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Python CTO

Claude Code correction guide. Updated January 2026.

Critical Corrections

  • Claude suggests pip install — use uv instead (10-100x faster)
  • Claude uses old setup.py patterns — use pyproject.toml (PEP 621)
  • Claude forgets free-threaded Python exists (3.13t/3.14t) for true parallelism
  • Claude defaults to requirements.txt — prefer uv.lock for reproducibility

Current Tooling (2026)

Tool Use NOT
uv Package management, venvs pip, pipenv, poetry
ruff Lint + format (one tool) flake8 + black + isort
mypy --strict Type checking Skipping types
pytest-cov Testing unittest alone
python3.14t Free-threaded builds GIL-bound threading

Patterns Claude Should Use

# Modern Python 3.14+ patterns
from typing import Self

class Config:
    def with_timeout(self, timeout: int) -> Self:
        self.timeout = timeout
        return self

# Use structural pattern matching
match response.status:
    case 200: handle_success(response)
    case 404: raise NotFound()
    case _: raise UnexpectedStatus(response.status)

Anti-Patterns Claude Generates

  • def f(items=[]): — mutable default argument
  • except: or except Exception: — swallows everything
  • import * — pollutes namespace
  • Missing if __name__ == "__main__": guard
  • os.system() — use subprocess.run() with shell=False

Version Gotchas

  • 3.14: Current stable (Jan 2026), free-threaded mode (python3.14t), improved error messages
  • 3.13: Free-threaded mode (python3.13t), requires pip>=24.1 for C extensions
  • 3.12: f-string parser rewrite, may break edge cases
  • With asyncio: Never time.sleep() — use await asyncio.sleep()
  • With typing: Use X | Y not Union[X, Y] (3.10+)