Writing Without Destroying

Writing a file is as easy as reading one, and far more dangerous. Open a file for writing in the usual mode and its old contents are gone before you have written a single character. This page shows the difference between replacing a file and adding to it, the one-letter slip that quietly erases a log, and the careful way to replace a file that matters.

Think of the difference between writing on a fresh page and adding a line at the bottom of a diary. Replacing a file is tearing out the old page first. Sometimes that is what you want; you should never do it by accident.

"w": Replace

The second argument to open is the mode. The letter w means write, and it empties the file the moment it is opened, or creates the file if it does not exist yet. Then write puts text into it. write adds exactly what you give it and nothing more, so each line needs its own line break, written as backslash-n inside the string.

with open("shopping.txt", "w", encoding="utf-8") as f:
    f.write("bread\n")
    f.write("milk\n")

with open("shopping.txt", encoding="utf-8") as f:
    print(f.read())

bread
milk

"a": Append

The letter a means append. The file keeps what it already has, and new text goes at the end. It is the right mode for anything that accumulates: a log, a diary, a record of payments.

The Mistake That Erases a Log

Here is the mistake to recognise. A small program keeps a daily log. Each day it opens the log and writes one line. The program below imitates three days with a loop. Opened with w, each day's opening empties the file, and at the end only Wednesday's line survives. Nothing warned anyone.

for day in ["Monday", "Tuesday", "Wednesday"]:
    with open("log.txt", "w", encoding="utf-8") as f:
        f.write(f"{day}: backed up the photos\n")

with open("log.txt", encoding="utf-8") as f:
    print(f.read())

Wednesday: backed up the photos

The fixed version differs by one letter, a instead of w, and writes to a fresh file, log2.txt, so the result is not mixed up with the first experiment. All three days are kept.

for day in ["Monday", "Tuesday", "Wednesday"]:
    with open("log2.txt", "a", encoding="utf-8") as f:
        f.write(f"{day}: backed up the photos\n")

with open("log2.txt", encoding="utf-8") as f:
    print(f.read())

Monday: backed up the photos
Tuesday: backed up the photos
Wednesday: backed up the photos

Replacing Safely

Sometimes you really do want to replace a file: a report rebuilt from scratch, a settings file saved with new values. The risk is the moment in between. If the program crashes after w has emptied the file but before the new content is written, the old content is gone and the new content never arrived.

The careful pattern is to write the new version to a separate file first and, only once it is complete, swap it into place. The pathlib module, which the next topic introduces properly, has a replace method that does the swap in one step: the finished file takes the old one's name. If the program fails while writing, the original is still whole.

from pathlib import Path

report = "Total spent: 177.75\nFood: 58.50\n"

new_file = Path("report.txt.new")
new_file.write_text(report, encoding="utf-8")
new_file.replace("report.txt")

print(Path("report.txt").read_text(encoding="utf-8"))

Total spent: 177.75
Food: 58.50

write_text and read_text are shortcuts that open, write or read, and close a file in one call. They are handy when you want the whole file at once.