8.1 Python File I/O

File handling is a fundamental aspect of Python programming, allowing you to interact with files directly on your system. This guide covers the essential operations for reading, writing, and managing files in Python, along with best practices.

1. Opening a File

To interact with a file, you use Python's built-in open() function.

Syntax

open(file, mode='r', buffering=-1, encoding=None)

• file: The path to the file you want to open.
• mode: A string specifying how the file will be used. Defaults to 'r' (read).
• buffering: Controls the buffering strategy. -1 uses the system default.
• encoding: The encoding to use for text files. Defaults to the system's default encoding.

Common File Modes

Example

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

# It's best practice to use the 'with' statement for automatic file closing
with open("example.txt", "r") as file:
    # File operations go here
    pass # Placeholder for file operations

2. Reading from a File

Python provides several methods for reading content from a file:

a. read(): Read Entire File

Reads the entire content of the file as a single string.

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

b. readline(): Read Line by Line

Reads a single line from the file, including the newline character (\n) at the end.

with open("example.txt", "r") as file:
    line1 = file.readline()
    line2 = file.readline()
    print(f"Line 1: {line1.strip()}") # .strip() removes leading/trailing whitespace, including newline
    print(f"Line 2: {line2.strip()}")

c. readlines(): Read All Lines into a List

Reads all lines from the file and returns them as a list of strings, with each string representing a line.

with open("example.txt", "r") as file:
    lines = file.readlines()
    for line in lines:
        print(line.strip())

3. Writing to a File

You can write data to a file using these methods:

a. write(): Write a String

Writes a single string to the file. You need to explicitly include newline characters (\n) if you want lines to be separate.

with open("output.txt", "w") as file:
    file.write("Hello, World!\n")
    file.write("Welcome to Python file handling.\n")

b. writelines(): Write a List of Strings

Writes a list of strings to the file. Each string in the list should ideally end with a newline character.

lines_to_write = ["First line\n", "Second line\n", "Third line\n"]

with open("lines.txt", "w") as file:
    file.writelines(lines_to_write)

4. Appending Data to a File

To add new content to the end of a file without deleting its existing content, use the append mode ('a').

with open("output.txt", "a") as file:
    file.write("\nThis is an appended line.")
    file.write("\nAnother appended line.")

5. Closing a File

When you open a file, it's crucial to close it to release system resources and ensure all buffered data is written. The with statement handles this automatically, making it the recommended approach.

If you don't use with, you must manually call the close() method:

file = open("example.txt", "r")
# Perform file operations...
file.close()

Recommendation: Always prefer using the with open(...) as ...: syntax.

6. Working with Binary Files

For non-text files like images, audio, or executable files, you must open them in binary mode ('b').

a. Reading a Binary File

with open("image.png", "rb") as file:
    binary_data = file.read()
    # Process binary_data (e.g., store it, display it, etc.)

b. Writing a Binary File

with open("copy.png", "wb") as file:
    file.write(binary_data) # Assuming binary_data was read from another file

7. Checking if a File Exists

Before performing operations that might fail if a file doesn't exist (like reading), it's good practice to check for its existence. The os module provides tools for this.

import os

file_path = "data.txt"

if os.path.exists(file_path):
    print(f"File '{file_path}' found.")
    # Proceed with file operations
else:
    print(f"File '{file_path}' does not exist.")

8. Useful File Object Methods

File objects returned by open() have several useful methods:

Conclusion

Python's file handling capabilities are both straightforward and powerful. Whether you're managing text files, binary files, or simply checking for file existence, Python offers flexible tools for efficient file operations. Mastering these I/O operations is crucial for tasks like data processing, report generation, and general interaction with external files in real-world applications.

Interview Questions on Python File Handling

• How do you open a file in Python, and what are the common file modes?
• Explain the difference between the read(), readline(), and readlines() methods.
• How do you write data to a file in Python?
• What is the difference between write mode ('w') and append mode ('a')?
• Why is it recommended to use the with statement when working with files?
• How can you read from and write to binary files in Python?
• How do you check if a file exists before performing file operations?
• What is the purpose of the seek() and tell() methods in file handling?
• How do you properly close a file, and what happens if you don't?
• Can you explain how to handle file exceptions or errors during file operations in Python? (Note: Exception handling wasn't explicitly covered in the basic guide but is a common follow-up.)