [email protected]. i Python Table of Contents About the Tutorial.......................................................................................................................................... i Audience........................................................................................................................................................ i Prerequisites.................................................................................................................................................. i Disclaimer & Copyright................................................................................................................................... i Table of Contents.......................................................................................................................................... ii 1. PYTHON ─ OVERVIEW............................................................................................................... 1 History of Python.......................................................................................................................................... 1 Python Features............................................................................................................................................ 1 2. PYTHON ─ ENVIRONMENT........................................................................................................ 3 Local Environment Setup............................................................................................................................... 3 Getting Python.............................................................................................................................................. 3 Installing Python........................................................................................................................................... 4 Setting up PATH............................................................................................................................................ 5 Setting path at Unix/Linux............................................................................................................................ 5 Setting path at Windows............................................................................................................................... 6 Python Environment Variables...................................................................................................................... 6 Running Python............................................................................................................................................. 6 3. PYTHON ─ BASIC SYNTAX.......................................................................................................... 9 First Python Program.................................................................................................................................... 9 Python Identifiers........................................................................................................................................ 10 Python Keywords........................................................................................................................................ 11 Lines and Indentation.................................................................................................................................. 11 Multi-Line Statements................................................................................................................................. 13 ii Python Quotation in Python.................................................................................................................................... 13 Comments in Python................................................................................................................................... 14 Using Blank Lines........................................................................................................................................ 14 Waiting for the User.................................................................................................................................... 15 Multiple Statements on a Single Line.......................................................................................................... 15 Multiple Statement Groups as Suites.......................................................................................................... 15 Command Line Arguments.......................................................................................................................... 15 Accessing Command-Line Arguments.......................................................................................................... 16 Parsing Command-Line Arguments............................................................................................................. 17 getopt.getopt method................................................................................................................................. 17 Exception getopt.GetoptError..................................................................................................................... 17 4. PYTHON ─ VARIABLE TYPES.................................................................................................... 20 Assigning Values to Variables...................................................................................................................... 20 Multiple Assignment................................................................................................................................... 21 Standard Data Types................................................................................................................................... 21 Python Numbers......................................................................................................................................... 21 Python Strings............................................................................................................................................. 23 Python Lists................................................................................................................................................. 24 Python Tuples............................................................................................................................................. 24 Python Dictionary....................................................................................................................................... 26 Data Type Conversion................................................................................................................................. 27 5. PYTHON ─ BASIC OPERATORS................................................................................................. 29 Types of Operators...................................................................................................................................... 29 Python Arithmetic Operators...................................................................................................................... 29 Python Comparison Operators.................................................................................................................... 31 iii Python Python Assignment Operators.................................................................................................................... 34 Python Bitwise Operators........................................................................................................................... 36 Python Logical Operators............................................................................................................................ 38 Python Membership Operators................................................................................................................... 38 Python Identity Operators........................................................................................................................... 40 Python Operators Precedence..................................................................................................................... 41 6. PYTHON ─ DECISION MAKING................................................................................................. 44 If Statement................................................................................................................................................ 45 If…else Statement....................................................................................................................................... 46 The elif Statement....................................................................................................................................... 48 Single Statement Suites............................................................................................................................... 49 7. PYTHON ─ LOOPS................................................................................................................... 51 While Loop.................................................................................................................................................. 52 The Infinite Loop......................................................................................................................................... 53 Using else Statement with Loops................................................................................................................ 54 Single Statement Suites............................................................................................................................... 55 For Loop...................................................................................................................................................... 56 Iterating by Sequence Index........................................................................................................................ 57 Using else Statement with Loops................................................................................................................ 58 Nested Loops.............................................................................................................................................. 59 Loop Control Statements............................................................................................................................. 60 Break Statement......................................................................................................................................... 