Python Tutorial · Working with Data · Lesson 37 of 42

Python File Handling

Read and write text files, CSV and paths safely with with-statements and pathlib.

Open, read and write

Use with open(...) — it closes the file automatically, even if an error occurs.

Example 1Try it yourself

with open("notes.txt", "w", encoding="utf-8") as f:
    f.write("first line\n")
    f.write("second line\n")

with open("notes.txt", "a", encoding="utf-8") as f:
    f.write("appended line\n")

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

Output

first line
second line
appended line

Mode	Meaning
`"r"`	read (default)
`"w"`	write (overwrites)
`"a"`	append
`"x"`	create, fail if it exists
`"b"`	binary, e.g. `"rb"`

Always pass encoding="utf-8" for text files.

Reading line by line

Iterating over a file reads one line at a time — memory-friendly for large files.

Example 2Try it yourself

with open("log.txt", "w", encoding="utf-8") as f:
    f.write("INFO start\nERROR disk full\nINFO retry\nERROR timeout\n")

with open("log.txt", encoding="utf-8") as f:
    errors = [line.strip() for line in f if line.startswith("ERROR")]
print(errors)

Output

['ERROR disk full', 'ERROR timeout']

CSV and pathlib

Example 3Try it yourself

import csv
from pathlib import Path

path = Path("people.csv")
with path.open("w", newline="", encoding="utf-8") as f:
    writer = csv.writer(f)
    writer.writerow(["name", "city"])
    writer.writerows([["Ada", "London"], ["Linus", "Helsinki"]])

with path.open(encoding="utf-8") as f:
    for row in csv.DictReader(f):
        print(row["name"], "lives in", row["city"])
print(path.exists(), path.stat().st_size, "bytes")

Output

Ada lives in London
Linus lives in Helsinki
True 39 bytes

Test yourself

Which mode opens a file for appending?

"r"
"w"
"a"
"x"

Show answer

Answer: "a"