Working with files is a fundamental skill for every Python developer. Whether you want to read from a file, write logs, or manage datasets, Python’s built-in open() function is the key.

This tutorial will teach you:

• What the open() function is and how it works

• File modes (read, write, append, etc.)

• How to read and write files

• Best practices using the with statement

• Common mistakes and how to avoid them

• A complete real-world example

What is the open() Function?

The open() function opens a file and returns a file object, which gives you access to methods like .read(), .write(), .close() and more.

Syntax:

open(file, mode='r', encoding=None)

• file: The name (or path) of the file to open

• mode: The file access mode (default is 'r' for read)

• encoding: Optional, but usually set to "utf-8" for text files

File Modes Explained

You can combine modes like 'rb', 'w+', 'a+' etc.

✏️ Writing to a File in Python

Using 'w' mode — Overwrites the file

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

This will create example.txt if it doesn’t exist, or overwrite it if it does.

Using 'a' mode — Appends to the file

with open("example.txt", "a") as file:
    file.write("\nAppended line.")

Reading from a File in Python

1. .read() — Read the entire file as a single string

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

2. .readline() — Read one line at a time

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

3. .readlines() — Read all lines as a list

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

✅ Why Use the with Statement?

Using with open(...) automatically:

• Opens the file

• Closes the file (even if an error occurs)

• Is cleaner and safer than manually using .close()

Example:

with open("file.txt", "r") as f:
    data = f.read()
# No need to call f.close()

Complete Example: File Logger

Let’s create a logger that writes and reads logs from a file.

def log_event(message):
    with open("log.txt", "a") as f:
        f.write(message + "\n")

def read_log():
    try:
        with open("log.txt", "r") as f:
            print("Log Contents:")
            print(f.read())
    except FileNotFoundError:
        print("No log file found.")

# Usage
log_event("Application started")
log_event("User logged in")
read_log()

Sample Output:

Log Contents:
Application started
User logged in

Pro Tips for Using open()

• ✅ Always use with to handle files safely

• ✅ Use encoding="utf-8" for text files to support special characters

• ✅ Check if a file exists with os.path.exists() before reading

• ✅ Prefer 'a' mode when logging to avoid overwriting old data

⚠️ Common Pitfalls and Mistakes

Summary

Final Thoughts

Python’s open() function is powerful yet simple. With a good understanding of modes and best practices like using with, you can confidently handle files in any project—logs, config files, data import/export, and more.