python

name	python
description	Write clean, readable Pythonic code. Use this skill any time you write or change a Python file.

General guidelines

Coding Style

• Always write clean, readable, well separated, well organized, Pythonic code.
• Don't run linters or formatters, they will run automatically after every edit by the harness.
• When using datetime, import it like this: import datetime as dt
• CRITICAL: ALWAYS put ALL imports at the top of the file, NEVER inside functions, methods, or any other block. No exceptions. Use TYPE_CHECKING guard for type-only imports to avoid circular dependencies.

Type hints

• ALWAYS type hint every function and method signature precisely even in tests.
• NEVER write "-> None" type hint for return values, they are not necessary.
• No need to type hint variables from functions call return values inside function bodies, they are inferred from the method return type, except extreme cases when the type is a huge help or a generic and it's concrete type cannot be determined.
• Only use APIs, class names, variables or objects which you already read or know exist for sure.
• Use the new pipe operator for Optional variables like this: value | None
• Use Any for "all types" instead of object.

Testing

• ALWAYS use pytest for all tests
• ALWAYS assert on whole ouptut or full results in tests, not just tiny parts
• For mocking, use pytest monkeypatch fixture, NEVER unittest.mock and NEVER any of the Mock classes or patch function
• IMPORTANT: NEVER import from conftest.py
• ALWAYS type hint test function parameters correctly.
• Don't make a test class with only one function, a module-level test function is enough
• NEVER use unittest.mock or any of the Mock classes or patch function, never assert on method calls, use real objects and assert on full results. When absolutely necessary and can't be avoided, use pytest.monkeypatch, when the test setup would be too difficult or the code have side effects, but consider writing a fake.
• ALWAYS assert on the full result of the function call, never just list lengths or containment, except where that's the point.

Managing dependencies

• Use uv add for adding dependencies, never directly edit pyproject.toml files.
• For CLI scripts and apps, ALWAYS use click library instead of argparse, add it to dependencies if necessary.