Python File and Exception Handling

Topic 11: File and Exception Handling

• Covers Python programming, specifically file and exception handling

Topic Learning Outcomes

• Understand opening files with different modes
• Demonstrate an understanding of file manipulation
• Implement reading from and writing to text files

Contents Overview

• Creating a text file
• Opening files in different modes
• Writing data into a file
• Reading data from a file
• Searching through a file
• Exception handling

Introduction to File Handling

• File handling is a mechanism to read data from or write data to disk files in Python programs.
• Input typically comes from the keyboard, output displayed on the monitor.
• Without file handling, data isn't stored permanently
• File handling lets one store data entered through a program permanently in a disk file.

Advantages of Files

• Large amounts of data storage is possible
• Data is stored permanently
• Data can be easily shared across various programs
• Updates to data are easily implemented

Data Files Overview

• Data files contain data for specific applications, and are intended for later use
• Data files may be stored in two formats: text files and binary files

Text Files

• Text files store information in ASCII or Unicode characters
• Everything is stored as a character
• Text files are terminated by a special character called EOL (End Of Line)
• An EOL error means the Python interpreter reached the end of a line while scanning a string literal
• String literals or constants should be enclosed in single or double quotes

Binary files

• Binary files store information in the same format as it is stored in memory
• Binary files have no delimiter for newline characters
• Programs can read and write binary faster / easier than text
• Data in binary files cannot be directly read and requires a Python program

File I/O

• One can read information from a file or write information to a file via File I/O
• File I/O refers to "input/output"
• Python contains built-in functions to simplify file I/O operations

Steps in Data File Handling

• Opening a file involves associating an external file with a program object
• Reading or writing to a file allows manipulation of the file object
• Closing a file should be done after operations are complete

Opening Files

• Before reading a file's contents, Python needs to know which file the operations will apply to
• The process is done via the open() function
• open() returns a "file handle," which is a variable for performing operations on the file
• Opening a file is like navigating to "File -> Open" in a word processor

File Opening Modes and Syntax

• Files can be opened for reading, writing, or appending
• file = open(filename) is the syntax
• file = open(filename, mode) can define the mode
• The default mode for opening a file is read

Syntax for Open() - Details

• myFile = open(FILE_NAME, ACCESS_MODE)
• FILE_NAME is a string containing the name of the file you want to access
• Examples: "input.txt", "numbers.dat", "roster.txt"
• ACCESS_MODE is an optional argument string that determines the mode files open in -"r": open for reading -"w": open for writing -"a": open for appending

File Access Modes Table

• w: Opens in write-only mode and creates the file if it doesn't exist, overwrites if the file exists
• r: Opens in read-only mode, file I/O error if the file doesn't exist
• a: Opens in append mode to add new data at the end, creates the file if it doesn't exist
• w+: Opens a file in both read/write mode, overwrites the file if it exists already or creates a new file if it does not exist
• r+: Opens a file in both read and write mode, and raises an i/o error if the file does not exist
• a+: Opens in append and read mode, can read and write to the file, creates the file if it doesn't exist, adds text to the existing data
• x: Exclusive creation mode; opens a file for writing only if it doesn’t already exist

Examples of Using open()

• testFile = open("scores.txt")
• dataIn = open("old_stats.dat")
• dataOut = open("stats.dat", "w")
• The scores.txt file contains lines of floating point numbers

Writing Files

• name = open("filename", "w"): Opens the file to be able to write on it, as well as delete all previous content
• name = open("filename", "a"): Opens the file to append to it, i.e. add data behind all previous data
• name.write(str): Writes a given string to the file
• name.close(): Closes the file once writing is complete

Handling Missing Files

• Trying to open a file that doesn't exist causes a FileNotFoundError exception
• e.g. No such file or directory: 'fileName.txt'

File Processing

• A text file could be considered a sequence of lines
• Text files have newline characters at the end of each line

File Handle as a Sequence

• A file handle opened for reading can be treated as a sequence of strings
• Each line in the file is a string in this sequence.
• The for statement is used to iterate through a sequence

Counting Lines

• Open a file in read-only mode
• Use a for loop to read each line
• Count the lines and get a total count

Searching Through a File

• Put an if statement in a for loop to print out lines that meet certain criteria

Searching Through a File (Fixed)

• The rstrip() function, a string library function, strips white space from the string's right-hand side
• Newlines are considered "white space" and are stripped

Skipping with Continue

• The continue statement helps skip a line

Using in to Select Lines

• Searches a string anywhere in the line and uses this search as our selection criteria

Prompt for File Name

• Take a filename as input
• Count the number of lines that meet a certain criteria
• Print the filename alongside the number of lines

