File I/O Guide

Python provides several built-in functions to work with files, making it easy to read from and write to files on your system. Here’s a summary of key file operations in Python.

• Using a for loop with a file open:

  • Example:

    # Open the file for reading.
    with open("sample.txt", "r") as file:   
        # For each line in a file.
        for line in file:
            # Print the line.
            print(line)
    

• open(file, mode):
  • Description: Opens a file and returns a file object. This is the first step in working with a file.
  • Parameters:
    • file: The name of the file (including path if needed).
    • mode: Specifies the mode in which to open the file.
      • 'r': Read (default). Opens a file for reading.
      • 'w': Write. Creates a new file or overwrites if the file exists.
      • 'a': Append. Adds to the end of the file if it exists.
      • 'r+': Read and write.
  • Example:

    file = open("example.txt", "r")  # Opens a file in read mode
    print(file.read())  # Reads file content
    file.close()  # Closes the file
    

• read(size):
  • Description: Reads up to size bytes from the file. If size is not specified, reads the entire file.
  • Parameters:
    • size (optional): The number of bytes to read. Reads the entire file if not specified.
  • Example:

    with open("example.txt", "r") as file:
        content = file.read(10)  # Reads the first 10 characters
        print(content)
    

• readline():
  • Description: Reads a single line from the file.
  • Example:

    with open("example.txt", "r") as file:
        first_line = file.readline()
        print(first_line)  # Outputs the first line
    

• readlines():
  • Description: Reads all lines in a file and returns them as a list.
  • Example:

    with open("example.txt", "r") as file:
        lines = file.readlines()
        print(lines)  # Outputs a list of lines
    

• write(data):
  • Description: Writes data to a file. If the file is opened in write mode ('w'), it will overwrite existing content.
  • Parameters:
    • data: The string to write to the file.
  • Example:

    with open("example.txt", "w") as file:
        file.write("Hello, world!")
    

• writelines(lines):
  • Description: Writes a list of strings to a file.
  • Parameters:
    • lines: A list of strings to write.
  • Example:

    with open("example.txt", "w") as file:
        lines = ["Line 1
    ", "Line 2
    ", "Line 3
    "]
        file.writelines(lines)
    

• close():
  • Description: Closes the file. Always close files to free up system resources.
  • Example:

    file = open("example.txt", "r")
    print(file.read())
    file.close()
    

• with statement:
  • Description: A context manager that automatically closes the file after the block of code runs, making code cleaner and safer.
  • Example:

    with open("example.txt", "r") as file:
        content = file.read()
        print(content)
    # File is automatically closed after this block
    

• File Modes Summary:
  • 'r': Read only.
  • 'w': Write only (overwrites if file exists).
  • 'a': Append only.
  • 'r+': Read and write.