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Dependencies with yield

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FastAPI supports dependencies that do some extra steps after finishing.

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

Tip

Make sure to use yield one single time.

Technical Details

Any function that is valid to use with:

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 creating a response:

async def get_db():
    db = DBSession()
    try:
        yield db
    finally:
        db.close()

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

async def get_db():
    db = DBSession()
    try:
        yield db
    finally:
        db.close()

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

async def get_db():
    db = DBSession()
    try:
        yield db
    finally:
        db.close()

Tip

You can use async or regular 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.

async def get_db():
    db = DBSession()
    try:
        yield db
    finally:
        db.close()

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:

from typing import Annotated

from fastapi import Depends


async def dependency_a():
    dep_a = generate_dep_a()
    try:
        yield dep_a
    finally:
        dep_a.close()


async def dependency_b(dep_a: Annotated[DepA, Depends(dependency_a)]):
    dep_b = generate_dep_b()
    try:
        yield dep_b
    finally:
        dep_b.close(dep_a)


async def dependency_c(dep_b: Annotated[DepB, Depends(dependency_b)]):
    dep_c = generate_dep_c()
    try:
        yield dep_c
    finally:
        dep_c.close(dep_b)

from fastapi import Depends
from typing_extensions import Annotated


async def dependency_a():
    dep_a = generate_dep_a()
    try:
        yield dep_a
    finally:
        dep_a.close()


async def dependency_b(dep_a: Annotated[DepA, Depends(dependency_a)]):
    dep_b = generate_dep_b()
    try:
        yield dep_b
    finally:
        dep_b.close(dep_a)


async def dependency_c(dep_b: Annotated[DepB, Depends(dependency_b)]):
    dep_c = generate_dep_c()
    try:
        yield dep_c
    finally:
        dep_c.close(dep_b)

Tip

Prefer to use the Annotated version if possible.

from fastapi import Depends


async def dependency_a():
    dep_a = generate_dep_a()
    try:
        yield dep_a
    finally:
        dep_a.close()


async def dependency_b(dep_a=Depends(dependency_a)):
    dep_b = generate_dep_b()
    try:
        yield dep_b
    finally:
        dep_b.close(dep_a)


async def dependency_c(dep_b=Depends(dependency_b)):
    dep_c = generate_dep_c()
    try:
        yield dep_c
    finally:
        dep_c.close(dep_b)

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.

from typing import Annotated

from fastapi import Depends


async def dependency_a():
    dep_a = generate_dep_a()
    try:
        yield dep_a
    finally:
        dep_a.close()


async def dependency_b(dep_a: Annotated[DepA, Depends(dependency_a)]):
    dep_b = generate_dep_b()
    try:
        yield dep_b
    finally:
        dep_b.close(dep_a)


async def dependency_c(dep_b: Annotated[DepB, Depends(dependency_b)]):
    dep_c = generate_dep_c()
    try:
        yield dep_c
    finally:
        dep_c.close(dep_b)

from fastapi import Depends
from typing_extensions import Annotated


async def dependency_a():
    dep_a = generate_dep_a()
    try:
        yield dep_a
    finally:
        dep_a.close()


async def dependency_b(dep_a: Annotated[DepA, Depends(dependency_a)]):
    dep_b = generate_dep_b()
    try:
        yield dep_b
    finally:
        dep_b.close(dep_a)


async def dependency_c(dep_b: Annotated[DepB, Depends(dependency_b)]):
    dep_c = generate_dep_c()
    try:
        yield dep_c
    finally:
        dep_c.close(dep_b)

Tip

Prefer to use the Annotated version if possible.

from fastapi import Depends


async def dependency_a():
    dep_a = generate_dep_a()
    try:
        yield dep_a
    finally:
        dep_a.close()


async def dependency_b(dep_a=Depends(dependency_a)):
    dep_b = generate_dep_b()
    try:
        yield dep_b
    finally:
        dep_b.close(dep_a)


async def dependency_c(dep_b=Depends(dependency_b)):
    dep_c = generate_dep_c()
    try:
        yield dep_c
    finally:
        dep_c.close(dep_b)

The same way, you could have some dependencies with yield and some other dependencies with return, and have some of those depend on some of the others.

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.

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.

The same way, you could raise an HTTPException or similar in the exit code, after the yield.

Tip

This is a somewhat advanced technique, and in most of the cases you won't really need it, as you can raise exceptions (including HTTPException) from inside of the rest of your application code, for example, in the path operation function.

