Sub-dependencies¶
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You can create dependencies that have sub-dependencies.
They can be as deep as you need them to be.
FastAPI will take care of solving them.
First dependency "dependable"¶
You could create a first dependency ("dependable") like:
from typing import Annotated
from fastapi import Cookie, Depends, FastAPI
app = FastAPI()
def query_extractor(q: str | None = None):
return q
def query_or_cookie_extractor(
q: Annotated[str, Depends(query_extractor)],
last_query: Annotated[str | None, Cookie()] = None,
):
if not q:
return last_query
return q
@app.get("/items/")
async def read_query(
query_or_default: Annotated[str, Depends(query_or_cookie_extractor)]
):
return {"q_or_cookie": query_or_default}
from typing import Annotated, Union
from fastapi import Cookie, Depends, FastAPI
app = FastAPI()
def query_extractor(q: Union[str, None] = None):
return q
def query_or_cookie_extractor(
q: Annotated[str, Depends(query_extractor)],
last_query: Annotated[Union[str, None], Cookie()] = None,
):
if not q:
return last_query
return q
@app.get("/items/")
async def read_query(
query_or_default: Annotated[str, Depends(query_or_cookie_extractor)]
):
return {"q_or_cookie": query_or_default}
from typing import Union
from fastapi import Cookie, Depends, FastAPI
from typing_extensions import Annotated
app = FastAPI()
def query_extractor(q: Union[str, None] = None):
return q
def query_or_cookie_extractor(
q: Annotated[str, Depends(query_extractor)],
last_query: Annotated[Union[str, None], Cookie()] = None,
):
if not q:
return last_query
return q
@app.get("/items/")
async def read_query(
query_or_default: Annotated[str, Depends(query_or_cookie_extractor)]
):
return {"q_or_cookie": query_or_default}
Tip
Prefer to use the Annotated version if possible.
from fastapi import Cookie, Depends, FastAPI
app = FastAPI()
def query_extractor(q: str | None = None):
return q
def query_or_cookie_extractor(
q: str = Depends(query_extractor), last_query: str | None = Cookie(default=None)
):
if not q:
return last_query
return q
@app.get("/items/")
async def read_query(query_or_default: str = Depends(query_or_cookie_extractor)):
return {"q_or_cookie": query_or_default}
Tip
Prefer to use the Annotated version if possible.
from typing import Union
from fastapi import Cookie, Depends, FastAPI
app = FastAPI()
def query_extractor(q: Union[str, None] = None):
return q
def query_or_cookie_extractor(
q: str = Depends(query_extractor),
last_query: Union[str, None] = Cookie(default=None),
):
if not q:
return last_query
return q
@app.get("/items/")
async def read_query(query_or_default: str = Depends(query_or_cookie_extractor)):
return {"q_or_cookie": query_or_default}
It declares an optional query parameter q as a str, and then it just returns it.
This is quite simple (not very useful), but will help us focus on how the sub-dependencies work.
Second dependency, "dependable" and "dependant"¶
Then you can create another dependency function (a "dependable") that at the same time declares a dependency of its own (so it is a "dependant" too):
from typing import Annotated
from fastapi import Cookie, Depends, FastAPI
app = FastAPI()
def query_extractor(q: str | None = None):
return q
def query_or_cookie_extractor(
q: Annotated[str, Depends(query_extractor)],
last_query: Annotated[str | None, Cookie()] = None,
):
if not q:
return last_query
return q
@app.get("/items/")
async def read_query(
query_or_default: Annotated[str, Depends(query_or_cookie_extractor)]
):
return {"q_or_cookie": query_or_default}
from typing import Annotated, Union
from fastapi import Cookie, Depends, FastAPI
app = FastAPI()
def query_extractor(q: Union[str, None] = None):
return q
def query_or_cookie_extractor(
q: Annotated[str, Depends(query_extractor)],
last_query: Annotated[Union[str, None], Cookie()] = None,
):
if not q:
return last_query
return q
@app.get("/items/")
async def read_query(
query_or_default: Annotated[str, Depends(query_or_cookie_extractor)]
):
return {"q_or_cookie": query_or_default}
from typing import Union
from fastapi import Cookie, Depends, FastAPI
from typing_extensions import Annotated
app = FastAPI()
def query_extractor(q: Union[str, None] = None):
return q
def query_or_cookie_extractor(
q: Annotated[str, Depends(query_extractor)],
last_query: Annotated[Union[str, None], Cookie()] = None,
):
if not q:
return last_query
return q
@app.get("/items/")
async def read_query(
query_or_default: Annotated[str, Depends(query_or_cookie_extractor)]
):
return {"q_or_cookie": query_or_default}
Tip
Prefer to use the Annotated version if possible.
from fastapi import Cookie, Depends, FastAPI
app = FastAPI()
def query_extractor(q: str | None = None):
return q
def query_or_cookie_extractor(
q: str = Depends(query_extractor), last_query: str | None = Cookie(default=None)
):
if not q:
return last_query
return q
@app.get("/items/")
async def read_query(query_or_default: str = Depends(query_or_cookie_extractor)):
return {"q_or_cookie": query_or_default}
Tip
Prefer to use the Annotated version if possible.
from typing import Union
from fastapi import Cookie, Depends, FastAPI
app = FastAPI()
def query_extractor(q: Union[str, None] = None):
return q
def query_or_cookie_extractor(
q: str = Depends(query_extractor),
last_query: Union[str, None] = Cookie(default=None),
):
if not q:
return last_query
return q
@app.get("/items/")
async def read_query(query_or_default: str = Depends(query_or_cookie_extractor)):
return {"q_or_cookie": query_or_default}
Let's focus on the parameters declared:
- Even though this function is a dependency ("dependable") itself, it also declares another dependency (it "depends" on something else).