61 Continue Statement.................................................................................................................................... 63 Pass Statement........................................................................................................................................... 65 iv Python 8. PYTHON ─ NUMBERS.............................................................................................................. 66 Number Type Conversion............................................................................................................................ 67 Random Number Functions......................................................................................................................... 69 Trigonometric Functions............................................................................................................................. 69 Mathematical Constants............................................................................................................................. 70 9. PYTHON ─ STRINGS................................................................................................................. 71 Accessing Values in Strings.......................................................................................................................... 71 Updating Strings.......................................................................................................................................... 71 Escape Characters....................................................................................................................................... 72 String Special Operators.............................................................................................................................. 73 String Formatting Operator......................................................................................................................... 74 Triple Quotes.............................................................................................................................................. 76 Unicode String............................................................................................................................................. 77 Built-in String Methods............................................................................................................................... 78 capitalize() Method..................................................................................................................................... 82 center(width, fillchar) Method.................................................................................................................... 82 count(str, beg= 0,end=len(string)) Method................................................................................................. 83 decode(encoding='UTF-8',errors='strict') Method....................................................................................... 84 encode(encoding='UTF-8',errors='strict') Method....................................................................................... 85 endswith(suffix, beg=0, end=len(string)) Method....................................................................................... 86 expandtabs(tabsize=8)................................................................................................................................ 87 find(str, beg=0 end=len(string))................................................................................................................... 88 index(str, beg=0, end=len(string))............................................................................................................... 89 isalnum() Method....................................................................................................................................... 90 isalpha()...................................................................................................................................................... 90 v Python isdigit()........................................................................................................................................................ 91 islower()...................................................................................................................................................... 92 isnumeric().................................................................................................................................................. 93 isspace() Method......................................................................................................................................... 94 istitle()......................................................................................................................................................... 95 isupper()...................................................................................................................................................... 96 join(seq)...................................................................................................................................................... 96 len(string).................................................................................................................................................... 97 ljust(width[, fillchar]).................................................................................................................................. 98 lower()........................................................................................................................................................ 99 lstrip()....................................................................................................................................................... 100 maketrans()............................................................................................................................................... 100 max(str).................................................................................................................................................... 102 min(str)..................................................................................................................................................... 102 replace(old, new [, max]).......................................................................................................................... 103 rfind(str, beg=0,end=len(string))............................................................................................................... 104 rindex(str, beg=0, end=len(string))............................................................................................................ 105 rjust(width,[, fillchar])............................................................................................................................... 106 rstrip()....................................................................................................................................................... 107 split(str="", num=string.count(str))........................................................................................................... 108 splitlines(num=string.count('\n')).............................................................................................................. 109 startswith(str, beg=0,end=len(string))....................................................................................................... 110 strip([chars])............................................................................................................................................. 111 swapcase()................................................................................................................................................ 111 title()......................................................................................................................................................... 112 translate(table, deletechars="")................................................................................................................ 113 vi Python upper()...................................................................................................................................................... 114 zfill (width)................................................................................................................................................ 115 isdecimal()................................................................................................................................................. 116 10. PYTHON ─ LISTS.................................................................................................................... 118 Python Lists............................................................................................................................................... 118 Accessing Values in Lists............................................................................................................................ 118 Updating Lists........................................................................................................................................... 119 Deleting List Elements............................................................................................................................... 119 Basic List Operations................................................................................................................................. 120 Indexing, Slicing, and Matrixes.................................................................................................................. 121 Built-in List Functions and Methods.......................................................................................................... 121 Cmp(list1, list2)......................................................................................................................................... 122 len(List)..................................................................................................................................................... 123 max(list).................................................................................................................................................... 124 min(list)..................................................................................................................................................... 