Python 3 Development Skill

Assist with all aspects of Python 3 development including creating files, writing code, running scripts, debugging, and testing.

Creating New Files

When creating a new Python file, always include the file header from CLAUDE.md using the Python template. The shebang and encoding lines are part of the template:

#!/usr/bin/env python3
# -*- coding: utf-8 -*-

Running and Testing

• •Run scripts with: python3 <script.py>
• •Run module: python3 -m <module_name>
• •Check syntax: python3 -m py_compile <script.py>
• •Run tests with: python3 -m pytest -v or python3 -m unittest discover
• •Type checking: mypy <script.py> (if available)
• •Linting: ruff check <script.py> or flake8 <script.py> (if available)

Code Quality

• •Target Python 3.10+ unless the project specifies otherwise
• •Use type hints for function signatures
• •Follow PEP 8 naming conventions:
  • •snake_case for functions and variables
  • •PascalCase for classes
  • •UPPER_SNAKE_CASE for constants
• •Use f-strings for string formatting
• •Use pathlib.Path over os.path for file operations
• •Use context managers (with statements) for resource management
• •Prefer list/dict/set comprehensions where they improve readability
• •Use dataclasses or NamedTuple for data containers

Project Structure

• •For packages, ensure __init__.py exists
• •Use pyproject.toml for project configuration when applicable
• •Virtual environments: python3 -m venv .venv
• •Install dependencies: pip install -r requirements.txt

Debugging

Built-in Python debugging

• •Use pprint for readable data structure output
• •Use traceback module for detailed exception information
• •Check for common issues: indentation errors, mutable default arguments, variable scope

pdb / breakpoint() (Python debugger)

• •Insert breakpoint() (Python 3.7+) to drop into the debugger at that line
• •Launch from command line: python3 -m pdb <script.py>
• •Key commands:
  • •n (next line), s (step into), c (continue), r (return from function)
  • •b <line> (set breakpoint), cl <line> (clear breakpoint)
  • •p <expr> (print expression), pp <expr> (pretty-print)
  • •l (list source), w (where/backtrace), u/d (up/down frame)
  • •q (quit)
• •Use PYTHONBREAKPOINT=0 to disable all breakpoints without removing them

Ruff (linter and formatter)

• •Fast Python linter and formatter written in Rust
• •Lint: ruff check <script.py> or ruff check .
• •Auto-fix: ruff check --fix <script.py>
• •Format: ruff format <script.py>
• •Configure in pyproject.toml under [tool.ruff]
• •Install with: pip install ruff

mypy (static type checker)

• •Checks type annotations for correctness
• •Run with: mypy <script.py> or mypy .
• •Strict mode: mypy --strict <script.py>
• •Ignore specific lines: # type: ignore[error-code]
• •Configure in pyproject.toml under [tool.mypy]
• •Install with: pip install mypy

pytest (testing framework)

• •Python's most widely used test framework
• •Run tests with: python3 -m pytest -v
• •Run a single file: python3 -m pytest -v test_specific.py
• •Run a single test: python3 -m pytest -v test_file.py::test_name
• •Test file structure:
  python

  import pytest
  
  def test_addition():
      assert my_function(1, 2) == 3
  
  def test_raises():
      with pytest.raises(ValueError):
          my_function(-1)
  
  @pytest.fixture
  def sample_data():
      return {"key": "value"}
  
  def test_with_fixture(sample_data):
      assert sample_data["key"] == "value"
  

• •Key plugins:
  • •pytest-cov -- code coverage reporting (--cov=src)
  • •pytest-mock -- mock objects via mocker fixture
  • •pytest-xdist -- parallel test execution (-n auto)

Argument Handling

• •If $ARGUMENTS is a filename ending in .py, work with that file
• •If $ARGUMENTS is "new <filename>", scaffold a new file with proper headers
• •If $ARGUMENTS is "run <filename>", execute the script and report output
• •If $ARGUMENTS is "test", run the project's test suite
• •Otherwise, treat $ARGUMENTS as a general Python development request