Bad File Names

• A try...except statement can handle cases where a file cannot be opened
• The except statement can print an error message and exit the program

Using File Objects to Read Files

• myFile = open("myStuff.txt")
• The line of code opens the "myStuff.txt" file in reading mode, and assigns the opened file to the myFile variable
• myFile is a variable of type file object
• Once the file is open, reading of it may begin

3 Ways to Read Files

• name = open("filename") opens the file, returning a file object
• name.read(): Reads the entire file, returning it as a string
• name.readline(): Reads the next line from the file as a string
• name.readlines(): Reads all the file content, returning it as a list of lines.
• Lines may also be read with a for loop

Closing Files

• f.close() closes the file

Exception handling

• Ensures, if an error happens we handle it instead of crashing the program

Introduction to Exception Handling

• Python programs often encounter errors
• Errors caused by not following proper language structure are syntax or parsing errors
• Errors can also occur at runtime, also known as exceptions
• FileNotFoundError occurs if Python can't find the file, ZeroDivisionError if dividing by zero
• If Python creates an exception object but isn't handled, the error will print along with details as to why the error occurred

Python Errors and Exceptions

• Encountering errors and exceptions can make coding frustrating
• Knowing about different types of errors helps
• Errors are classified into syntax errors, runtime errors or logical errors

Syntax Errors (Compile Time)

• Syntax errors result from not following proper language syntax
• Syntax errors happen when programming language rules are misused
• These are detected when the code is compiled

Runtime Errors (Exceptions)

• Exceptions happen when an program is run
• Caused by illegal operations, which may not be realized until running
• A syntactically correct program may cause Python to raise an exception
• e.g., trying to open nonexistent or corrupted files
• e.g., dividing a number by zero (ZeroDivisionError)

Logical Errors

• Logical errors produce incorrect results even when the program runs without crashing
• Logical errors result from a mistake in the program's logic
• Logical errors wont generate error messages because no syntax or runtime errors are encountered

Exceptions Defined

• Even if a statement or expression has proper syntax, it may cause an error at execution time
• Errors detected during execution are called exceptions
• An exception is an event disrupting the normal flow of program instructions

Error Examples

• Dividing by zero: results in ZeroDivisionError
• Referring to an undefined variable: results in a NameError
• Performing an operation on incompatible data types: results in a TypeError

Python Exception Handling

• When a Python script can't cope with a situation, it raises an exception.
• A Python script has to handle the exception immediately, or will terminate
• Python exception handling uses:
• The try block: Tests a block of code for errors.
• The except block: Handles errors.
• The finally block: Executed code regardless of try/except block outcomes
• When an error occurs, Python normally stops and generates an error message

Python Exception Handling – Try Block

• Exceptions can be handled with the try statement
• If the try block raises an error, the except block will be executed
• Without the try block, the program will crash and raise an error

Exception Handling - Example

• Perform division and understand exception handling
• If an exception occurs, "Exception handled" prints instead of error

The try / except Structure

• Surround a dangerous section of code with try and except
• If the try block works, the except block is skipped
• If the code in the try block fails, Python jumps to the except block
• A try statement can have multiple except blocks to handle different exceptions

Try with Different Exceptions

• Include multiple exceptions in the try and except block to give tailored responses

The try / except Structure - Continued

• Multiple exceptions are put into a single except block using parentheses
• This means the except block can handle multiple exceptions

More Examples

Try… Finally

• A finally block can be used with a try block
• the code placed in the finally block is executed no matter Exception is caused or caught
• Cannot use except clause and finally clause together with a try block
• It is also not possible to use else clause and finally together with try block.

Try-Finally Example

• If there is an error trying to write the file, the code in the finally clause will be executed

Built-in Exceptions

• ZeroDivisionError: Raised when division or modulo by zero takes place for all numeric types.
• NameError: Raised when name of keywords, identifiers, etc are wrong
• IndentationError: Raised when indentation is not properly given
• IOError: Raised when an input/output operation fails, such as print or open() function
• EOFError: Raised when there is no input from raw_input() or input() and end of file reached

Logging Exceptions

• Logging tracks events that happen when software runs
• Logging is important for developing, debugging, and running programs
• Log exceptions with logging.exception() method
• It logs a message with ERROR level
• Arguments are interpreted for debug()
• The method should only be called from an exception handler

Logging the Exceptions

• Logging an exception in Python with an error can be done in the logging.exception() method.
• Exception info is added to the logging message
• Only call logging.exception() from an exception handler

Summary

• Creating a text file
• Opening files in different modes
• Writing data into a file
• Reading from a file
• Searching through a file
• Exception handling