124 List.append(obj)........................................................................................................................................ 126 list.count(obj)............................................................................................................................................ 127 list.extend(seq)......................................................................................................................................... 128 list.index(obj)............................................................................................................................................ 128 list.insert(index,obj).................................................................................................................................. 129 list.pop(obj=list[-1]).................................................................................................................................. 130 List.remove(obj)........................................................................................................................................ 131 List.reverse()............................................................................................................................................. 131 list.sort([func]).......................................................................................................................................... 132 vii Python 11. PYTHON ─ TUPLES................................................................................................................ 134 Accessing Values in Tuples........................................................................................................................ 134 Updating Tuples........................................................................................................................................ 135 Deleting Tuple Elements........................................................................................................................... 135 Basic Tuples Operations............................................................................................................................ 136 Indexing, Slicing, and Matrixes.................................................................................................................. 136 No Enclosing Delimiters............................................................................................................................. 137 Built-in Tuple Functions............................................................................................................................. 137 Cmp(tuple1, tuple2).................................................................................................................................. 138 Len(tuple).................................................................................................................................................. 139 Max(tuple)................................................................................................................................................ 140 Min(tuple)................................................................................................................................................. 141 Tuple(seg)................................................................................................................................................. 141 12. PYTHON ─ DICTIONARY......................................................................................................... 143 Accessing Values in Dictionary.................................................................................................................. 143 Updating Dictionary.................................................................................................................................. 144 Delete Dictionary Elements....................................................................................................................... 144 Properties of Dictionary Keys.................................................................................................................... 145 Built-in Dictionary Functions and Methods............................................................................................... 146 Cmp(dict1, dict2)....................................................................................................................................... 146 len(dict)..................................................................................................................................................... 147 str(dict)..................................................................................................................................................... 148 type()........................................................................................................................................................ 149 dict.clear()................................................................................................................................................. 151 Dict.copy()................................................................................................................................................. 151 viii Python Dict.fromkeys()......................................................................................................................................... 152 Dict.get(key,default=none)....................................................................................................................... 153 Dict.has_key(key)...................................................................................................................................... 154 Dict.items()............................................................................................................................................... 155 Dict.keys()................................................................................................................................................. 156 dict.setdefault(key, default=None)........................................................................................................... 156 dict.update(dict2)...................................................................................................................................... 157 dict.values().............................................................................................................................................. 158 13. PYTHON ─ DATE AND TIME................................................................................................... 160 What is Tick?............................................................................................................................................. 160 What is TimeTuple?................................................................................................................................... 160 Getting Current Time................................................................................................................................. 162 Getting Formatted Time............................................................................................................................ 162 Getting Calendar for a Month................................................................................................................... 163 The time Module...................................................................................................................................... 163 time.altzone.............................................................................................................................................. 165 time.actime([tupletime])........................................................................................................................... 166 time.clock( ).............................................................................................................................................. 166 time.ctime([secs])..................................................................................................................................... 168 time.gmtime([secs]).................................................................................................................................. 168 time.localtime([secs])................................................................................................................................ 169 time.mktime(tupletime)............................................................................................................................ 170 time.sleep(secs)........................................................................................................................................ 171 time.strftime(fmt[,tupletime]).................................................................................................................. 172 time.strptime(str,fmt='%a %b %d %H:%M:%S %Y')................................................................................... 174 ix Python time.time( )............................................................................................................................................... 176 time.tzset()................................................................................................................................................ 177 The calendar Module................................................................................................................................ 179 Other Modules and Functions................................................................................................................... 181 14. PYTHON ─ FUNCTIONS......................................................................................................... 182 Defining a Function................................................................................................................................... 182 Calling a Function...................................................................................................................................... 183 Passing by Reference Versus Passing by Value.......................................................................................... 184 Function Arguments.................................................................................................................................. 185 Required Arguments................................................................................................................................. 