When using postponed annotations (from __future__ import annotations), class based dependencies fail to resolve their dependencies with NameErrorr

[waweber]

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Example Code

from __future__ import annotations

from fastapi import FastAPI, Depends

app = FastAPI()

CoolType = int

def get_cool_type() -> int:
    return 123

class MyClass:
    def __init__(self, val: CoolType = Depends(get_cool_type)):
        self.val = val

def get_my_class(val: CoolType = Depends(get_cool_type)):
    return MyClass

@app.get("/")
def my_view(mycls: MyClass = Depends()):
    return mycls.val

@app.get("/test2")
def my_view_working(mycls: MyClass = Depends(get_my_class)):
    return mycls.val

Description

Run the example app, and you will get a NameError because CoolType cannot be resolved. If you comment out the first view, the program will work correctly. The second view just uses a function to construct and return MyClass instead using MyClass as a dependency directly.

A possible root of the problem is in fastapi.dependency.utils:get_typed_signature:

def get_typed_signature(call: Callable[..., Any]) -> inspect.Signature:
    signature = inspect.signature(call)
    globalns = getattr(call, "__globals__", {})
    ...

To resolve the annotations, this function uses the dependency's __globals__ property to look up the names. When call is a function, this works just fine. When call is a class, it doesn't have a __globals__, so we get an empty namespace as a fallback.

However, when we specify a class as a dependency, what we really care about is MyClass.__init__. This is a function, and it has a __globals__ containing all the types we need to resolve its dependencies.

A possible fix for this would be to make this function check if call is a class and use it's __init__ method instead.

Operating System

Linux

Operating System Details

No response

FastAPI Version

0.73.0

Python Version

Python 3.10.2

Additional Context

Also using pydantic 1.9.0

See also this related issue: #1654

[waweber]

As of version 0.78.0, this bug still exists. For anyone needing a workaround, you have two options:

1. Use a function dependency instead, e.g.

def use_my_dependency(some_param: str = Query(...)):
    return MyDependency(some_param)

2. Have all your class-based dependencies subclass something that looks like this:

class DependencyBase:
    """Class-based dependency base.

    This class serves as a workaround to https://github.com/tiangolo/fastapi/issues/4557
    """

    def __init_subclass__(cls, **kw) -> None:
        super().__init_subclass__(**kw)
        cls.__globals__ = cls.__init__.__globals__

This sets up the class to have the __globals__ property that FastAPI is expecting. Your class dependencies will need to have __init__ explicitly defined in the same scope as the dependencies they refer to.

[ndevenish]

Slightly smaller example that I think is the same issue?

from __future__ import annotations

from fastapi import Request, FastAPI, Depends

app = FastAPI()

class SomeDependency:
    async def __call__(self, request: Request):
        return request

handler = SomeDependency()

@app.get("/")
def get_view(injected=Depends(handler)):
    return {"success": True}

Giving on 0.85.0:

....
  File "/path/to/site-packages/fastapi/dependencies/utils.py", line 268, in get_typed_annotation
    annotation = evaluate_forwardref(annotation, globalns, globalns)
  File "pydantic/typing.py", line 70, in pydantic.typing.evaluate_forwardref
    'ValuesView',
  File "/path/to/python/3.10.2/lib/python3.10/typing.py", line 691, in _evaluate
    eval(self.__forward_code__, globalns, localns),
  File "<string>", line 1, in <module>
NameError: name 'Request' is not defined

There is also a third workaround: Remove from __future__ import annotations from the file affected by this. That of course means that you have to follow the previous rules in terms of self-referencing, not using newer union syntax before 3.10 etc.

[mastercoms]

I am experiencing this bug as well, but when using nested decorators.

[mastercoms]

From what I can tell, the original bug can be fixed by updating globalns in get_typed_signature to vars(sys.modules[call.__module__]) when call is a class rather than a function (as mentioned in PEP-563).