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Standard Library, Packaging and Tooling

Know how imports really work, how to isolate and pin dependencies, and the standard-library traps (datetimes, decimals, JSON, regex, subprocess, encodings) that interviewers love.

Key points

1
A module runs once per process and is cached in sys.modules. A script runs as __main__, so importing it by name executes it a second time.
2
Use python -m pkg.module for code that uses relative imports: it imports the package first and puts the working directory on sys.path[0].
3
Isolate projects in a virtual environment, use pipx for CLI tools, and pin transitive dependencies with a lock file (uv.lock, poetry.lock or pip-tools output).
4
Keep datetimes timezone-aware (datetime.now(UTC), zoneinfo). Same-tzinfo arithmetic is wall-clock, so convert to UTC to measure real elapsed time.
5
Build Decimal values from strings, and choose the rounding mode explicitly; the default is half-to-even, like round().
6
Run subprocesses with a list of arguments, check=True and communicate()/run(), never shell=True with user input.
7
Always pass encoding="utf-8" to open(); the default is the locale encoding until UTF-8 mode becomes the default in 3.15 (PEP 686).

Common traps

from config import X copies the value, so importlib.reload(config) does not update X.
$ in a regex matches before a trailing newline; use re.fullmatch or \Z for validation.
Path("/srv") / user_value drops the base when user_value is absolute.

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