185 Keyword Arguments.................................................................................................................................. 186 Default Arguments.................................................................................................................................... 187 Variable Length Arguments....................................................................................................................... 188 The Anonymous Functions........................................................................................................................ 189 The return Statement............................................................................................................................... 190 Scope of Variables..................................................................................................................................... 190 Global vs. Local variables.......................................................................................................................... 191 15. PYTHON ─ MODULES............................................................................................................ 192 The import Statement............................................................................................................................... 192 The from...import Statement..................................................................................................................... 193 The from...import * Statement:................................................................................................................. 193 Locating Modules:..................................................................................................................................... 193 The PYTHONPATH Variable....................................................................................................................... 194 Namespaces and Scoping.......................................................................................................................... 194 The dir( ) Function..................................................................................................................................... 195 x Python The globals() and locals() Functions........................................................................................................... 196 The reload() Function................................................................................................................................ 196 Packages in Python................................................................................................................................... 196 16. PYTHON ─ FILES I/O.............................................................................................................. 198 Printing to the Screen................................................................................................................................ 198 Reading Keyboard Input............................................................................................................................ 198 The raw_input Function............................................................................................................................ 198 The input Function.................................................................................................................................... 199 Opening and Closing Files.......................................................................................................................... 199 The open Function.................................................................................................................................... 199 The file Object Attributes.......................................................................................................................... 201 The close() Method................................................................................................................................... 202 Reading and Writing Files.......................................................................................................................... 203 The write() Method................................................................................................................................... 203 The read() Method.................................................................................................................................... 204 File Positions............................................................................................................................................. 204 Renaming and Deleting Files..................................................................................................................... 205 The rename() Method............................................................................................................................... 206 The remove() Method............................................................................................................................... 206 Directories in Python................................................................................................................................. 207 The mkdir() Method.................................................................................................................................. 207 The chdir() Method................................................................................................................................... 207 The getcwd() Method................................................................................................................................ 208 The rmdir() Method................................................................................................................................... 208 File and Directory Related Methods.......................................................................................................... 209 xi Python file.close()................................................................................................................................................. 210 File.flush()................................................................................................................................................. 211 File.fileno()................................................................................................................................................ 212 File.isatty()................................................................................................................................................ 213 File.next().................................................................................................................................................. 214 File.read([size])......................................................................................................................................... 215 File.readline([size]).................................................................................................................................... 216 file.readline([sizehint]).............................................................................................................................. 218 file.seek(offset[,whence])......................................................................................................................... 219 file.tell().................................................................................................................................................... 221 file.truncate([size]).................................................................................................................................... 222 file.write(str)............................................................................................................................................. 224 file.writelines(sequence)........................................................................................................................... 225 OS Object Methods................................................................................................................................... 227 17. PYTHON ─ EXCEPTIONS........................................................................................................ 233 Assertions in Python................................................................................................................................. 235 The assert Statement................................................................................................................................ 235 What is Exception?.................................................................................................................................... 236 Handling an Exception............................................................................................................................... 236 The except Clause with No Exceptions....................................................................................................... 238 The except Clause with Multiple Exceptions.............................................................................................. 239 The try-finally Clause................................................................................................................................. 239 Argument of an Exception......................................................................................................................... 240 Raising an Exception................................................................................................................................. 241 User-Defined Exceptions........................................................................................................................... 242 xii Python 18. PYTHON ─ CLASSES AND OBJECTS........................................................................................ 244 Overview of OOP Terminology.................................................................................................................. 244 Creating Classes........................................................................................................................................ 245 Creating Instance Objects.......................................................................................................................... 246 Accessing Attributes.................................................................................................................................. 