Python Intermediate Lesson 1: Project Folders, pathlib and Virtual Environments

📎 This article includes 3 downloadable practice files ↓

📘 Python Intermediate Course · Lesson 1 of 10

Scripts that “work on my machine” usually fail for two reasons: a missing library, or a file path that only exists on one PC. Fix both before writing more code.

A project folder

sales-report/
  .venv/              <- the virtual environment (never edit, never share)
  data/               <- input files
  output/             <- what the script creates
  report.py
  requirements.txt

Virtual environment

cd sales-report
python -m venv .venv
.venv\Scripts\activate          # Windows   (macOS/Linux: source .venv/bin/activate)
pip install pandas openpyxl
pip freeze > requirements.txt   # a colleague runs: pip install -r requirements.txt

Each project gets its own libraries and versions, so upgrading pandas for one project can’t break another.

pathlib: paths that travel

from pathlib import Path
HERE = Path(__file__).resolve().parent     # the script's own folder
data = HERE / "sales.csv"                  # / joins paths on every OS
for f in sorted((HERE / "monthly").glob("*.csv")):
    print(f.name, f.stem, f.suffix)
(HERE / "output").mkdir(exist_ok=True)

Common mistakes

• Forgetting to activate the venv: pip install goes to the global Python. Check the (.venv) prefix in the prompt.
• Committing .venv to Git or OneDrive: share requirements.txt instead.

Practice

Unzip the practice data into a folder, put this lesson’s script in the same folder and run it (python pyi-NN.py). Compare with the expected output file. Then change one thing (a filter, a column, a format) and predict the new output before you run it.