File Handling

Introduction

Every real program eventually needs to save information somewhere that survives after it finishes running. Variables disappear the moment your script ends, but files stick around on disk. Python's built-in file handling tools let you read data from files and write data back to them with just a few lines of code.

In this lesson, you will learn how to open files in different modes, read their contents in several ways, write and append new content, and safely check whether a file exists before working with it.

Why File Handling Matters

Without file handling, a program can only remember things while it is running. As soon as it closes, everything is lost. Files let you store settings, logs, user data, reports, and results permanently, and share that data between different runs of a program or even between different programs.

Opening a File with open()

The built-in open() function is how Python connects to a file on disk. It takes a file path and a mode, and returns a file object you can use to read or write data.

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

<_io.TextIOWrapper name='notes.txt' mode='r' encoding='cp1252'>

The output shows a file object, not the file's contents. To see the text inside, you need to call a reading method, which is covered shortly. Notice the call to close() at the end — forgetting this is a very common mistake, which is exactly why the with statement exists.

File Modes

The second argument to open() is the mode, which tells Python what you intend to do with the file.

The with Statement (Context Manager)

Manually calling open() and close() works, but it is risky: if an error happens between those two lines, close() never runs, and the file can stay locked or lose unsaved data. The with statement solves this by automatically closing the file for you, even if an exception occurs inside the block.

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

# file is automatically closed here, even if an error happened above
print("Is file closed?", file.closed)

Hello from notes.txt!
Is file closed? True

The with statement works because open() returns what Python calls a context manager — an object that knows how to set up a resource and clean it up automatically. You will learn exactly how context managers work under the hood, and how to build your own, in the dedicated Context Managers lesson later in this course.

Reading Files

Python gives you three main tools to read file content: read(), readline(), and readlines(). Assume notes.txt contains three lines: "First line", "Second line", and "Third line".

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

First line
Second line
Third line

read() loads the entire file into a single string, including newline characters. This is convenient for small files but can use a lot of memory for very large ones.

with open("notes.txt", "r") as file:
    first = file.readline()
    second = file.readline()
    print(repr(first))
    print(repr(second))

'First line\n'
'Second line\n'

readline() reads just one line each time it is called, including the trailing newline character, and remembers where it left off for the next call.

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

['First line\n', 'Second line\n', 'Third line']

readlines() returns a list where each element is one line of the file, again including newline characters.

Reading Line by Line with a Loop

For large files, the most memory-efficient way to read is to loop directly over the file object. Python reads one line at a time instead of loading the whole file into memory at once.

with open("notes.txt", "r") as file:
    for line in file:
        print(line.strip())

First line
Second line
Third line

Calling .strip() removes the trailing newline character so each printed line does not have an extra blank line after it.

Writing to a File

Use mode "w" to write new content to a file. Remember that this erases any existing content in the file first.

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

with open("greeting.txt", "r") as file:
    print(file.read())

Hello, Python!
This is a new file.

The write() method does not add a newline automatically, so you must include \n yourself when you want the next write to start on a new line.

Appending to a File

Use mode "a" when you want to add new content to the end of a file without deleting what is already there.

with open("greeting.txt", "a") as file:
    file.write("This line was appended.\n")

with open("greeting.txt", "r") as file:
    print(file.read())

Hello, Python!
This is a new file.
This line was appended.

Checking if a File Exists

Opening a file that does not exist in "r" mode raises a FileNotFoundError. You can avoid this by checking whether the file exists first, using the os.path module.

import os

filename = "notes.txt"

if os.path.exists(filename):
    print(f"{filename} exists, reading it now.")
    with open(filename, "r") as file:
        print(file.read())
else:
    print(f"{filename} was not found.")

notes.txt exists, reading it now.
First line
Second line
Third line

You will learn a more robust way to handle a missing file — using try/except to catch FileNotFoundError directly — in the very next lesson on exception handling.

Common Mistakes

Best Practices

Frequently Asked Questions

Key Takeaways

Summary

File handling lets your Python programs save and retrieve data that outlives a single run of the script. You learned how to open files safely with the with statement, read their contents in several ways, write and append new content, and check for a file's existence before working with it.