246 Built-In Class Attributes............................................................................................................................. 248 Destroying Objects (Garbage Collection)................................................................................................... 249 Class Inheritance....................................................................................................................................... 251 Overriding Methods.................................................................................................................................. 252 Base Overloading Methods....................................................................................................................... 253 Overloading Operators.............................................................................................................................. 254 Data Hiding............................................................................................................................................... 255 19. PYTHON ─ REGULAR EXPRESSIONS....................................................................................... 257 The match Function.................................................................................................................................. 257 The search Function.................................................................................................................................. 259 Matching Versus Searching....................................................................................................................... 260 Search and Replace................................................................................................................................... 261 Regular-Expression Modifiers: Option Flags.............................................................................................. 261 Regular-Expression Patterns..................................................................................................................... 262 Regular-Expression Examples.................................................................................................................... 265 Grouping with Parentheses....................................................................................................................... 267 Backreferences.......................................................................................................................................... 267 20. PYTHON ─ CGI PROGRAMMING............................................................................................ 270 What is CGI?.............................................................................................................................................. 270 Web Browsing........................................................................................................................................... 270 xiii Python CGI Architecture........................................................................................................................................ 271 Web Server Support and Configuration..................................................................................................... 271 First CGI Program...................................................................................................................................... 272 HTTP Header............................................................................................................................................. 273 CGI Environment Variables........................................................................................................................ 274 GET and POST Methods............................................................................................................................. 275 Passing Information using GET method:.................................................................................................... 276 Simple URL Example : Get Method............................................................................................................ 276 Simple FORM Example: GET Method......................................................................................................... 277 Passing Information Using POST Method.................................................................................................. 278 Passing Checkbox Data to CGI Program..................................................................................................... 279 Passing Radio Button Data to CGI Program............................................................................................... 280 Passing Text Area Data to CGI Program..................................................................................................... 281 Passing Drop Down Box Data to CGI Program........................................................................................... 283 Using Cookies in CGI.................................................................................................................................. 284 How It Works?........................................................................................................................................... 284 Setting up Cookies..................................................................................................................................... 285 Retrieving Cookies..................................................................................................................................... 285 File Upload Example.................................................................................................................................. 286 How To Raise a "File Download" Dialog Box?............................................................................................ 288 21. PYTHON ─ DATABASE ACCESS............................................................................................... 289 What is MySQLdb?.................................................................................................................................... 289 How do I Install MySQLdb?....................................................................................................................... 290 Database Connection................................................................................................................................ 290 Creating Database Table........................................................................................................................... 292 xiv Python INSERT Operation...................................................................................................................................... 293 READ Operation........................................................................................................................................ 295 Update Operation..................................................................................................................................... 296 DELETE Operation..................................................................................................................................... 297 Performing Transactions........................................................................................................................... 298 COMMIT Operation................................................................................................................................... 299 ROLLBACK Operation................................................................................................................................ 299 Disconnecting Database............................................................................................................................ 299 Handling Errors......................................................................................................................................... 300 22. PYTHON ─ NETWORK PROGRAMMING................................................................................. 302 What is Sockets?....................................................................................................................................... 302 The socket Module.................................................................................................................................... 303 Server Socket Methods............................................................................................................................. 303 Client Socket Methods.............................................................................................................................. 304 General Socket Methods........................................................................................................................... 304 A Simple Server......................................................................................................................................... 304 A Simple Client.......................................................................................................................................... 305 Python Internet modules.......................................................................................................................... 306 Further Readings....................................................................................................................................... 307 23. PYTHON ─ SENDING EMAIL................................................................................................... 308 Sending an HTML e-mail using Python...................................................................................................... 309 Sending Attachments as an E-mail............................................................................................................ 310 24. PYTHON ─ MULTITHREADING............................................................................................... 313 Starting a New Thread............................................................................................................................... 313 The Threading Module.............................................................................................................................. 314 xv Python Creating Thread Using Threading Module:................................................................................................ 