python-type-system

# Python Type System

Master Python's type system to write type-safe, maintainable code. This
skill covers type hints, static type checking with mypy, and advanced
typing features.

## Type Checking Tools

```bash
# Install mypy for static type checking
pip install mypy

# Run mypy on a file or directory
mypy my_module.py
mypy src/

# Run with specific configuration
mypy --config-file mypy.ini src/

# Run with strict mode
mypy --strict src/

# Show type coverage report
mypy --html-report mypy-report src/
```

## mypy Configuration

**mypy.ini configuration file:**

```ini
[mypy]
# Global options
python_version = 3.11
warn_return_any = True
warn_unused_configs = True
disallow_untyped_defs = True
disallow_any_unimported = True
no_implicit_optional = True
warn_redundant_casts = True
warn_unused_ignores = True
warn_no_return = True
check_untyped_defs = True
strict_equality = True

# Per-module options
[mypy-tests.*]
disallow_untyped_defs = False

[mypy-third_party.*]
ignore_missing_imports = True
```

**pyproject.toml configuration:**

```toml
[tool.mypy]
python_version = "3.11"
warn_return_any = true
warn_unused_configs = true
disallow_untyped_defs = true
disallow_any_unimported = true
no_implicit_optional = true
warn_redundant_casts = true
warn_unused_ignores = true
warn_no_return = true
check_untyped_defs = true
strict_equality = true

[[tool.mypy.overrides]]
module = "tests.*"
disallow_untyped_defs = false
```

## Basic Type Hints

**Primitive types and collections:**

```python
from typing import List, Dict, Set, Tuple, Optional, Union, Any

# Basic types
def greet(name: str) -> str:
    return f"Hello, {name}"

# Collections
def process_items(items: List[str]) -> Dict[str, int]:
    return {item: len(item) for item in items}

# Optional (can be None)
def find_user(user_id: int) -> Optional[str]:
    users = {1: "Alice", 2: "Bob"}
    return users.get(user_id)

# Union types (multiple possible types)
def process_value(value: Union[int, str]) -> str:
    return str(