Python Development

Purpose: Production-ready Python development standards for building secure, performant, maintainable applications. Audience: Engineers building Python applications, APIs, data pipelines, or AI/ML systems. Standard: Follows github/awesome-copilot Python development patterns.

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When to Use This Skill

• Building Python applications or APIs
• Adding type hints to Python code
• Writing async/await patterns in Python
• Testing with pytest
• Structuring Python project layouts

Prerequisites

• Python 3.11+ installed
• pip or poetry package manager
• pytest for testing

Quick Reference

Need	Solution	Pattern
Type hints	Use everywhere	def get_user(id: int) -> Optional[User]:
Async code	Use async/await	async def fetch_data() -> str:
Error handling	Specific exceptions	try-except ValueError
Testing	pytest	def test_user_creation():
Logging	Standard library	logger.info("User %s created", user_id)
Docstrings	Google style	"""Gets user by ID.\n\nArgs:\n id: User identifier

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Python Version

Current: Python 3.11+ Minimum: Python 3.9+

Modern Python Features (Use These)

# Type hints (PEP 484) - Use everywhere
from typing import Optional, List, Dict, Any
from dataclasses import dataclass

def get_user(user_id: int) -> Optional[dict[str, Any]]:
 """Get user by ID."""
 return users.get(user_id)

# Dataclasses for data structures
@dataclass
class User:
 id: int
 name: str
 email: str
 is_active: bool = True

# f-strings for formatting
name = "Alice"
age = 30
message = f"User {name} is {age} years old"

# Walrus operator (:=) in Python 3.8+
if (user := get_user(123)) is not None:
 print(f"Found user: {user['name']}")

# Pattern matching (Python 3.10+)
def process_response(status: int) -> str:
 match status:
 case 200:
 return "Success"
 case 404:
 return "Not found"
 case 500:
 return "Server error"
 case _:
 return "Unknown status"

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Type Hints

Always use type hints for function parameters, return values, and class attributes.

from typing import Optional, List, Dict, Any, Union, TypeVar, Generic

# Basic types
def calculate_total(price: float, quantity: int) -> float:
 return price * quantity

# Optional types
def find_user(user_id: int) -> Optional[User]:
 """Returns None if user not found."""
 return db.query(User).filter_by(id=user_id).first()

# Collections
def get_active_users() -> List[User]:
 return [u for u in users if u.is_active]

def get_user_map() -> Dict[int, User]:
 return {u.id: u for u in users}

# Union types
def process_data(data: Union[str, bytes]) -> str:
 if isinstance(data, bytes):
 return data.decode('utf-8')
 return data

# Generic types
T = TypeVar('T')

def first_or_none(items: List[T]) -> Optional[T]:
 """Get first item or None if list is empty."""
 return items[0] if items else None

# Type aliases for complex types
UserId = int
UserData = Dict[str, Any]

def create_user(user_id: UserId, data: UserData) -> User:
 return User(id=user_id, **data)

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Best Practices Summary

Code Style (PEP 8)

# [PASS] GOOD: Follow PEP 8
def calculate_total(items: List[Item]) -> float:
 """Calculate total price of items."""
 return sum(item.price * item.quantity for item in items)

# Variable naming
user_count = 10 # snake_case for variables
MAX_RETRIES = 3 # UPPER_CASE for constants
UserService # PascalCase for classes

# [PASS] GOOD: List comprehensions
active_users = [u for u in users if u.is_active]

# [FAIL] BAD: Mutable default arguments
def add_item(item, items=[]): # Don't do this!
 items.append(item)
 return items

# [PASS] GOOD: Use None as default
def add_item(item, items=None):
 if items is None:
 items = []
 items.append(item)
 return items

Performance

# [PASS] GOOD: Use generators for large datasets
def process_large_file(filename: str):
 """Process large file line by line."""
 with open(filename) as f:
 for line in f: # Generator - memory efficient
 yield process_line(line)

# [PASS] GOOD: Use collections.defaultdict
from collections import defaultdict

user_groups = defaultdict(list)
for user in users:
 user_groups[user.group].append(user)

# [PASS] GOOD: Use set for membership testing
valid_ids = {1, 2, 3, 4, 5}
if user_id in valid_ids: # O(1) lookup
 process_user(user_id)

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Common Pitfalls

Issue	Problem	Solution
Mutable defaults	def func(items=[]):	Use items=None then if items is None: items = []
Missing type hints	No type information	Add types everywhere
Broad exceptions	except Exception:	Catch specific exceptions
No docstrings	Undocumented code	Add Google-style docstrings
String concatenation	s = s + "text" in loop	Use "".join(list) or f-strings
Not using context managers	Manual file.close()	Use with open(...) as f:

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Project Structure

my_project/
+-- src/
| +-- my_project/
| | +-- __init__.py
| | +-- models/
| | | +-- __init__.py
| | | -- user.py
| | +-- services/
| | | +-- __init__.py
| | | -- user_service.py
| | +-- repositories/
| | | +-- __init__.py
| | | -- user_repository.py
| | -- utils/
| | +-- __init__.py
| | -- helpers.py
+-- tests/
| +-- __init__.py
| +-- test_models.py
| +-- test_services.py
| -- test_repositories.py
+-- requirements.txt
+-- pyproject.toml
+-- README.md
-- .gitignore

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Resources

• Official Docs: docs.python.org
• PEP 8: pep8.org
• Type Hints: PEP 484
• pytest: pytest.org
• Async: docs.python.org/asyncio
• Awesome Copilot: github.com/github/awesome-copilot

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See Also: Skills.md - AGENTS.md

Last Updated: January 27, 2026

Scripts

Script	Purpose	Usage
scaffold-project.py	Generate Python project with pyproject.toml, ruff, mypy, pre-commit	python scripts/scaffold-project.py --name myapp [--fastapi]

Troubleshooting

Issue	Solution
Type hint errors with mypy	Install type stubs, use type: ignore sparingly
Async event loop already running	Use asyncio.run() at top level only, use await inside async functions
pytest not finding tests	Name test files test_*.py and functions test_*, check pytest.ini paths

References

• Async Errors Context
• Docs Testing Logging
• Dataclasses Patterns