315 Synchronizing Threads.............................................................................................................................. 317 Multithreaded Priority Queue................................................................................................................... 319 25. PYTHON ─ XML PROCESSING................................................................................................ 323 What is XML?............................................................................................................................................ 323 XML Parser Architectures and APIs:.......................................................................................................... 323 Parsing XML with SAX APIs........................................................................................................................ 325 The make_parser Method......................................................................................................................... 325 The parse Method..................................................................................................................................... 325 The parseString Method............................................................................................................................ 326 Parsing XML with DOM APIs...................................................................................................................... 329 26. PYTHON ─ GUI PROGRAMMING........................................................................................... 332 Tkinter Programming................................................................................................................................ 332 Tkinter Widgets......................................................................................................................................... 333 Button....................................................................................................................................................... 335 Canvas....................................................................................................................................................... 338 Checkbutton.............................................................................................................................................. 340 Entry......................................................................................................................................................... 344 Frame........................................................................................................................................................ 349 Label......................................................................................................................................................... 351 Listbox....................................................................................................................................................... 354 MenuButton.............................................................................................................................................. 358 Menu........................................................................................................................................................ 362 Message.................................................................................................................................................... 366 Radiobutton.............................................................................................................................................. 369 xvi Python Scale.......................................................................................................................................................... 373 Scrollbar.................................................................................................................................................... 378 Text........................................................................................................................................................... 381 TopLevel.................................................................................................................................................... 387 SpinBox..................................................................................................................................................... 390 PanelWindow............................................................................................................................................ 393 LabelFrame............................................................................................................................................... 396 tkMessageBox........................................................................................................................................... 398 Standard Attributes................................................................................................................................... 400 Dimensions............................................................................................................................................... 400 Colors........................................................................................................................................................ 401 Fonts......................................................................................................................................................... 402 Anchors..................................................................................................................................................... 403 Relief styles............................................................................................................................................... 404 Bitmaps..................................................................................................................................................... 406 Cursors...................................................................................................................................................... 407 Geometry Management............................................................................................................................ 408 Python Tkinter pack() Method.................................................................................................................. 409 Python Tkinter grid() Method.................................................................................................................... 410 Python Tkinter place() Method................................................................................................................. 411 27. PYTHON ─ FURTHER EXTENSIONS......................................................................................... 413 Pre-Requisites for Writing Extensions....................................................................................................... 413 First Look at a Python Extension............................................................................................................... 413 The Header File Python.h.......................................................................................................................... 413 The C Functions......................................................................................................................................... 414 xvii Python The Method Mapping Table...................................................................................................................... 415 The Initialization Function......................................................................................................................... 416 Building and Installing Extensions............................................................................................................. 418 Importing Extensions................................................................................................................................ 418 Passing Function Parameters.................................................................................................................... 418 The PyArg_ParseTuple Function............................................................................................................... 420 Returning Values....................................................................................................................................... 421 The Py_BuildValue Function..................................................................................................................... 423 xviii Python 1. Python ─ Overview Python is a high-level, interpreted, interactive and object-oriented scripting language. Python is designed to be highly readable. It uses English keywords frequently where as other languages use punctuation, and it has fewer syntactical constructions than other languages.  Python is Interpreted: Python is processed at runtime by the interpreter. You do not need to compile your program before executing it. This is similar to PERL and PHP.  Python is Interactive: You can actually sit at a Python prompt and interact with the interpreter directly to write your programs.  Python is Object-Oriented: Python supports Object-Oriented style or technique of programming that encapsulates code within objects.  Python is a Beginner's Language: Python is a great language for the beginner-level programmers and supports the development of a wide range of applications from simple text processing to WWW browsers to games. History of Python Python was developed by Guido van Rossum in the late eighties and early nineties at the National Research Institute for Mathematics and Computer Science in the Netherlands. Python is derived from many other languages, including ABC, Modula-3, C, C++, Algol-68, SmallTalk, Unix shell, and other scripting languages. Python is copyrighted. Like Perl, Python source code is now available under the GNU General Public License (GPL). Python is now maintained by a core development team at the institute, although Guido van Rossum still holds a vital role in directing its progress. Python Features Python's features include:  Easy-to-learn: Python has few keywords, simple structure, and a clearly defined syntax. This allows the student to pick up the language quickly.  Easy-to-read: Python code is more clearly defined and visible to the eyes.  