reading-files.md

Python by Example: Reading Files

Reading files is a core skill. Use open() with mode 'r' (default) and a context manager (with) so the file is closed automatically. Read the whole file with read(), all lines with readlines(), or iterate line by line for large files. The example uses a temp file so it works cross-platform.

What you'll learn:

• Opening files with open()
• read(), readlines(), and iteration
• Context managers for automatic cleanup

import tempfile
import os

# Create a temp file for demo (cross-platform)
fd, path = tempfile.mkstemp(suffix=".txt")
try:
    with os.fdopen(fd, "w") as f:
        f.write("line 1\nline 2\nline 3\n")

    # Read entire file
    with open(path) as f:
        content = f.read()
    print(content)

    # Read lines
    with open(path) as f:
        lines = f.readlines()
    print(lines)

    # Iterate line by line (memory efficient)
    with open(path) as f:
        for line in f:
            print(line.rstrip())
finally:
    os.unlink(path)

Iterating with for line in f is memory-efficient—it doesn't load the whole file. readlines() returns a list of lines, including newlines.

To run this program:

$ python source/reading-files.py
line 1
line 2
line 3

['line 1\n', 'line 2\n', 'line 3\n']
line 1
line 2
line 3

Tip: For very large files, iterate line by line. For small files, read() or readlines() is fine.

Try it: Read a file you create and print each line with its line number.

Source: reading-files.py

Next: Writing Files