Environments and Lockfiles

Your Beginner course used venv + pip install + requirements.txt. Intermediate adds the why and the reproducibility layer:

• venv isolates per-project so package versions can't clash between projects and your system Python stays clean. Never pip install globally.
• requirements.txt pins what you install, but it says nothing about which builds your builds depend on (transitive pins).
• Lockfiles (pip-compile output, uv.lock, pdm.lock) record the full resolved graph so that today's install and next year's install are byte-identical. Rule: hand-edit constraints in pyproject.toml; let a lockfile freeze reality.

The professional flow: constraints live in pyproject.toml (loose, readable: httpx>=0.27), the lockfile is generated and committed (exact), and CI installs from the lockfile so tests run against what production runs.

In this sandbox there is no network and no pip — so the gradable skill here is reasoning about the files: reading dependency declarations, spotting unpinned ranges, and writing a lockfile-aware workflow. The next lesson's audit drills exactly that.