Easy-to-maintain: Python's source code is fairly easy-to-maintain. 1 Python  A broad standard library: Python's bulk of the library is very portable and cross-platform compatible on UNIX, Windows, and Macintosh.  Interactive Mode: Python has support for an interactive mode which allows interactive testing and debugging of snippets of code.  Portable: Python can run on a wide variety of hardware platforms and has the same interface on all platforms.  Extendable: You can add low-level modules to the Python interpreter. These modules enable programmers to add to or customize their tools to be more efficient.  Databases: Python provides interfaces to all major commercial databases.  GUI Programming: Python supports GUI applications that can be created and ported to many system calls, libraries, and windows systems, such as Windows MFC, Macintosh, and the X Window system of Unix.  Scalable: Python provides a better structure and support for large programs than shell scripting. Apart from the above-mentioned features, Python has a big list of good features, few are listed below:  It supports functional and structured programming methods as well as OOP.  It can be used as a scripting language or can be compiled to byte-code for building large applications.  It provides very high-level dynamic data types and supports dynamic type checking.  It supports automatic garbage collection.  It can be easily integrated with C, C++, COM, ActiveX, CORBA, and Java. 2 Python 2. Python ─ Environment Python is available on a wide variety of platforms including Linux and Mac OS X. Let's understand how to set up our Python environment. Local Environment Setup Open a terminal window and type "python" to find out if it is already installed and which version is installed.  Unix (Solaris, Linux, FreeBSD, AIX, HP/UX, SunOS, IRIX, etc.)  Win 9x/NT/2000  Macintosh (Intel, PPC, 68K)  OS/2  DOS (multiple versions)  PalmOS  Nokia mobile phones  Windows CE  Acorn/RISC OS  BeOS  Amiga  VMS/OpenVMS  QNX  VxWorks  Psion  Python has also been ported to the Java and.NET virtual machines Getting Python The most up-to-date and current source code, binaries, documentation, news, etc., is available on the official website of Python: http://www.python.org/. You can download Python documentation from www.python.org/doc/. The documentation is available in HTML, PDF, and PostScript formats. 3 Python Installing Python Python distribution is available for a wide variety of platforms. You need to download only the binary code applicable for your platform and install Python. If the binary code for your platform is not available, you need a C compiler to compile the source code manually. Compiling the source code offers more flexibility in terms of choice of features that you require in your installation. Here is a quick overview of installing Python on various platforms: Unix and Linux Installation Here are the simple steps to install Python on Unix/Linux machine.  Open a Web browser and go to http://www.python.org/download/.  Follow the link to download zipped source code available for Unix/Linux.  Download and extract files.  Editing the Modules/Setup file if you want to customize some options.  run./configure script  make  make install This installs Python at standard location /usr/local/bin and its libraries at /usr/local/lib/pythonXX where XX is the version of Python. Windows Installation Here are the steps to install Python on Windows machine.  Open a Web browser and go to http://www.python.org/download/  Follow the link for the Windows installer python-XYZ.msi file where XYZ is the version you need to install.  To use this installer python-XYZ.msi, the Windows system must support Microsoft Installer 2.0. Save the installer file to your local machine and then run it to find out if your machine supports MSI.  Run the downloaded file. This brings up the Python install wizard, which is really easy to use. Just accept the default settings, wait until the install is finished, and you are done. 4 Python Macintosh Installation Recent Macs come with Python installed, but it may be several years out of date. See http://www.python.org/download/mac/ for instructions on getting the current version along with extra tools to support development on the Mac. For older Mac OS's before Mac OS X 10.3 (released in 2003), MacPython is available. Jack Jansen maintains it and you can have full access to the entire documentation at his website - http://www.cwi.nl/~jack/macpython.html. You can find complete installation details for Mac OS installation. Setting up PATH Programs and other executable files can be in many directories, so operating systems provide a search path that lists the directories that the OS searches for executables. The path is stored in an environment variable, which is a named string maintained by the operating system. This variable contains information available to the command shell and other programs. The path variable is named as PATH in Unix or Path in Windows (Unix is case- sensitive; Windows is not). In Mac OS, the installer handles the path details. To invoke the Python interpreter from any particular directory, you must add the Python directory to your path. Setting path at Unix/Linux To add the Python directory to the path for a particular session in Unix:  In the csh shell: type setenv PATH "$PATH:/usr/local/bin/python" and press Enter.  In the bash shell (Linux): type export ATH="$PATH:/usr/local/bin/python" and press Enter.  In the sh or ksh shell: type PATH="$PATH:/usr/local/bin/python" and press Enter. Note: /usr/local/bin/python is the path of the Python directory 5 Python Setting path at Windows To add the Python directory to the path for a particular session in Windows: At the command prompt: type path %path%;C:\Python and press Enter. Note: C:\Python is the path of the Python directory Python Environment Variables Here are important environment variables, which can be recognized by Python: Variable Description It has a role similar to PATH. This variable tells the Python interpreter where to locate the module files imported into a PYTHONPATH program. It should include the Python source library directory and the directories containing Python source code. PYTHONPATH is sometimes preset by the Python installer. It contains the path of an initialization file containing Python source code. It is executed every time you start the interpreter. PYTHONSTARTUP It is named as.pythonrc.py in Unix and it contains commands that load utilities or modify PYTHONPATH. It is used in Windows to instruct Python to find the first case- PYTHONCASEOK insensitive match in an import statement. Set this variable to any value to activate it. It is an alternative module search path. It is usually embedded PYTHONHOME in the PYTHONSTARTUP or PYTHONPATH directories to make switching module libraries easy. Running Python There are three different ways to start Python: (1) Interactive Interpreter You can start Python from Unix, DOS, or any other system that provides you a command-line interpreter or shell window. 6 Python Enter python the command line. Start coding right away in the interactive interpreter. $python # Unix/Linux or python% # Unix/Linux or C:>python # Windows/DOS Here is the list of all the available command line options: Option Description -d It provides debug output. -O It generates optimized bytecode (resulting in.pyo files). -S Do not run import site to look for Python paths on startup. -v verbose output (detailed trace on import statements). -X disable class-based built-in exceptions (just use strings); obsolete starting with version 1.6. -c cmd run Python script sent in as cmd string file run Python script from given file 7 Python (2) Script from the Command-line A Python script can be executed at command line by invoking the interpreter on your application, as in the following: $python script.py # Unix/Linuxor python% script.py # Unix/Linuxor C:>python script.py # Windows/DOS Note: Be sure the file permission mode allows execution. (3) Integrated Development Environment You can run Python from a Graphical User Interface (GUI) environment as well, if you have a GUI application on your system that supports Python.  Unix: IDLE is the very first Unix IDE for Python.  Windows: PythonWin is the first Windows interface for Python and is an IDE with a GUI.  Macintosh: The Macintosh version of Python along with the IDLE IDE is available from the main website, downloadable as either MacBinary or BinHex'd files. If you are not able to set up the environment properly, then you can take help from your system admin. Make sure the Python environment is properly set up and working perfectly fine. Note: All the examples given in subsequent chapters are executed with Python 2.4.3 version available on CentOS flavor of Linux. We already have set up Python Programming environment online, so that you can execute all the available examples online at the same time when you are learning theory. Feel free to modify any example and execute it online. 8 Python 3. Python ─ Basic Syntax The Python language has many similarities to Perl, C, and Java. However, there are some definite differences between the languages. First Python Program Let us execute programs in different modes of programming. Interactive Mode Programming: Invoking the interpreter without passing a script file as a parameter brings up the following prompt: $ python Python 2.4.3 (#1, Nov 11 2010, 13:34:43) [GCC 4.1.2 20080704 (Red Hat 4.1.2-48)] on linux2 Type "help", "copyright", "credits" or "license" for more information. >>> Type the following text at the Python prompt and press the Enter: >>> print "Hello, Python!"; If you are running new version of Python, then you need to use print statement with parenthesis as in print ("Hello, Python!");. However in Python version 2.4.3, this produces the following result: Hello, Python! Script Mode Programming Invoking the interpreter with a script parameter begins execution of the script and continues until the script is finished. When the script is finished, the interpreter is no longer active. Let us write a simple Python program in a script. Python files have extension.py. Type the following source code in a test.py file: print "Hello, Python!"; 9 Python We assume that you have Python interpreter set in PATH variable. Now, try to run this program as follows: $ python test.py This produces the following result: Hello, Python! Let us try another way to execute a Python script. Here is the modified test.py file: #!