Python Programming Basics

• Python is a programming language, lecture 2 introduces programming in it
• The lecture is given by Dr. Alexandra Simanovsky, [email protected]

Recap on Lesson 1

• Covered variables, assignment, types, and operators
• Types include: int, float, bool
• Boolean logic was explained
• The syntax was revised

Basic Syntax

• Lines and indentation are important
• Python allows multi-line statements using the line continuation character ()

Comments in Python

• Comments begin with a hash sign (#) outside a string literal
• All characters after the # on the physical line are part of the comment
• Python interpreter ignores comments

Variable Types

• Variables are reserved memory locations for storing values
• Creating a variable reserves space in memory
• The interpreter allocates memory based on the variable's data type

Multiple Assignment

• Python can assign a single value to multiple variables simultaneously
• Multiple objects can be assigned to multiple variables
• Two integer objects with values 1 and 2 are assigned to variables a and b and the string "john" is assigned to variable c

Standard Data Types

• Data stored in memory can be of many types
• Python has five standard data types: numbers, string, list, tuple, and dictionary.

Python Numbers

• Supports three numerical types:
  • int (signed integers)
  • float (floating point real values)
  • complex (complex numbers)

The Python String

• A string is a sequence of characters used to save text
• Strings are literals, there's no difference between single or double quoted strings, so the use of each is up to the programmer in accordance to the text to be saved

The Empty String

• "" is a valid string that is not always intuitive, but important
• It exists and is invisible
• It is to strings what 0 is to numbers

The "len" function

• Python's built-in "len" function finds the length of a string

String comparison (==, !=)

• Strings can be compared against each other
• Returns True or False

Python Strings

• Strings are contiguous sets of characters represented in quotation marks
• Subsets are retrievable using the slice operator ([ ] and [:])
• Indexes start at 0
• The operator [n:m] gets the part of the string from the "n-eth" character to the "m-eth" character, including the first but excluding the last
• Omitting the first index starts the slice at the beginning of the string
• Omitting the second index causes the slice to proceed to the end of the string

String Slicing

• String Slicing is where various locations in a defined python string can be called

Upper, Lower, Replace

• Strings have many built-in methods
• Methods can perform actions like converting the string into uppercase or lowercase, replacing specific subsections, or concatenating
• Methods use the prefix "str." indicating they operate on the class "string".

Functions

• Functions (like print) are standalone and reusable pieces of code, performing a specific task
• Functions are called using parentheses
• Functions can take arguments and return values

Methods

• Methods (like str.upper) belong to an object
• Methods are usually associated with specific data type
• Methods are called using dot notation on an object
• Methods usually act on whatever Object they belong to

Accessing String characters with bracket operator []

• Strings can be accessed with a bracket operator
• Note there is 0 based indexing

Bracket operator [] with negative indexes

• Characters can also be accessed with negative indexes
• Returns different parts of the string

String Comparison

• Strings can be compared

String Comparison

• "0">"" # the empty string is True
• "">" " # a string containing exactly one space is False
• "0">0 gives an error because it can't compare a string to an int

ASCII

• Characters are mapped to a specific number based on the ASCII table
• Strings are sequences of numbers in binary form
• Common special characters have ASCII representations
  • "\n" represents new line
  • "\t" represents tab

Ord, Chr

• chr: returns the string representing a character whose Unicode code point is the integer i.
• ord: returns an integer representing the Unicode code point of a character
• Unicode of a is 97, while Euro sign is 8364

The "in" Function

• The "in" operator checks if a string contains another string
• Returns True if a specific substring exits, or False if not

Other Useful String Methods

• find
• startswith, endswith
• isalpha, isdigit, islower
• strip, rstrip

Lists

• Lists are an important part of Python

Lists

• The most basic data structure in Python is the sequence
• Each element of a sequence is assigned a number as its position or index
• The first index is zero, the second index is one
• Python has six built-in types of sequences, but the most common ones are "lists" and "tuples"
• The list is versatile and can be written as comma-separated values inside square brackets
• It is important to know that items in a list don't have to be the same type.

