Python File Handling

Reading from and writing to files lets a Python program persist data beyond a single run, or process data that already exists on disk. Python's built-in open() function is the entry point for all file operations, returning a file object that supports reading, writing, and iteration.

The recommended way to work with files in Python is the with statement, which guarantees the file is properly closed as soon as the block finishes — even if an error occurs partway through — without requiring you to remember to call close() manually.

Why it matters

Programs keep data between runs by writing it to files: reports, exports, configuration, logs. The rules are few, but mistakes are costly: opening a file in the wrong mode erases it, and forgetting to close it can lose data. The with statement shown in this lesson is the form used in all professional code.

Opening Files and Modes

open(filename, mode) opens a file and returns a file object. The mode string controls how the file is accessed.

• "r" — read (default); the file must already exist, or a FileNotFoundError is raised.
• "w" — write; creates the file if it does not exist, and completely overwrites (truncates) it if it does.
• "a" — append; creates the file if it does not exist, and adds new content to the end without erasing what is already there.
• "r+" — read and write, without truncating the existing content.
• Adding "b" to any mode (e.g. "rb", "wb") opens the file in binary mode instead of text mode.

The with Statement (Context Manager)

Writing with open("data.txt", "r") as f: opens the file and binds it to the name f for the duration of the indented block. When the block ends — normally or due to an exception — Python automatically calls f.close() for you. This pattern is strongly preferred over manually calling open() and close(), because a forgotten close() can leave a file locked or its buffered writes unflushed.

Reading and Writing Content

Inside a "with" block opened for reading, f.read() returns the entire file's contents as one string, f.readline() reads a single line at a time, and f.readlines() returns a list of all lines; you can also iterate directly over the file object line by line with a for loop, which is the most memory-efficient option for large files. For writing, f.write(text) writes a string to the file (it does not add a newline automatically, unlike print()).

Examples

with open("notes.txt", "w") as f:
    f.write("First line\n")
    f.write("Second line\n")
    f.write("Third line\n")

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

First line
Second line
Third line

with open("notes.txt", "a") as f:
    f.write("Fourth line\n")

with open("notes.txt", "r") as f:
    contents = f.read()

print(contents)

First line
Second line
Third line
Fourth line

Exercise

In a temporary folder, write three lines to a text file, read them back, and print how many lines there are and the first line. Then append a fourth line and print the new number of lines.

Expected output: 3, first, 4 (one per line)

import tempfile
from pathlib import Path

with tempfile.TemporaryDirectory() as folder:
    path = Path(folder) / "notes.txt"

    # TODO: write the lines "first", "second" and "third" to the file

    # TODO: read the lines back, print how many there are and the first one

    # TODO: append the line "fourth", read the file again and print the line count
    pass

import tempfile
from pathlib import Path

with tempfile.TemporaryDirectory() as folder:
    path = Path(folder) / "notes.txt"

    with open(path, "w") as file:
        file.write("first\nsecond\nthird\n")

    with open(path) as file:
        lines = file.read().splitlines()
    print(len(lines))
    print(lines[0])

    with open(path, "a") as file:
        file.write("fourth\n")

    with open(path) as file:
        print(len(file.read().splitlines()))

Quick quiz

Interview questions