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 after.

Tip

Make sure to use yield one single time.

Technical Details

Any function that is valid to use with:

• @contextlib.contextmanager or
• @contextlib.asynccontextmanager

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:

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 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.

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 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 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 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.

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 the response is sent, so Exception Handlers will have already run. 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.

Tip

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

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.

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 for which you create 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.

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:

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:

• @contextlib.contextmanager or
• @contextlib.asynccontextmanager

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.