Reading and Writing Files

Reading and Writing Files

Variables die with the program. Files are how data survives — and Python’s with open(...) pattern makes reading and writing them one of the cleanest parts of the language.

Writing a file

with open("notes.txt", "w") as f:
    f.write("Line one\n")
    f.write("Line two\n")

print("saved!")

Decode the line:

• open("notes.txt", "w") — open the file in mode "w" (write)
• as f — the open file is called f inside the block
• f.write("...") — write text (no automatic newline — hence the \ns)
• with — the magic: when the block ends, the file is closed automatically, even if an error happened

Always use with. Manual open()/close() works but forgetfulness causes real data loss; with makes closing impossible to forget.

The modes (there are only three you need)

The "w" erasure surprises people:

with open("notes.txt", "w") as f:
    f.write("just this")

# notes.txt now contains ONLY "just this" — previous content gone!

Adding to a log or history? "a". Creating a fresh report? "w".

Reading a file — three ways

with open("notes.txt", "r") as f:
    # Way 1: everything as one string
    content = f.read()
    print(content)

with open("notes.txt") as f:          # "r" is the default
    # Way 2: list of lines (newlines included)
    lines = f.readlines()
    print(lines)                       # ['Line one\n', 'Line two\n']

# Way 3: line by line — the memory-friendly way
with open("notes.txt") as f:
    for line in f:
        print(line.strip())            # strip removes the trailing \n

Way 3 is the professional default — it processes one line at a time, so even a million-line file never overloads memory. The .strip() in the loop matters: each line arrives with its invisible \n attached.

The classic combo: write then read back

# Save
with open("highscore.txt", "w") as f:
    f.write("1250")

# Later... load
with open("highscore.txt") as f:
    score = int(f.read().strip())

print(f"High score: {score}")      # High score: 1250

Notice int() — files give back strings, always. Numbers round-trip through str() and int().

FileNotFoundError — your first handled file error

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

Reading a missing file raises FileNotFoundError — wrap it when the file might legitimately be absent (first run of an app, optional config).

Common Errors & Fixes

• FileNotFoundError — wrong filename or wrong folder. Files open relative to where you ran the script, not where the script lives. Use full paths or run from the right directory.
• File is empty after writing — you forgot f.write() entirely, or wrote after the with block closed.
• io.UnsupportedOperation: not readable — opened with "w" (write-only) and tried to read. Reopen with "r" or "w+".
• Weird \n at the end of every line — that’s the line’s real newline; .strip() it.

✅ Checkpoint

1. Which mode erases the file first? (“w”)
2. Why with instead of open/close? (Auto-closes even on errors — can’t forget)
3. What does each line from a for-loop read arrive with? (A trailing \n — strip it)
4. What type is everything read from a file? (String — convert numbers yourself)

Next: JSON and CSV — the two file formats that power real applications.