classes-as-dependencies.md

Classes as Dependencies

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

A dict from the previous example

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

{!../../../docs_src/dependencies/tutorial001.py!}

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

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:

something()

or

something(some_argument, some_keyword_argument="foo")

then it is a "callable".

Classes as dependencies

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

For example:

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:

{!../../../docs_src/dependencies/tutorial002.py!}

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

{!../../../docs_src/dependencies/tutorial002.py!}

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

{!../../../docs_src/dependencies/tutorial001.py!}

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

Now you can declare your dependency using this class.

{!../../../docs_src/dependencies/tutorial002.py!}

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

Notice how we write CommonQueryParams twice in the above code:

commons: CommonQueryParams = Depends(CommonQueryParams)

The last CommonQueryParams, in:

... = Depends(CommonQueryParams)

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

From it is that FastAPI will extract the declared parameters and that is what FastAPI will actually call.

---

In this case, the first CommonQueryParams, in:

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:

commons = Depends(CommonQueryParams)

..as in:

{!../../../docs_src/dependencies/tutorial003.py!}

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

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

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:

commons: CommonQueryParams = Depends(CommonQueryParams)

...you write:

commons: CommonQueryParams = Depends()

You declare the dependency as the type of the parameter, and you use Depends() as its "default" value (that after the =) for that function's parameter, without any parameter in Depends(), instead of having to write the full class again inside of Depends(CommonQueryParams).

The same example would then look like:

{!../../../docs_src/dependencies/tutorial004.py!}

...and FastAPI will know what to do.

!!! tip If that seems more confusing than helpful, disregard it, you don't need it.

It is just a shortcut. Because **FastAPI** cares about helping you minimize code repetition.