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Classes as Dependencies

Before diving deeper into the Dependency Injection system, let's upgrade the previous example.

A dict from the previous example

Section titled “A dict from the previous example”

In the previous example, we were returning a dict from our dependency ("dependable"):

Python
from typing import Annotated

from fastapi import Depends, FastAPI

app = FastAPI()


async def common_parameters(q: str | None = None, skip: int = 0, limit: int = 100):
    return {"q": q, "skip": skip, "limit": limit}


@app.get("/items/")
async def read_items(commons: Annotated[dict, Depends(common_parameters)]):
    return commons


@app.get("/users/")
async def read_users(commons: Annotated[dict, Depends(common_parameters)]):
    return commons

But then we get a dict in the parameter commons of the path operation function.

And we know that editors can't provide a lot of support (like completion) for dicts, because they can't know their keys and value types.

We can do better...

What makes a dependency

Section titled “What makes a dependency”

Up to now you have seen dependencies declared as functions.

But that's not the only way to declare dependencies (although it would probably be the more common).

The key factor is that a dependency should be a "callable".

A "callable" in Python is anything that Python can "call" like a function.

So, if you have an object something (that might not be a function) and you can "call" it (execute it) like:

Python
something()

or

Python
something(some_argument, some_keyword_argument="foo")

then it is a "callable".

Classes as dependencies

Section titled “Classes as dependencies”

You might notice that to create an instance of a Python class, you use that same syntax.

For example:

Python
class Cat:
    def __init__(self, name: str):
        self.name = name


fluffy = Cat(name="Mr Fluffy")

In this case, fluffy is an instance of the class Cat.

And to create fluffy, you are "calling" Cat.

So, a Python class is also a callable.

Then, in FastAPI, you could use a Python class as a dependency.

What FastAPI actually checks is that it is a "callable" (function, class or anything else) and the parameters defined.

If you pass a "callable" as a dependency in FastAPI, it will analyze the parameters for that "callable", and process them in the same way as the parameters for a path operation function. Including sub-dependencies.

That also applies to callables with no parameters at all. The same as it would be for path operation functions with no parameters.

Then, we can change the dependency "dependable" common_parameters from above to the class CommonQueryParams:

Python
from typing import Annotated

from fastapi import Depends, FastAPI

app = FastAPI()


fake_items_db = [{"item_name": "Foo"}, {"item_name": "Bar"}, {"item_name": "Baz"}]


class CommonQueryParams:
    def __init__(self, q: str | None = None, skip: int = 0, limit: int = 100):
        self.q = q
        self.skip = skip
        self.limit = limit


@app.get("/items/")
async def read_items(commons: Annotated[CommonQueryParams, Depends(CommonQueryParams)]):
    response = {}
    if commons.q:
        response.update({"q": commons.q})
    items = fake_items_db[commons.skip : commons.skip + commons.limit]
    response.update({"items": items})
    return response

Pay attention to the __init__ method used to create the instance of the class:

Python
from typing import Annotated

from fastapi import Depends, FastAPI

app = FastAPI()


fake_items_db = [{"item_name": "Foo"}, {"item_name": "Bar"}, {"item_name": "Baz"}]


class CommonQueryParams:
    def __init__(self, q: str | None = None, skip: int = 0, limit: int = 100):
        self.q = q
        self.skip = skip
        self.limit = limit


@app.get("/items/")
async def read_items(commons: Annotated[CommonQueryParams, Depends(CommonQueryParams)]):
    response = {}
    if commons.q:
        response.update({"q": commons.q})
    items = fake_items_db[commons.skip : commons.skip + commons.limit]
    response.update({"items": items})
    return response

...it has the same parameters as our previous common_parameters:

Python
from typing import Annotated

from fastapi import Depends, FastAPI

app = FastAPI()


async def common_parameters(q: str | None = None, skip: int = 0, limit: int = 100):
    return {"q": q, "skip": skip, "limit": limit}


@app.get("/items/")
async def read_items(commons: Annotated[dict, Depends(common_parameters)]):
    return commons


@app.get("/users/")
async def read_users(commons: Annotated[dict, Depends(common_parameters)]):
    return commons

Those parameters are what FastAPI will use to "solve" the dependency.

In both cases, it will have:

  • An optional q query parameter that is a str.
  • A skip query parameter that is an int, with a default of 0.
  • A limit query parameter that is an int, with a default of 100.

