What is the typing.cast function?

Question

Accepted Answer

**cast** tells the type checker to treat a value as a **specific type**.

```python
from typing import cast

data = get_data()  # type: Any

# Without cast — type checker warns
name = data["name"]  # Warning: Key "name" not known

# With cast — type checker trusts you
from typing import TypedDict

class UserData(TypedDict):
    name: str
    age: int

user = cast(UserData, data)
print(user["name"])  # No warning!

# With FastAPI
from fastapi import Request

async def get_current_user(request: Request):
    user = request.state.user
    return cast(User, user)  # Type checker knows it's User

# Runtime: cast is just identity function
value = cast(int, "hello")  # Returns "hello" unchanged!
print(type(value))  # <class 'str'>
```

**When to use:**
- After type narrowing fails.
- When you know more than the type checker.
- After isinstance checks.
- With dynamic attribute access.

Python: What is the typing.cast function?