Reading and Writing Files

Python provides powerful built-in functions for reading from and writing to files. File handling is an essential skill for interacting with external data.

Opening a File

Use the open() function to open a file. The mode determines how the file is accessed.

Common Modes:

• "r": Read (default).
• "w": Write (overwrites the file).
• "a": Append (adds to the end of the file).
• "b": Binary mode (e.g., "rb", "wb").
• "x": Create (fails if the file exists).

Example:

# Open a file for reading
file = open("example.txt", "r")

# Close the file
file.close()

Reading a File

Read Entire File:

with open("example.txt", "r") as file:
    content = file.read()
    print(content)

Read Line by Line:

with open("example.txt", "r") as file:
    for line in file:
        print(line.strip())  # Removes trailing newline

Read Specific Number of Characters:

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

Writing to a File

Writing Text:

with open("output.txt", "w") as file:
    file.write("Hello, World!\n")
    file.write("This is a new file.")

Appending to a File:

with open("output.txt", "a") as file:
    file.write("\nAppending this line.")

Working with Files Using with

Using the with statement ensures the file is properly closed, even if an error occurs.

Example:

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

File Methods

Working with Binary Files

For non-text files like images or audio, use binary mode ("b").

Example:

# Copy an image
with open("source.jpg", "rb") as source:
    content = source.read()

with open("copy.jpg", "wb") as target:
    target.write(content)

Checking if a File Exists

Use the os module to check for a file’s existence.

Example:

import os

if os.path.exists("example.txt"):
    print("File exists!")
else:
    print("File not found!")

Practice Exercises

1. Read a File:
   • Create a file data.txt with some text.
   • Write a script to read and print its contents.
2. Write to a File:
   • Create a file notes.txt and write three lines of text.
   • Append two more lines to the same file.
3. Copy a File:
   • Write a script to copy the contents of one file to another.
4. File Checker:
   • Write a script that checks if a file exists and prints its size if it does.

File handling in Python is essential for working with data from external sources, saving results, and managing files efficiently!