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18 · File Handling

Part 4 — Real-World Python · Estimated time: 25–35 min · Prerequisite: 07 · Loops

Programs become genuinely useful when they can save and load data — notes, scores, settings, logs. Python reads and writes files with the open() function and the with statement. This lesson covers writing, reading, appending, and handling a missing file.

ℹ️ Every example below cleans up after itself (it deletes the file it made), so running them won't litter your folder. In the in-browser version, files live in a temporary sandbox and vanish when you reload.

What you'll learn

  • Opening files with open() and the safe with statement
  • Writing ("w"), reading ("r"), and appending ("a")
  • Reading a file line by line
  • Handling a missing file with FileNotFoundError

1. Write and read

The with open(...) statement opens a file and automatically closes it when the block ends. Mode "w" writes (creating or overwriting); mode "r" reads.

with open("sample.txt", "w") as f:
    f.write("Hello, file!\n")
    f.write("Second line.\n")

with open("sample.txt", "r") as f:   # "r" is the default mode
    contents = f.read()
print(contents)

▶️ Run it: python examples/01_write_and_read.py

2. Read line by line

Looping over a file gives you one line at a time — memory-friendly for big files. Each line keeps its trailing newline, so .strip() is handy.

with open("fruits.txt") as f:
    for line in f:
        print(line.strip())

▶️ Run it: python examples/02_reading_lines.py

💡 Try it yourself: read a file into a list of lines with f.readlines() and print how many there are.

3. Append instead of overwrite

Mode "w" erases the file first. Mode "a" appends to the end, keeping what's already there.

with open("log.txt", "w") as f:
    f.write("line 1\n")
with open("log.txt", "a") as f:   # add without erasing
    f.write("line 2\n")

▶️ Run it: python examples/03_append.py

4. Handling a missing file

Opening a file that doesn't exist for reading raises FileNotFoundError. Catch it (you met try/except in the previous lesson):

try:
    with open("missing.txt") as f:
        print(f.read())
except FileNotFoundError:
    print("That file doesn't exist yet.")

▶️ Run it: python examples/04_missing_file.py

Common mistakes

Mistake What happens Fix
Opening with "w" to keep old data the file is wiped first Use "a" to append.
Forgetting to close the file data may not be saved Use with open(...) — it closes for you.
Reading a missing file FileNotFoundError Wrap it in try/except, or check it exists.
Expecting lines without \n each line keeps its newline Use line.strip() when printing.

Recap / cheat-sheet

with open("f.txt", "w") as f:   # write (overwrite/create)
    f.write("text\n")

with open("f.txt", "a") as f:   # append
    f.write("more\n")

with open("f.txt") as f:        # read (default mode "r")
    all_text = f.read()         # whole file as one string
    # or:
    for line in f:              # one line at a time
        print(line.strip())

import os
os.remove("f.txt")              # delete a file

Exercises

Run a file with python exercises/<file>.py, then compare with the matching file in solutions/. Each solution creates its file, uses it, then deletes it.

  1. exercises/01_save_note.py — write a note to a file and read it back.
  2. exercises/02_count_lines.py — write several lines, then count them.
  3. exercises/03_append_entries.py — write then append, and read the result.

Try each yourself before opening the solution.

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