Python File Handling - CMPE 30032 Module 2

File Handling in Python

• File is a named location on disk to store related information, used to permanently store data in a non-volatile memory (e.g., hard disk).
• File handling involves opening, reading, writing, appending, and closing a file.

File Types

• There are two types of files: text files and binary files.
• Text files store data in the form of characters, used to store characters or strings.
• Binary files store entire data in the form of bytes, used to store text, images, audio, and video.

File Permissions

• Read (r): file can be 'looked at' by the program, but not changed.
• Append (a): file can be added to, file is created if it doesn't already exist.
• Write (w): overwrites existing data, starting from the beginning of the file, file is created if it doesn't already exist.

Reading from a File

• The algorithm: connect to and open the file, read the contents into a variable, output the variable, and close the file.
• Read functions: read(n), readline(), readlines()
• read(n): read at most n characters from the file, reads till end of file if it is negative or None.
• readline(): read and return one line from the file, reads in at most n bytes if specified.
• readlines(): read and return a list of lines from the file, reads in at most n bytes/characters if specified.

Writing & Appending to a File

• Write to a file: open the file in write mode (w), use the write() function to write data, and close the file.
• Append to a file: open the file in append mode (a), use the write() function to write data, and close the file.

Lab Exercises

• Lab Exercise 4: write a Python program to read a text file, extract even and odd numbers, and write them to two separate files.
• Lab Exercise 5:
  • 5.1: write a method to write multiple lines of text into a text file.
  • 5.2: write a method to create two separate text files containing the square of even integers and the cube of odd numbers found in a source text file.
  • 5.3: write a Python program to read a binary file, extract the name of the student with the highest GWA, and output the result.

String Manipulation

• A string in Python is created by enclosing characters in quotes (either single quotes or double quotes)
• Indexing: Python uses zero-based indexing, where the first character of a string has an index of 0
• Accessing characters in a string: s[0], s[1], etc.
• Slicing: s[3:5] returns a substring from index 3 to 5 (excluding 5)
• String methods:
  • len(s): returns the length of the string
  • s.find("substring"): returns the index of the first occurrence of the substring
  • s.count("substring"): returns the number of occurrences of the substring
  • s.startswith("prefix"): returns True if the string starts with the prefix
  • s.endswith("suffix"): returns True if the string ends with the suffix
  • s.isdigit(): returns True if the string consists of digits only
  • s.isalpha(): returns True if the string consists of alphabets only
  • s.isalnum(): returns True if the string consists of alphanumeric characters only
  • s.title(): returns the string with the first character of each word capitalized
  • s.upper(): returns the string in uppercase
  • s.lower(): returns the string in lowercase
  • s.strip(): returns the string with leading and trailing whitespace removed
  • s.lstrip(): returns the string with leading whitespace removed
  • s.rstrip(): returns the string with trailing whitespace removed
  • s.replace("old", "new"): returns the string with all occurrences of "old" replaced with "new"

File Handling

• File types:
  • Text files: used to store text data
  • Binary files: used to store binary data (such as images)
• File operations:
  • Reading from a file: open("file.txt", "r")
  • Writing to a file: open("file.txt", "w")
  • Appending to a file: open("file.txt", "a")
• File modes:
  • r: read mode
  • w: write mode
  • a: append mode
• File permissions:
  • Read permission: allows reading from the file
  • Write permission: allows writing to the file
  • Execute permission: allows executing the file
• Reading from a file:
  • read(n): reads at most n characters from the file
  • readline(): reads and returns one line from the file
  • readlines(): reads and returns a list of lines from the file
• Writing to a file:
  • write(string): writes the string to the file
  • writelines(list): writes the list of strings to the file

Exception Handling

• Types of exceptions:
  • Checked exceptions: occur at compile-time (e.g. syntax errors)
  • Unchecked exceptions: occur at runtime (e.g. division by zero)
• Handling exceptions:
  • try block: contains the code that might raise an exception
  • except block: catches and handles the exception
  • else block: executes if no exception is raised
  • finally block: executes regardless of whether an exception is raised or not
• Example of exception handling:

try:
    # code that might raise an exception
except ExceptionType:
    # handle the exception
else:
    # execute if no exception is raised
finally:
    # execute regardless of whether an exception is raised or not

Object-Oriented Programming

• Programming paradigms:
  • Structured programming: focuses on procedures and functions
  • Object-oriented programming: focuses on objects and classes
• Object-oriented programming concepts:
  • Classes: define the structure and behavior of objects
  • Objects: instances of classes
  • Inheritance: a class can inherit properties and behavior from another class
  • Polymorphism: objects of different classes can be treated as if they were of the same class
  • Encapsulation: hiding the implementation details of an object from the outside world
• Class attributes:
  • Instance variables: attributes that belong to an instance of a class
  • Class variables: attributes that belong to a class
• Class methods:
  • Instance methods: methods that belong to an instance of a class
  • Class methods: methods that belong to a class
  • Static methods: methods that can be called without creating an instance of a class
• Constructors:
  • __init__: a special method that is called when an object is created
  • Default constructors: constructors that are called when no arguments are provided
  • Parameterized constructors: constructors that take arguments
• Inheritance:
  • Single inheritance: a class inherits from one parent class
  • Multiple inheritance: a class inherits from multiple parent classes
  • Multilevel inheritance: a class inherits from a parent class that itself inherits from another parent class
  • Hybrid inheritance: a combination of multiple and multilevel inheritance
• Method overriding: a child class provides a different implementation of a method that is already defined in its parent class