Reading and Writing Text Files

Why this matters

Variables vanish when your program ends. Files are how programs remember: saved documents, logs, settings, data to analyse. Reading and writing text files takes only a few lines.

The idea

with open("notes.txt", "w") as file:   # "w" = write (creates or replaces the file)
    file.write("first line\n")
    file.write("second line\n")

with open("notes.txt") as file:        # default mode is "r" = read
    for line in file:
        print(line.rstrip("\n"))       # each line still ends with "\n"

• with open(...) as file: opens the file and closes it automatically when the block ends, even if something goes wrong. Always use it.
• Modes: "r" read (the default), "w" write (erases existing content), "a" append (adds to the end).
• file.write(text) writes exactly what you give it. It does not add a newline, so add "\n" yourself.
• file.read() returns the whole text; looping over file gives one line at a time (including its "\n").

Reading a file that does not exist is a FileNotFoundError. (A later lesson shows how to handle errors gracefully.)

Try it

Write three lines to a file, then read them back and print them with line numbers using enumerate. Open the file again in "a" mode and add a fourth line.

Your turn

The checks give your functions a file path (so they never touch your own files). Write three functions:

• save_lines(path, lines): write each string in lines on its own line, replacing any existing content.
• load_lines(path): return the lines of the file as a list of strings without the trailing newline characters.
• append_line(path, line): add one line to the end of the file, keeping what was there.

Recap

• with open(path, mode) as file: opens a file and always closes it.
• "r" reads, "w" replaces, "a" appends; write needs explicit "\n".
• Iterate over a file to get lines, and rstrip("\n") to remove the line ending.

Common mistakes

• Opening with "w" when you meant "a", which erases the file.
• Forgetting "\n", so every line is joined into one.
• Using the file after the with block, when it is already closed (ValueError: I/O operation on closed file).