Professional Python: tools and habits

From scripts to real projects

The habits in this topic are what separate a working script from code a team can trust: each project in its own environment, a clear folder layout, consistent style, checked types, proper logs, automatic tests and version control.

venv: a separate box of packages for each project

Different projects need different packages, sometimes different versions of the same one. A virtual environment gives each project its own private set of packages.

python -m venv .venv                  # create it, once per project
.venv\Scripts\activate                # Windows
source .venv/bin/activate             # macOS and Linux
pip install requests pandas           # installs into this project only
pip freeze > requirements.txt         # record the exact versions
pip install -r requirements.txt       # recreate them on another computer
deactivate

Do not copy the .venv folder or add it to Git; share requirements.txt instead.

Project structure and PEP 8 style

school_app/
    .venv/
    school_app/
        __init__.py
        marks.py
        reports.py
    tests/
        test_marks.py
    requirements.txt
    README.md
    main.py

PEP 8 is Python's official style guide. The key rules:

Tools such as black or ruff can format and check your code automatically.

Type hints: say what types a function expects

def average(marks: list[float]) -> float:
    return sum(marks) / len(marks)

name: str = "Asha"
age: int | None = None          # int or None

Type hints do not change how the code runs; Python ignores them at run time. They help your editor warn you, document the code, and let checkers find bugs.

Catch bugs before running: mypy

pip install mypy
mypy school_app

def grade(marks: int) -> str:
    return "Pass" if marks >= 33 else "Fail"

grade("90")      # mypy: Argument 1 has incompatible type "str"; expected "int"

logging: better than print for real apps

import logging

logging.basicConfig(
    filename="bank.log",
    level=logging.INFO,
    format="%(asctime)s %(levelname)s %(message)s",
)
log = logging.getLogger(__name__)

log.info("Deposit of Rs %s to account %s", 500, "A101")
log.warning("Balance below minimum")
log.error("Withdrawal failed: insufficient balance")

Unlike print, logs carry the time and level, can go to a file, and can be turned up or down without deleting lines of code.

Automated tests with pytest

A test is code that checks your code. Write tests once, then run them after every change to catch anything you broke.

# calculator.py
def divide(a, b):
    if b == 0:
        raise ValueError("cannot divide by zero")
    return a / b

# tests/test_calculator.py
import pytest
from calculator import divide

def test_divide():
    assert divide(10, 2) == 5

def test_divide_by_zero():
    with pytest.raises(ValueError):
        divide(1, 0)

pip install pytest
pytest              # finds files named test_*.py and runs every test_ function

Git: save versions and share your code

git init                              # start tracking a project
git add .                             # stage changes
git commit -m "Add marks report"      # save a version with a message
git log --oneline                     # see the history
git push                              # upload to GitHub

Add a .gitignore file listing .venv/, __pycache__/ and secret files such as .env, so they are never committed. Our separate Git & GitHub course covers this in depth.

Common mistakes

• Installing every package globally, until two projects need different versions.
• Committing passwords or API keys to Git.
• Leaving debugging print() calls in finished code instead of using logging.
• Writing tests that only check the happy path and never the errors.

Key points

• One venv per project; record packages in requirements.txt.
• Follow PEP 8 so code looks the same across a team.
• Type hints and mypy catch type mistakes before the code runs.
• Use logging for real apps, pytest for automatic checks and Git for history.

Practice questions

def test_add():
    assert add(2, 3) == 5