- It depends on the
query_extractor, and assigns the value returned by it to the parameterq.
- It depends on the
- It also declares an optional
last_querycookie, as astr.- If the user didn't provide any query
q, we use the last query used, which we saved to a cookie before.
- If the user didn't provide any query
Use the dependency¶
Then we can use the dependency with:
from typing import Annotated
from fastapi import Cookie, Depends, FastAPI
app = FastAPI()
def query_extractor(q: str | None = None):
return q
def query_or_cookie_extractor(
q: Annotated[str, Depends(query_extractor)],
last_query: Annotated[str | None, Cookie()] = None,
):
if not q:
return last_query
return q
@app.get("/items/")
async def read_query(
query_or_default: Annotated[str, Depends(query_or_cookie_extractor)]
):
return {"q_or_cookie": query_or_default}
from typing import Annotated, Union
from fastapi import Cookie, Depends, FastAPI
app = FastAPI()
def query_extractor(q: Union[str, None] = None):
return q
def query_or_cookie_extractor(
q: Annotated[str, Depends(query_extractor)],
last_query: Annotated[Union[str, None], Cookie()] = None,
):
if not q:
return last_query
return q
@app.get("/items/")
async def read_query(
query_or_default: Annotated[str, Depends(query_or_cookie_extractor)]
):
return {"q_or_cookie": query_or_default}
from typing import Union
from fastapi import Cookie, Depends, FastAPI
from typing_extensions import Annotated
app = FastAPI()
def query_extractor(q: Union[str, None] = None):
return q
def query_or_cookie_extractor(
q: Annotated[str, Depends(query_extractor)],
last_query: Annotated[Union[str, None], Cookie()] = None,
):
if not q:
return last_query
return q
@app.get("/items/")
async def read_query(
query_or_default: Annotated[str, Depends(query_or_cookie_extractor)]
):
return {"q_or_cookie": query_or_default}
Tip
Prefer to use the Annotated version if possible.
from fastapi import Cookie, Depends, FastAPI
app = FastAPI()
def query_extractor(q: str | None = None):
return q
def query_or_cookie_extractor(
q: str = Depends(query_extractor), last_query: str | None = Cookie(default=None)
):
if not q:
return last_query
return q
@app.get("/items/")
async def read_query(query_or_default: str = Depends(query_or_cookie_extractor)):
return {"q_or_cookie": query_or_default}
Tip
Prefer to use the Annotated version if possible.
from typing import Union
from fastapi import Cookie, Depends, FastAPI
app = FastAPI()
def query_extractor(q: Union[str, None] = None):
return q
def query_or_cookie_extractor(
q: str = Depends(query_extractor),
last_query: Union[str, None] = Cookie(default=None),
):
if not q:
return last_query
return q
@app.get("/items/")
async def read_query(query_or_default: str = Depends(query_or_cookie_extractor)):
return {"q_or_cookie": query_or_default}
Info
Notice that we are only declaring one dependency in the path operation function, the query_or_cookie_extractor.
But FastAPI will know that it has to solve query_extractor first, to pass the results of that to query_or_cookie_extractor while calling it.
graph TB
query_extractor(["query_extractor"])
query_or_cookie_extractor(["query_or_cookie_extractor"])
read_query["/items/"]
query_extractor --> query_or_cookie_extractor --> read_queryUsing the same dependency multiple times¶
If one of your dependencies is declared multiple times for the same path operation, for example, multiple dependencies have a common sub-dependency, FastAPI will know to call that sub-dependency only once per request.
And it will save the returned value in a "cache" and pass it to all the "dependants" that need it in that specific request, instead of calling the dependency multiple times for the same request.
In an advanced scenario where you know you need the dependency to be called at every step (possibly multiple times) in the same request instead of using the "cached" value, you can set the parameter use_cache=False when using Depends:
async def needy_dependency(fresh_value: Annotated[str, Depends(get_value, use_cache=False)]):
return {"fresh_value": fresh_value}
Tip
Prefer to use the Annotated version if possible.
async def needy_dependency(fresh_value: str = Depends(get_value, use_cache=False)):
return {"fresh_value": fresh_value}
Recap¶
Apart from all the fancy words used here, the Dependency Injection system is quite simple.
Just functions that look the same as the path operation functions.
But still, it is very powerful, and allows you to declare arbitrarily deeply nested dependency "graphs" (trees).
Tip
All this might not seem as useful with these simple examples.
But you will see how useful it is in the chapters about security.
And you will also see the amounts of code it will save you.