Files, Modules & pip

Open files with with open(...) as f: — the with block closes the file automatically, even if an error happens. Use mode "w" to write, "a" to append and the default "r" to read, and always pass encoding="utf-8".

Any .py file is a module. Put related functions in one file and import them elsewhere. The check if __name__ == "__main__": runs code only when the file is executed directly, not when it is imported.

Python's standard library covers a lot (csv, json, datetime, pathlib, random...). For everything else, install packages from PyPI with pip install inside your virtual environment, and record them in requirements.txt.

grading.pyPython

PASS_MARK = 30


def average(values: list[float]) -> float:
    return sum(values) / len(values) if values else 0.0


def passed(values: list[float]) -> int:
    return sum(1 for v in values if v >= PASS_MARK)

main.pyPython

import csv

from grading import average, passed


def main() -> None:
    with open("scores.csv", "w", newline="", encoding="utf-8") as f:
        writer = csv.writer(f)
        writer.writerow(["name", "score"])
        writer.writerows([["Amina", 88], ["Juma", 25], ["Neema", 71]])

    with open("scores.csv", encoding="utf-8") as f:
        rows = list(csv.DictReader(f))

    scores = [float(r["score"]) for r in rows]
    print(f"Average: {average(scores):.1f}")
    print(f"Passed: {passed(scores)} of {len(scores)}")


if __name__ == "__main__":
    main()

Runs in your browser · Python

TerminalShell

python main.py
pip install requests            # install a package from PyPI
pip freeze > requirements.txt   # record exact versions
pip install -r requirements.txt # recreate on another machine

Key points

Always use with open(...) and encoding="utf-8".
Use if __name__ == "__main__": for a module's entry point.
Install packages inside a virtual environment and pin them in requirements.txt.

Exercise

Create text_tools.py with word_count(text) and longest_word(text). In main.py, read a .txt file, print both results, and append a summary line to report.txt.

Show solution

Try the exercise yourself first — then compare your approach with this one.

text_tools.py holds the reusable functions. main.py imports them, reads the essay with with open(...), prints the results, and opens report.txt in append mode ("a") so each run adds a new line instead of overwriting.

text_tools.pyPython

def word_count(text: str) -> int:
    return len(text.split())


def longest_word(text: str) -> str:
    return max(text.split(), key=len, default="")

main.pyPython

from text_tools import longest_word, word_count


def main() -> None:
    with open("essay.txt", "w", encoding="utf-8") as f:
        f.write("Photosynthesis happens in the chloroplasts of green plants.")

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

    count = word_count(text)
    longest = longest_word(text)
    print(f"Words: {count}, longest: {longest}")

    with open("report.txt", "a", encoding="utf-8") as report:
        report.write(f"essay.txt: {count} words, longest '{longest}'\n")


if __name__ == "__main__":
    main()

Runs in your browser · Python

Key points

Exercise

Check your understanding