Overview

This project contains classes and decorators to use FastAPI with "class based routing". In particular this allows you to construct an instance of a class and have methods of that instance be route handlers. For example:

from dao import Dao
# Some fictional dao
from classy_fastapi import Routable, get, delete

def parse_arg() -> argparse.Namespace:
   """parse command line arguments."""
   ...


class UserRoutes(Routable):
   """Inherits from Routable."""

   # Note injection here by simply passing values to the constructor. Other injection frameworks also 
   # supported as there's nothing sepecial about this __init__ method.
   def __init__(self, dao: Dao) -> None:
      """Constructor. The Dao is injected here."""
      super().__init__()
      self.__dao = Dao

   @get('/user/{name}')
   def get_user_by_name(name: str) -> User:
      # Use our injected DAO instance.
      return self.__dao.get_user_by_name(name)

   @delete('/user/{name}')
   def delete_user(name: str) -> None:
      self.__dao.delete(name)


def main():
    args = parse_args()
    # Configure the DAO per command line arguments
    dao = Dao(args.url, args.user, args.password)
    # Simple intuitive injection
    user_routes = UserRoutes(dao)
    
    app = FastAPI()
    # router memeber inherited from cr.Routable and configured per the annotations.
    app.include_router(user_routes.router)

Note that there are no global variables and dependency injection is accomplished by simply passing arguments to the constructor.

Why

FastAPI generally has one define routes like:

app = FastAPI()

@app.get('/echo/{x}')
def echo(x: int) -> int:
   return x

Note that app is a global. Furthermore, FastAPI's suggested way of doing dependency injection is handy for things like pulling values out of header in the HTTP request. However, they don't work well for more standard dependency injection scenarios where we'd like to do something like inject a DAO or database connection. For that, FastAPI suggests their parameterized dependencies which might look something like:

app = FastAPI()

class ValueToInject:
   def __init__(self, y: int) -> None:
      self.y = y

   def __call__(self) -> int:
      return self.y

to_add = ValueToInject(2)

@app.get('/add/{x}')
def add(x: int, y: Depends(to_add)) -> int:
   return x + y

This works but there's a few issues:

• The Dependency must be a callable which requires an unfortunate amount of boilerplate.
• If we want to use the same dependency on several routes, as we would with something like a database connection, we have to repeat the Dependency(to_add) bit on each endpoint. Note that FastAPI lets you group endpoints your we can include the dependency on all of them but then there's no way to access the dependency from the router code so this really only works for things like authentication where the dependency can do some route handling (e.g. return a 402 if an auth header is missing).
• to_add is a global variable which is limiting.

Let's consider an expanded, more realistic example where we have a group of routes that operate on users to add them, delete them, change the password, etc. Those routes will need to access a database so we have a DAO that helps set that up. We're going to take the database URL, password, etc. via command line arguments and then set up our routes. Furthermore, we'll split up our application into a few separate files. Doing this without class routing looks like the following:

# main.py

import .user
from .deps import dao

def parse_arg() -> argparse.Namespace:
   """parse command line arguments."""
   ...

def main():
    args = parse_args()
    global dao
    dao = Dao(args.url, args.user, args.password)
    
    app = FastAPI()
    app.include_router(user.router)

####
# dao.py

from dao import Dao

# DAO for injection. We don't know the command line arguments yet but we need to make this global as we need to be able
# to access it in user.py below so it's None here and gets set in main()
dao: Optional[Dao] = None

#####
# user.py
from .deps import dao
from dao import Dao
from fastapi.routing import APIRouter

@router.get('/user/{name}')
def get_user_by_name(name: str, dao: Dao = Depends(dao)) -> User:
   return dao.get_user_by_name(name)

@router.delete('/user/{name}')
def delete_user(name: str, dao: Dao = Depends(dao)) -> None:
   dao.delete(name)

# ... additional user methods ...

That works but it's a bit verbose. Additionally, as noted above, it has some limitations. For example, suppose we've updated our API in a breaking way so we've added a /v2 set of routes. However, the users.py routes haven't changed at all except that we've changed how we store users (e.g. a new password hashing algorithm) so /v2 user routes need to use a different DAO. Ideally you'd call app.include_router twice with different prefixes but that won't work because the dependency on the DAO is to a specific DAO instance in user.py. You can add dependency overrides but it feels awkward.

By contrast the class based routing in this package does not have any global variables at all and injection can be performed by simply passing values to a constructor or via any other dependency injection framework.

Alternatives

FastAPI-utils has a class based views implementation but the routes are on the class itself rather than on instances of the class.

There's demand for this feature so a number of alternatives have been proposed in an open bug and on StackOverflow but all seem to require global injection or hacks like defining all the routes inside the constructor.

Older Versions of Python

Unfortunately this does not work with async routes with Python versions less than 3.8 due to bugs in inspect.iscoroutinefunction. Specifically with older versions of Python iscoroutinefunction incorrectly returns false so async routes aren't await'd. We therefore only support Python versions >= 3.8