But it's there for you if you need it. 🤓

from typing import Annotated

from fastapi import Depends, FastAPI, HTTPException

app = FastAPI()


data = {
    "plumbus": {"description": "Freshly pickled plumbus", "owner": "Morty"},
    "portal-gun": {"description": "Gun to create portals", "owner": "Rick"},
}


class OwnerError(Exception):
    pass


def get_username():
    try:
        yield "Rick"
    except OwnerError as e:
        raise HTTPException(status_code=400, detail=f"Owner error: {e}")


@app.get("/items/{item_id}")
def get_item(item_id: str, username: Annotated[str, Depends(get_username)]):
    if item_id not in data:
        raise HTTPException(status_code=404, detail="Item not found")
    item = data[item_id]
    if item["owner"] != username:
        raise OwnerError(username)
    return item

from fastapi import Depends, FastAPI, HTTPException
from typing_extensions import Annotated

app = FastAPI()


data = {
    "plumbus": {"description": "Freshly pickled plumbus", "owner": "Morty"},
    "portal-gun": {"description": "Gun to create portals", "owner": "Rick"},
}


class OwnerError(Exception):
    pass


def get_username():
    try:
        yield "Rick"
    except OwnerError as e:
        raise HTTPException(status_code=400, detail=f"Owner error: {e}")


@app.get("/items/{item_id}")
def get_item(item_id: str, username: Annotated[str, Depends(get_username)]):
    if item_id not in data:
        raise HTTPException(status_code=404, detail="Item not found")
    item = data[item_id]
    if item["owner"] != username:
        raise OwnerError(username)
    return item

Tip

Prefer to use the Annotated version if possible.

from fastapi import Depends, FastAPI, HTTPException

app = FastAPI()


data = {
    "plumbus": {"description": "Freshly pickled plumbus", "owner": "Morty"},
    "portal-gun": {"description": "Gun to create portals", "owner": "Rick"},
}


class OwnerError(Exception):
    pass


def get_username():
    try:
        yield "Rick"
    except OwnerError as e:
        raise HTTPException(status_code=400, detail=f"Owner error: {e}")


@app.get("/items/{item_id}")
def get_item(item_id: str, username: str = Depends(get_username)):
    if item_id not in data:
        raise HTTPException(status_code=404, detail="Item not found")
    item = data[item_id]
    if item["owner"] != username:
        raise OwnerError(username)
    return item

An alternative you could use to catch exceptions (and possibly also raise another HTTPException) is to create a Custom Exception Handler.

Execution of dependencies with yield

The sequence of execution is more or less like this diagram. Time flows from top to bottom. And each column is one of the parts interacting or executing code.

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,operation: Can raise exceptions, including HTTPException
    client ->> dep: Start request
    Note over dep: Run code up to yield
    opt raise Exception
        dep -->> handler: Raise Exception
        handler -->> client: HTTP error response
    end
    dep ->> operation: Run dependency, e.g. DB session
    opt raise
        operation -->> dep: Raise Exception (e.g. HTTPException)
        opt handle
            dep -->> dep: Can catch exception, raise a new HTTPException, raise other exception
            dep -->> handler: Auto forward exception
        end
        handler -->> client: HTTP error response
    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 -->> tasks: Handle exceptions in the background task code
    end

Info

Only one response will be sent to the client. It might be one of the error responses or it will be the response from the path operation.

After one of those responses is sent, no other response can be sent.

Tip

This diagram shows HTTPException, but you could also raise any other exception that you catch in a dependency with yield or with a Custom Exception Handler.

If you raise any exception, it will be passed to the dependencies with yield, including HTTPException, and then again to the exception handlers. If there's no exception handler for that exception, it will then be handled by the default internal ServerErrorMiddleware, returning a 500 HTTP status code, to let the client know that there was an error in the server.

Dependencies with yield, HTTPException and Background Tasks

Warning

You most probably don't need these technical details, you can skip this section and continue below.

These details are useful mainly if you were using a version of FastAPI prior to 0.106.0 and used resources from dependencies with yield in background tasks.

Before FastAPI 0.106.0, raising exceptions after yield was not possible, the exit code in dependencies with yield was executed after the response was sent, so Exception Handlers would have already run.

This was designed this way mainly to allow using the same objects "yielded" by dependencies inside of background tasks, because the exit code would be executed after the background tasks were finished.

Nevertheless, as this would mean waiting for the response to travel through the network while unnecessarily holding a resource in a dependency with yield (for example a database connection), this was changed in FastAPI 0.106.0.

Tip

Additionally, a background task is normally an independent set of logic that should be handled separately, with its own resources (e.g. its own database connection).

So, this way you will probably have cleaner code.

If you used to rely on this behavior, now you should create the resources for background tasks inside the background task itself, and use internally only data that doesn't depend on the resources of dependencies with yield.

For example, instead of using the same database session, you would create a new database session inside of the background task, and you would obtain the objects from the database using this new session. And then instead of passing the object from the database as a parameter to the background task function, you would pass the ID of that object and then obtain the object again inside the background task function.

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 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 create a context manager for it, 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:

class MySuperContextManager:
    def __init__(self):
        self.db = DBSession()

    def __enter__(self):
        return self.db

    def __exit__(self, exc_type, exc_value, traceback):
        self.db.close()


async def get_db():
    with MySuperContextManager() as db:
        yield db

Tip

Another way to create a context manager is with:

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.