Lists

• Lists are ordered sequences of elements and is mutable
• Python's list literals are in square brackets

Lists

• List elements can have direct access when indexing
• Like arrays in other programming languages
• Less efficient
• Denoted by [] operator for direct access and slicing
• Support for len, min, max, sum
• Support for +, *, in, not in

Lists example

• List of physics, chemistry, 1997, 2000 is a valid list
• [2, 3, 4, 5] is a valid list call

Updating lists

• Single or multiple list elements can be updated using the assignment operator

Delete List Elements

• A list element can be removed with a "del" statement if its index is known

Basic List Operations

• The + and * operators work like numbers to concatenate and repeat
• Result of the above is a new list.

Indexing, Slicing, and Matrices

• Code example of listing "C++", "Java", "Python"

Built-in List Functions and Methods

• Includes functions

List len() Methods

• len() returns the number of elements in a list

List max()/min() Method

• Methods to determine the maximum and minimum values from a defined list

List list() Methods

• Method that allows one to list various strings into Python
• Convert a tuple into list using this method

List Methods

• list.append(obj): Appends object obj to list
• list.count(obj): Returns the count of how many times obj occurs in list
• list.extend(seq): Appends the contents of seq to list
• list.index(obj): Returns the lowest index in list that obj appears
• list.insert(index, obj): Inserts object obj into list at offset index
• list.pop(obj=list[-1]): Removes and returns last object or obj from list
• list.remove(obj): Removes object obj from list
• list.reverse(): Reverses objects of list in place
• list.sort([func]): Sorts objects of list by compare func if given

List append() Method

• "append" method method appends a new object into the existing list

List count() Method

• "count()" method helps count number of times object occurs on list

List extend() Method

• "extend()" method appends the contexts of seq to list

List index() Method

• Method example of listing various chemistry functions

List Insert() Method

• insert(index, obj) method to insert one object into list at offset index
• index - Index for the object need
• obj: Object getting inserted into list

List pop() Method

• "Pop" lists last object from list

List reverse() and sort() Methods

• reverse() in lists will completely re-order the items
• sort() will order them according to type

List remove() Method

• removes specified object from list

The Stack and Queue Data Structure

• Lists can implement stacks or queues
• Stacks: LIFO (last in, first out) structures
• Queues: FIFO (first in, first out) structures
• Lists use "append", "insert", and "pop" methods

Nested Lists

• Lists can be nested

A nested list illustrated

• L = ['a', 'b', ['cc', 'dd', ['eee', 'fff']], 'g', 'h'] is a sample line of code

Tuples

• Tuples are a key factor in Python

Tuples

• Tuples are a sequence of immutable Python objects, like lists
• The main difference is that the tuples cannot be changed while lists can
• Tuples use parentheses (), while lists use square brackets [].

Tuple Literals

• Code examples of using Tuple Literals

Mixing types in a Tuple

• Tuple elements can be of any type

Accessing Values in Tuples

• Use square brackets [ ] for slicing and indexing

Updating Tuples

• Tuples are immutable, which means you cannot update or change the values of tuple elements
• You can take portions of existing tuples to create new tuples

Delete Tuple Elements

• Removing individual tuple elements is not possible
• Combine another tuple with the discarded elements
• To explicitly remove an entire tuple, use the del statement

Basic Tuples Operations

• Tuples respond to the + and * operators much like lists and strings
• Concatenation and repetition are supported, but is a new tuple

Indexing, Slicing, and Matrixes

• Tuples are sequences, indexing and slicing work the same way for tuples as they do for strings
• Code example T=('C++', 'Java', 'Python')

Built-in Tuple Functions

• Includes the following Tuple functions
• Python examples of how to conduct these

Tuple vs. String

• Both tuple and string are immutable sequence types
• Both support bracket operator for indexing and slicing, len, in and other common functions and operators
• A string is a sequence of unicode characters
• mystring = "string"
  • Has string specific operators and functions (e.g. lower())
• A tuple is an ordered sequence of objects
• mytuple = ("s", "t", "r", "i", "n", "g")
  • print (mytuple)
  • ('s', 't', 'r', 'i', 'n','g')
• You can cast a string to a tuple and vise versa

Tuples vs Lists

• tuples are immutable, lists are mutable
• "Why, if then, one should use a tuple"
  • Some operation are slightly faster over tuples
    • e.g iterating over items
  • In some cases, an immutable type is required -e.g. as keys to dictionaries - later in course) -If data doesn't and shouldn’t change, implementing it as tuple will guarantee that it remains write-protected. Some prefer as a convention to use tuples for heterogeneous(different) data types and lists for homogeneous (similar) data types.
• You can convert a tuple to a list

• list((1,2,3))

• [1, 2, 3]