File Input and Output (I/O) in Python

In Python, file I/O is used to read from or write to files. This is an essential part of any programming language when it comes to data processing, logging, or configuration.

Opening Files

To work with files in Python, you use the built-in open() function.

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

Modes:

Mode	Description
'r'	Read (default). Fails if the file doesn’t exist.
'w'	Write. Creates a new file or truncates existing one.
'a'	Append. Adds content to the end of the file.
'b'	Binary mode. Used with 'rb', 'wb', etc.
'x'	Create. Fails if the file already exists.

Reading from a File

read() – Reads entire content

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

readline() – Reads one line at a time

with open("example.txt", "r") as file:
    line = file.readline()
    print(line)

readlines() – Reads all lines into a list

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

Writing to a File

write() – Write string to file

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

writelines() – Write list of strings

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

Using with Statement (Best Practice)

The with block ensures the file is automatically closed after use:

with open("data.txt", "r") as file:
    data = file.read()

This is the recommended way to handle files in Python.

Error Handling in File I/O

Always handle file operations with care to avoid exceptions:

try:
    with open("config.txt", "r") as file:
        config = file.read()
except FileNotFoundError:
    print("File not found.")
except IOError:
    print("Error while handling the file.")

File Paths

You can also handle file paths using the os or pathlib module:

from pathlib import Path

file_path = Path("docs") / "myfile.txt"
with open(file_path, "r") as file:
    print(file.read())

Example: Reading & Writing

# Write to a file
with open("sample.txt", "w") as file:
    file.write("This is a test.")

# Read the file
with open("sample.txt", "r") as file:
    print(file.read())

Summary

• Use open() to access files.
• Use read(), readline(), or readlines() to read.
• Use write() or writelines() to write.
• Always use with to handle files safely.
• Handle exceptions for robustness.