tiangolo.fastapi/docs/tutorial/dependencies/dependencies-with-yield.md

Dependencies with yield

FastAPI supports dependencies that do some extra steps after finishing.

To do this, use yield instead of return, and write the extra steps after.

!!! tip Make sure to use yield one single time.

!!! info For this to work, you need to use Python 3.7 or above, or in Python 3.6, install the "backports":

```
pip install async-exit-stack async-generator
```

This installs <a href="https://github.com/sorcio/async_exit_stack" class="external-link" target="_blank">async-exit-stack</a> and <a href="https://github.com/python-trio/async_generator" class="external-link" target="_blank">async-generator</a>.

!!! note "Technical Details" Any function that is valid to use with:

* <a href="https://docs.python.org/3/library/contextlib.html#contextlib.contextmanager" class="external-link" target="_blank">`@contextlib.contextmanager`</a> or 
* <a href="https://docs.python.org/3/library/contextlib.html#contextlib.asynccontextmanager" class="external-link" target="_blank">`@contextlib.asynccontextmanager`</a>

would be valid to use as a **FastAPI** dependency.

In fact, FastAPI uses those two decorators internally.

A database dependency with yield

For example, you could use this to create a database session and close it after finishing.

Only the code prior to and including the yield statement is executed before sending a response:

{!./src/dependencies/tutorial007.py!}

The yielded value is what is injected into path operations and other dependencies:

{!./src/dependencies/tutorial007.py!}

The code following the yield statement is executed after the response has been delivered:

{!./src/dependencies/tutorial007.py!}

!!! tip You can use async or normal functions.

**FastAPI** will do the right thing with each, the same as with normal dependencies.

A dependency with yield and try

If you use a try block in a dependency with yield, you'll receive any exception that was thrown when using the dependency.

For example, if some code at some point in the middle, in another dependency or in a path operation, made a database transaction "rollback" or create any other error, you will receive the exception in your dependency.

So, you can look for that specific exception inside the dependency with except SomeException.

In the same way, you can use finally to make sure the exit steps are executed, no matter if there was an exception or not.

{!./src/dependencies/tutorial007.py!}

Sub-dependencies with yield

You can have sub-dependencies and "trees" of sub-dependencies of any size and shape, and any or all of them can use yield.

FastAPI will make sure that the "exit code" in each dependency with yield is run in the correct order.

For example, dependency_c can have a dependency on dependency_b, and dependency_b on dependency_a:

{!./src/dependencies/tutorial008.py!}

And all of them can use yield.

In this case dependency_c, to execute its exit code, needs the value from dependency_b (here named dep_b) to still be available.

And, in turn, dependency_b needs the value from dependency_a (here named dep_a) to be available for its exit code.

{!./src/dependencies/tutorial008.py!}

The same way, you could have dependencies with yield and return mixed.

And you could have a single dependency that requires several other dependencies with yield, etc.

You can have any combinations of dependencies that you want.

FastAPI will make sure everything is run in the correct order.

!!! note "Technical Details" This works thanks to Python's Context Managers.

**FastAPI** uses them internally to achieve this.

Dependencies with yield and HTTPException

You saw that you can use dependencies with yield and have try blocks that catch exceptions.

It might be tempting to raise an HTTPException or similar in the exit code, after the yield. But it won't work.

The exit code in dependencies with yield is executed after Exception Handlers{.internal-link target=_blank}. There's nothing catching exceptions thrown by your dependencies in the exit code (after the yield).

So, if you raise an HTTPException after the yield, the default (or any custom) exception handler that catches HTTPExceptions and returns an HTTP 400 response won't be there to catch that exception anymore.

This is what allows anything set in the dependency (e.g. a DB session) to, for example, be used by background tasks.

Background tasks are run after the response has been sent. So there's no way to raise an HTTPException because there's not even a way to change the response that is already sent.

But if a background task creates a DB error, at least you can rollback or cleanly close the session in the dependency with yield, and maybe log the error or report it to a remote tracking system.

If you have some code that you know could raise an exception, do the most normal/"Pythonic" thing and add a try block in that section of the code.

If you have custom exceptions that you would like to handle before returning the response and possibly modifying the response, maybe even raising an HTTPException, create a Custom Exception Handler{.internal-link target=_blank}.

!!! tip You can still raise exceptions including HTTPException before the yield. But not after.

The sequence of execution is more or less like this:

sequenceDiagram

participant client as Client
participant handler as Exception handler
participant dep as Dep with yield
participant operation as Path Operation
participant tasks as Background tasks

    Note over client,tasks: Can raise exception for dependency, handled after response is sent
    Note over client,operation: Can raise HTTPException and can change the response
    client ->> dep: Start request
    Note over dep: Code up to yield
    opt raise
        dep -->> handler: Raise HTTPException
        handler -->> client: HTTP error response
        dep -->> dep: Raise other exception
    end
    dep ->> operation: Run dependency, e.g. DB session
    opt raise
        operation -->> handler: Raise HTTPException
        handler -->> client: HTTP error response
        operation -->> dep: Raise other exception
    end
    operation ->> client: Return response to client
    Note over client,operation: Response is already sent, can't change it anymore
    opt Tasks
        operation -->> tasks: Send background tasks
    end
    opt Raise other exception
        tasks -->> dep: Raise other exception
    end
    Note over dep: After yield
    opt Handle other exception
        dep -->> dep: Handle exception, can't change response. E.g. close DB session.
    end

!!! tip This diagram shows HTTPException, but you could also raise any other exception that you create a Custom Exception Handler{.internal-link target=_blank} for. And that exception would be handled by that custom exception handler instead of the dependency exit code.

But if you raise an exception that is not handled by the exception handlers, it will be handled by the exit code of the dependency.

Context Managers

What are "Context Managers"

"Context Managers" are any of those Python objects that you can use in a with statement.

For example, you can use with to read a file:

with open("./somefile.txt") as f:
    contents = f.read()
    print(contents)

Underneath, the open("./somefile.txt") creates an object that is a called a "Context Manager".

When the with block finishes, it makes sure to close the file, even if there were exceptions.

When you create a dependency with yield, FastAPI will internally convert it to a context manager, and combine it with some other related tools.

Using context managers in dependencies with yield

!!! warning This is, more or less, an "advanced" idea.

If you are just starting with **FastAPI** you might want to skip it for now.

In Python, you can create Context Managers by creating a class with two methods: __enter__() and __exit__().

You can also use them inside of FastAPI dependencies with yield by using with or async with statements inside of the dependency function:

{!./src/dependencies/tutorial010.py!}

!!! tip Another way to create a context manager is with:

* <a href="https://docs.python.org/3/library/contextlib.html#contextlib.contextmanager" class="external-link" target="_blank">`@contextlib.contextmanager`</a> or 
* <a href="https://docs.python.org/3/library/contextlib.html#contextlib.asynccontextmanager" class="external-link" target="_blank">`@contextlib.asynccontextmanager`</a>

using them to decorate a function with a single `yield`.

That's what **FastAPI** uses internally for dependencies with `yield`.

But you don't have to use the decorators for FastAPI dependencies (and you shouldn't).

FastAPI will do it for you internally.