gravitas-python

GRAVITAS — Python Extension

Extends the base GRAVITAS skill with Python-specific patterns. Load skills/gravitas/SKILL.md first, then this file.

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Python Recon Additions

□ pyproject.toml — [tool.mypy], [tool.ruff], [tool.pytest.ini_options]
□ setup.py / setup.cfg — legacy config
□ requirements.txt / requirements-dev.txt — dependency versions
□ conftest.py — pytest fixtures (critical for understanding test patterns)
□ __init__.py — package structure
□ Type stubs — are there py.typed markers? stub packages installed?

Read pyproject.toml for type strictness

cat pyproject.toml | grep -A 10 "\[tool.mypy\]"
cat pyproject.toml | grep -A 10 "\[tool.ruff\]"

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Python Verification Chain

# Full chain — in this order
mypy . --strict                          # type correctness
ruff check .                             # lint (fast, Rust-based)
ruff format --check .                    # format check
pytest -v --tb=short                     # all tests

# With coverage
pytest --cov=src --cov-report=term-missing -v

# Type coverage
mypy . --strict 2>&1 | tail -5           # summary line shows error count

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Python-Specific Anti-Patterns

Never use bare except

BAD:  except:
GOOD: except ValueError as e:
GOOD: except (TypeError, ValueError) as e:

Never shadow builtins

BAD:  list = [1, 2, 3]     # shadows list()
BAD:  id = user.id         # shadows id()
GOOD: items = [1, 2, 3]
GOOD: user_id = user.id

Type annotations required (with mypy strict)

BAD:  def process(data):
GOOD: def process(data: dict[str, Any]) -> ProcessResult:

Use dataclasses or Pydantic over dicts for structured data

BAD:  return {"user_id": 1, "name": "Alice"}   # no type safety
GOOD: return UserResponse(user_id=1, name="Alice")

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Common Python Failure Modes

Failure	Root Cause	Fix
Optional[X] has no attribute Y	None not handled	Add if x is not None: guard
Incompatible return value type	Wrong return type	Check function signature
Missing return statement	Not all paths return	Add explicit return
Module has no attribute X	Wrong import	Check module with dir(module)
Cannot find implementation for X	Missing stub	Install types-X package
Circular import	Import at module level	Move to function level or restructure

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Test Patterns (pytest)

Fixture pattern

# conftest.py
import pytest
from myapp.db import get_db

@pytest.fixture
def db():
    conn = get_db(":memory:")
    yield conn
    conn.close()

@pytest.fixture
def client(db):
    from myapp import create_app
    app = create_app({"TESTING": True})
    with app.test_client() as c:
        yield c

Standard test structure

class TestUserAuth:
    def test_login_success(self, client, db):
        # Arrange
        db.execute("INSERT INTO users VALUES (1, 'alice', 'hash')")
        # Act
        response = client.post("/login", json={"username": "alice", "password": "pw"})
        # Assert
        assert response.status_code == 200
        assert response.json["token"] is not None

    def test_login_wrong_password(self, client, db):
        response = client.post("/login", json={"username": "alice", "password": "wrong"})
        assert response.status_code == 401

Parametrize for edge cases

@pytest.mark.parametrize("input,expected", [
    ("", ValueError),
    (None, TypeError),
    ("valid@email.com", None),
    ("not-an-email", ValueError),
])
def test_validate_email(input, expected):
    if expected:
        with pytest.raises(expected):
            validate_email(input)
    else:
        validate_email(input)  # should not raise

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Framework-Specific Notes

FastAPI

# Always test with TestClient, not raw function calls
from fastapi.testclient import TestClient
client = TestClient(app)

# Type all path/query params and bodies
@router.post("/users", response_model=UserResponse)
async def create_user(body: CreateUserRequest) -> UserResponse:
    ...

Django

# Use pytest-django
# settings for tests
@pytest.mark.django_db
def test_user_creation():
    user = User.objects.create_user(username="test", password="pass")
    assert user.id is not None

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Quick Reference

# Run single test file
pytest src/auth/test_session.py -v

# Run single test
pytest src/auth/test_session.py::TestSession::test_timeout -v

# Run with pdb on failure
pytest --pdb

# Show slowest tests
pytest --durations=10

# Type check single file
mypy src/auth/session.py --strict