What is the typing.assert_type function?

Question

Accepted Answer

**assert_type** tells the type checker to **verify a value has a specific type**.

```python
from typing import assert_type

data = get_data()  # type: Any

# Runtime: no-op (does nothing)
# Type checker: verifies type
assert_type(data, dict)  # Warns if data is not dict

# With specific types
from typing import Union

value: Union[int, str] = get_value()

if isinstance(value, int):
    assert_type(value, int)  # Type checker knows it's int
    print(value + 1)  # OK

# With Pydantic
from pydantic import BaseModel

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

data = parse_obj(User, {"name": "Alice", "age": 25})
assert_type(data, User)  # Verifies parse_obj returns User

# With FastAPI
from fastapi import FastAPI

app = FastAPI()

@app.get("/users/{user_id}")
async def get_user(user_id: int):
    user = await fetch_user(user_id)
    assert_type(user, User)  # Verifies fetch_user returns User
    return user
```

**Benefits:**
- Type verification at development time.
- No runtime cost.
- Better IDE support.
- Documentation of expected types.

Python: What is the typing.assert_type function?