Practice MyPy - Python Typing Exercises

A hands-on project for learning Python's type system through exercises.

Setup

# Add development dependencies
uv add --dev pytest mypy

# Verify installation
uv run pytest --version
uv run mypy --version

Exercises

Each exercise file contains:

• Docstrings explaining the concepts
• Stub functions/classes with TODO comments
• Working implementations that need type hints

Exercise 1: Generic Types Basics

exercises/ex01_generics_basics.py

• TypeVar basics
• Generic functions
• Generic classes with Generic[T]

Exercise 2: TypeVar Constraints and Bounds

exercises/ex02_typevar_constraints.py

• Constrained TypeVars (TypeVar('T', int, str))
• Bounded TypeVars (TypeVar('T', bound=SomeClass))
• Generic classes with bounds

Exercise 3: Protocols (Structural Subtyping)

exercises/ex03_protocols.py

• Basic Protocols
• Protocols with properties
• @runtime_checkable Protocols
• Generic Protocols

Exercise 4: Callable and @overload

exercises/ex04_callable_and_overload.py

• Callable[[Args], Return] type hints
• Higher-order functions
• @overload for multiple signatures
• Literal types with overloads

Exercise 5: ParamSpec and Concatenate

exercises/ex05_paramspec.py

• ParamSpec for preserving function signatures in decorators
• Concatenate for adding/removing parameters
• Decorator factories with full type safety
• Method decorators with self handling

Exercise 6: TypedDict and Type Guards

exercises/ex06_typeddict_and_guards.py

• TypedDict for typed dictionary structures (JSON, API responses)
• NotRequired for optional keys
• Nested TypedDict for complex structures
• TypeGuard for custom type narrowing functions
• Discriminated unions with Literal type fields

Exercise 7: ClassVar, Final, and Self

exercises/ex07_classvar_final_self.py

• ClassVar for class variables vs instance variables
• Final for immutable values and constants
• @final decorator to prevent subclassing/overriding
• Self type for fluent interfaces and factory methods
• Proper typing for method chaining and inheritance

Exercise 8: NewType, TypeAlias, Annotated, and cast

exercises/ex08_newtypes_aliases_annotated.py

• NewType for distinct types (UserId vs int)
• TypeAlias for readable complex type definitions
• Annotated for attaching metadata to types
• cast() for type assertions when you know better
• Building type-safe ID systems

Workflow

1. Read the exercise file and understand the concepts
2. Add type hints to functions/classes marked with TODO
3. Run tests to verify runtime behavior:

   uv run pytest tests/test_ex01.py -v

4. Run mypy to verify type correctness:

   uv run mypy exercises/ex01_generics_basics.py

5. Fix any type errors and repeat

Tips

• Start with Exercise 1 and progress in order
• Read the docstrings carefully - they explain what types are expected
• When stuck, check Python's typing module docs
• The tests show expected runtime behavior; mypy checks type safety