Python type hints: are they checked at runtime?

Question

I added `def greet(name: str) -> str:` but calling `greet(42)` doesn't raise any error. What's the point of type hints then?

Accepted Answer

Correct: Python **does not enforce** type hints at runtime. They're metadata for:

1. **Static type checkers** (`mypy`, `pyright`, the one built into VS Code/Pylance) that catch `greet(42)` *before* you run the code.
2. **Editor features**: autocomplete, go-to-definition, refactoring.
3. **Libraries that read them at runtime on purpose**: Pydantic, FastAPI and dataclasses use them for validation and serialization.

```python
from pydantic import BaseModel

class User(BaseModel):
    name: str
    age: int

User(name="Ruth", age="not a number")  # raises ValidationError
```

Add `mypy --strict` or `pyright` to CI to get real value from the hints.

1 Answer