/usr/bin/python print "Hello, Python!"; We assume that you have Python interpreter available in /usr/bin directory. Now, try to run this program as follows: $ chmod +x test.py # This is to make file executable $./test.py This produces the following result: Hello, Python! Python Identifiers A Python identifier is a name used to identify a variable, function, class, module, or other object. An identifier starts with a letter A to Z or a to z, or an underscore (_) followed by zero or more letters, underscores and digits (0 to 9). Python does not allow punctuation characters such as @, $, and % within identifiers. Python is a case sensitive programming language. Thus, Manpower and manpower are two different identifiers in Python. Here are naming conventions for Python identifiers:  Class names start with an uppercase letter. All other identifiers start with a lowercase letter.  Starting an identifier with a single leading underscore indicates that the identifier is private.  Starting an identifier with two leading underscores indicates a strongly private identifier. 10 Python  If the identifier also ends with two trailing underscores, the identifier is a language-defined special name. Python Keywords The following list shows the Python keywords. These are reserved words and you cannot use them as constant or variable or any other identifier names. All the Python keywords contain lowercase letters only. And exec Not Assert finally or Break for pass Class from print Continue global raise def if return del import try elif in while else is with except lambda yield Lines and Indentation Python provides no braces to indicate blocks of code for class and function definitions or flow control. Blocks of code are denoted by line indentation, which is rigidly enforced. The number of spaces in the indentation is variable, but all statements within the block must be indented the same amount. For example: 11 Python if True: print "True" else: print "False" However, the following block generates an error: if True: print "Answer" print "True" else: print "Answer" print "False" Thus, in Python all the continuous lines indented with same number of spaces would form a block. The following example has various statement blocks: Note: Do not try to understand the logic at this point of time. Just make sure you understood various blocks even if they are without braces. #!/usr/bin/python import sys try: # open file stream file = open(file_name, "w") except IOError: print "There was an error writing to", file_name sys.exit() print "Enter '", file_finish, print "' When finished" while file_text != file_finish: file_text = raw_input("Enter text: ") if file_text == file_finish: # close the file file.close break 12 Python file.write(file_text) file.write("\n") file.close() file_name = raw_input("Enter filename: ") if len(file_name) == 0: print "Next time please enter something" sys.exit() try: file = open(file_name, "r") except IOError: print "There was an error reading file" sys.exit() file_text = file.read() file.close() print file_text Multi-Line Statements Statements in Python typically end with a new line. Python does, however, allow the use of the line continuation character (\) to denote that the line should continue. For example: total = item_one + \ item_two + \ item_three Statements contained within the [], {}, or () brackets do not need to use the line continuation character. For example: days = ['Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday'] Quotation in Python Python accepts single ('), double (") and triple (''' or """) quotes to denote string literals, as long as the same type of quote starts and ends the string. 13 Python The triple quotes are used to span the string across multiple lines. For example, all the following are legal: word = 'word' sentence = "This is a sentence." paragraph = """This is a paragraph. It is made up of multiple lines and sentences.""" Comments in Python A hash sign (#) that is not inside a string literal begins a comment. All characters after the # and up to the end of the physical line are part of the comment and the Python interpreter ignores them. #!/usr/bin/python # First comment print "Hello, Python!"; # second comment This produces the following result: Hello, Python! You can type a comment on the same line after a statement or expression: name = "Madisetti" # This is again comment You can comment multiple lines as follows: # This is a comment. # This is a comment, too. # This is a comment, too. # I said that already. Using Blank Lines A line containing only whitespace, possibly with a comment, is known as a blank line and Python totally ignores it. In an interactive interpreter session, you must enter an empty physical line to terminate a multiline statement. 14 Python Waiting for the User The following line of the program displays the prompt, the statement saying “Press the enter key to exit”, and waits for the user to take action: #!/usr/bin/python raw_input("\n\nPress the enter key to exit.") Here, "\n\n" is used to create two new lines before displaying the actual line. Once the user presses the key, the program ends. This is a nice trick to keep a console window open until the user is done with an application. Multiple Statements on a Single Line The semicolon ( ; ) allows multiple statements on the single line given that neither statement starts a new code block. Here is a sample snip using the semicolon: import sys; x = 'foo'; sys.stdout.write(x + '\n') Multiple Statement Groups as Suites A group of individual statements, which make a single code block are called suites in Python. Compound or complex statements, such as if, while, def, and class require a header line and a suite. Header lines begin the statement (with the keyword) and terminate with a colon (:) and are followed by one or more lines which make up the suite. For example: if expression : suite elif expression : suite else : suite Command Line Arguments Many programs can be run to provide you with some basic information about how they should be run. Python enables you to do this with -h: $ python -h 15 Python usage: python [option]... [-c cmd | -m mod | file | -] [arg]... Options and arguments (and corresponding environment variables): -c cmd : program passed in as string (terminates option list) -d : debug output from parser (also PYTHONDEBUG=x) -E : ignore environment variables (such as PYTHONPATH) -h : print this help message and exit [ etc. ] You can also program your script in such a way that it should accept various options. Accessing Command-Line Arguments Python provides a getopt module that helps you parse command-line options and arguments. $ python test.py arg1 arg2 arg3 The Python sys module provides access to any command-line arguments via the sys.argv. This serves two purposes:  sys.argv is the list of command-line arguments.  len(sys.argv) is the number of command-line arguments. Here sys.argv is the program i.e. script name. Example Consider the following script test.py: #!/usr/bin/python import sys print 'Number of arguments:', len(sys.argv), 'arguments.' print 'Argument List:', str(sys.argv) Now run above script as follows: $ python test.py arg1 arg2 arg3 This produces the following result: 16 Python Number of arguments: 4 arguments. Argument List: ['test.py', 'arg1', 'arg2', 'arg3'] NOTE: As mentioned above, first argument is always script name and it is also being counted in number of arguments. Parsing Command-Line Arguments Python provided a getopt module that helps you parse command-line options and arguments. This module provides two functions and an exception to enable command line argument parsing. getopt.getopt method This method parses command line options and parameter list. Following is simple syntax for this method: getopt.getopt(args, options[, long_options]) Here is the detail of the parameters:  args: This is the argument list to be parsed.  options: This is the string of option letters that the script wants to recognize, with options that require an argument should be followed by a colon (:).  long_options: This is optional parameter and if specified, must be a list of strings with the names of the long options, which should be supported. Long options, which require an argument should be followed by an equal sign ('='). To accept only long options, options should be an empty string.  This method returns value consisting of two elements: the first is a list of (option, value) pairs. The second is the list of program arguments left after the option list was stripped.  Each option-and-value pair returned has the option as its first element, prefixed with a hyphen for short options (e.g., '-x') or two hyphens for long options (e.g., '--long-option'). Exception getopt.GetoptError This is raised when an unrecognized option is found in the argument list or when an option requiring an argument is given none. 17 Python The argument to the exception is a string indicating the cause of the error. The attributes msg and opt give the error message and related option. Example Consider we want to pass two file names through command line and we also want to give an option to check the usage of the script. Usage of the script is as follows: usage: test.py -i -o Here is the following script to test.py: #!/usr/bin/python import sys, getopt def main(argv): inputfile = '' outputfile = '' try: opts, args = getopt.getopt(argv,"hi:o:",["ifile=","ofile="]) except getopt.GetoptError: print 'test.py -i -o ' sys.exit(2) for opt, arg in opts: if opt == '-h': print 'test.py -i -o ' sys.exit() elif opt in ("-i", "--ifile"): inputfile = arg elif opt in ("-o", "--ofile"): outputfile = arg print 'Input file is "', inputfile print 'Output file is "', outputfile if __name__ == "__main__": main(sys.argv[1:]) Now, run above script as follows: 18 Python $ test.py -h usage: test.py -i -o $ test.py -i BMP -o usage: test.py -i -o $ test.py -i inputfile Input file is " inputfile Output file is " 19 Python 4. Python ─ Variable Types Variables are nothing but reserved memory locations to store values. This means when you create a variable, you reserve some space in memory. Based on the data type of a variable, the interpreter allocates memory and decides what can be stored in the reserved memory. Therefore, by assigning different data types to variables, you can store integers, decimals, or characters in these variables. Assigning Values to Variables Python variables do not need explicit declaration to reserve memory space. The declaration happens automatically when you assign a value to a variable. The equal sign (=) is used to assign v

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