FastAPI/Pydantic in a project with MyPy

Question

Im currently working through the fastAPI tutorial, and my environment is setup with black, flake8, bandit and mypy. Everything in the tutorial is working fine, but I keep having to # type: ignore things to make mypy cooperate.

class Item(BaseModel):
    name: str
    description: str = None
    price: float
    tax: float = None


@app.post("/items/")
async def create_items(item: Item) -> Item:
    return item

Mypy then errors:

 ❯ mypy main.py                                                                                                                                                                                                 [14:34:08]
main.py:9: error: Incompatible types in assignment (expression has type "None", variable has type "str")
main.py:11: error: Incompatible types in assignment (expression has type "None", variable has type "float")

I could # type: ignore, but then I lose the type hints and validation in my editor. Am I missing something obvious, or should I just disable mypy for FastAPI projects?

tiangolo · Accepted Answer · 2019-05-21 19:57:22Z

10

You can use Optional:

from typing import Optional

class Item(BaseModel):
    name: str
    description: Optional[str] = None
    price: float
    tax: Optional[float] = None

That tells mypy that the value should be of that type but None is acceptable.

answered May 21, 2019 at 19:57

tiangolo

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Babu Reddy · Accepted Answer · 2019-10-22 14:20:58Z

0

If you are using mypy it could complain with type declarations like:

tax: float = None

With an error like: Incompatible types in assignment (expression has type "None", variable has type "float") In those cases you can use Optional to tell mypy that the value could be None, like:

tax: Optional[float] = None

In the above code, Check out this video, its been explained in this one Base Model explained here

edited Oct 22, 2019 at 14:20

answered Oct 22, 2019 at 14:14

Babu Reddy

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