In both cases the data will be converted, validated, documented on the OpenAPI schema, etc.

Use it

Section titled “Use it”

Now you can declare your dependency using this class.

Python
from typing import Annotated

from fastapi import Depends, FastAPI

app = FastAPI()


fake_items_db = [{"item_name": "Foo"}, {"item_name": "Bar"}, {"item_name": "Baz"}]


class CommonQueryParams:
    def __init__(self, q: str | None = None, skip: int = 0, limit: int = 100):
        self.q = q
        self.skip = skip
        self.limit = limit


@app.get("/items/")
async def read_items(commons: Annotated[CommonQueryParams, Depends(CommonQueryParams)]):
    response = {}
    if commons.q:
        response.update({"q": commons.q})
    items = fake_items_db[commons.skip : commons.skip + commons.limit]
    response.update({"items": items})
    return response

FastAPI calls the CommonQueryParams class. This creates an "instance" of that class and the instance will be passed as the parameter commons to your function.

Type annotation vs Depends

Section titled “Type annotation vs Depends”

Notice how we write CommonQueryParams twice in the above code:

Python
commons: Annotated[CommonQueryParams, Depends(CommonQueryParams)]
Python
commons: CommonQueryParams = Depends(CommonQueryParams)

The last CommonQueryParams, in:

Python
... Depends(CommonQueryParams)

...is what FastAPI will actually use to know what is the dependency.

It is from this one that FastAPI will extract the declared parameters and that is what FastAPI will actually call.

In this case, the first CommonQueryParams, in:

Python
commons: Annotated[CommonQueryParams, ...
Python
commons: CommonQueryParams ...

...doesn't have any special meaning for FastAPI. FastAPI won't use it for data conversion, validation, etc. (as it is using the Depends(CommonQueryParams) for that).

You could actually write just:

Python
commons: Annotated[Any, Depends(CommonQueryParams)]
Python
commons = Depends(CommonQueryParams)

...as in:

Python
from typing import Annotated, Any

from fastapi import Depends, FastAPI

app = FastAPI()


fake_items_db = [{"item_name": "Foo"}, {"item_name": "Bar"}, {"item_name": "Baz"}]


class CommonQueryParams:
    def __init__(self, q: str | None = None, skip: int = 0, limit: int = 100):
        self.q = q
        self.skip = skip
        self.limit = limit


@app.get("/items/")
async def read_items(commons: Annotated[Any, Depends(CommonQueryParams)]):
    response = {}
    if commons.q:
        response.update({"q": commons.q})
    items = fake_items_db[commons.skip : commons.skip + commons.limit]
    response.update({"items": items})
    return response

But declaring the type is encouraged as that way your editor will know what will be passed as the parameter commons, and then it can help you with code completion, type checks, etc:

Shortcut

Section titled “Shortcut”

But you see that we are having some code repetition here, writing CommonQueryParams twice:

Python
commons: Annotated[CommonQueryParams, Depends(CommonQueryParams)]
Python
commons: CommonQueryParams = Depends(CommonQueryParams)

FastAPI provides a shortcut for these cases, in where the dependency is specifically a class that FastAPI will "call" to create an instance of the class itself.

For those specific cases, you can do the following:

Instead of writing:

Python
commons: Annotated[CommonQueryParams, Depends(CommonQueryParams)]
Python
commons: CommonQueryParams = Depends(CommonQueryParams)

...you write:

Python
commons: Annotated[CommonQueryParams, Depends()]
Python
commons: CommonQueryParams = Depends()

You declare the dependency as the type of the parameter, and you use Depends() without any parameter, instead of having to write the full class again inside of Depends(CommonQueryParams).

The same example would then look like:

Python
from typing import Annotated

from fastapi import Depends, FastAPI

app = FastAPI()


fake_items_db = [{"item_name": "Foo"}, {"item_name": "Bar"}, {"item_name": "Baz"}]


class CommonQueryParams:
    def __init__(self, q: str | None = None, skip: int = 0, limit: int = 100):
        self.q = q
        self.skip = skip
        self.limit = limit


@app.get("/items/")
async def read_items(commons: Annotated[CommonQueryParams, Depends()]):
    response = {}
    if commons.q:
        response.update({"q": commons.q})
    items = fake_items_db[commons.skip : commons.skip + commons.limit]
    response.update({"items": items})
    return response

...and FastAPI will know what to do.

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