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WIP: FastAPI Best Practices

1. Project Structure. Group files by module domain, not file types.

I didn't like the project structure presented by @tiangolo, where we separate files by their type (e.g. api, crud, models, schemas). Structure that I find more scalable and evolvable is inspired by Netflix's Dispatch with some little modifications.

fastapi-project
├── alembic/
├── src
│   ├── auth
│   │   ├── router.py
│   │   ├── schemas.py  # pydantic models
│   │   ├── models.py  # db models
│   │   ├── dependencies.py
│   │   ├── config.py  # local configs
│   │   ├── constants.py
│   │   ├── exceptions.py
│   │   ├── service.py
│   │   └── utils.py
│   ├── aws
│   │   ├── client.py  # client model for external service communication
│   │   ├── schemas.py
│   │   ├── config.py
│   │   ├── constants.py
│   │   ├── exceptions.py
│   │   └── utils.py
│   └── posts
│   │   ├── router.py
│   │   ├── schemas.py
│   │   ├── models.py
│   │   ├── dependencies.py
│   │   ├── constants.py
│   │   ├── exceptions.py
│   │   ├── service.py
│   │   └── utils.py
│   ├── config.py  # global configs
│   ├── models.py  # global models
│   ├── exceptions.py  # global exceptions
│   ├── pagination.py  # global module e.g. pagination
│   ├── database.py  # db connection related stuff
│   └── main.py
├── tests/
│   ├── auth
│   ├── aws
│   └── posts
├── templates/
│   └── index.html
├── requirements
│   ├── base.txt
│   ├── dev.txt
│   └── prod.txt
├── .env
├── .gitignore
├── logging.ini
└── alembic.ini
  1. Store all the module directories inside src folder
    1. src/ - highest level of an app, contains common models, configs, and constants, etc.
    2. src/main.py - root of the project, which inits the FastAPI app
  2. Each package has its own router, schemas, models, etc.
    1. router.py - is a core of each module with all the endpoints
    2. schemas.py - for pydantic models
    3. models.py - for db models
    4. service.py - module specific business logic
    5. dependencies.py - router dependencies
    6. constants.py - module specific constants and error codes
    7. config.py - e.g. env vars
    8. utils.py - non-business logic functions, e.g. response normalization, data enrichment, etc.
    9. exceptions - module specific exceptions, e.g. PostNotFound, InvalidUserData
  3. When package requires services or dependencies or constants from other packages - import them with explicit module name
from src.auth import constants as auth_constants
from src.notifictions import service as notification_service
from src.posts.constants import ErrorCode as PostsErrorCode  # standard ErrorCode class with constant literals

2. Excessively use Pydantic

Pydantic has a rich set of features to validate and transform data.

In addition to regular features like required, non-required fields and default data, it has built-in comprehensive data processing params like regex, enums for limited allowed options, length validation, email validation, etc.

from enum import Enum
from pydantic import BaseModel, constr, EmailStr, Field, AnyUrl

class MusicBand(str, Enum):
   AEROSMITH = "AEROSMITH"
   QUEEN = "QUEEN"
   ACDC = "AC/DC"


class UserBase(BaseModel):
    first_name: str = Field(min_length=1, max_length=128)
    username: constr(regex="^[A-Za-z0-9-_]+$", to_lower=True, strip_whitespace=True)
    email: EmailStr
    age: int = Field(ge=18, default=None)  # must be greater or equal to 18
    favorite_band: MusicBand = None  # only "AEROSMITH", "QUEEN", "AC/DC" values are allowed to be inputted
    website: AnyUrl = None

3. Use dependencies for data validation vs DB

Pydantic can only validate the values of client input. Use dependencies to validate data against database requirements like email already exists, user not found, etc.

# dependencies.py
async def valid_post_id(post_id: UUID4) -> Mapping:
    post = await service.get_by_id(post_id)
    if not post:
        raise PostNotFound()

    return post


# router.py
@router.get("/posts/{post_id}", response_model=PostResponse)
async def get_post_by_id(post: Mapping = Depends(valid_post_id)):
    return post


@router.put("/posts/{post_id}", response_model=PostResponse)
async def update_post(
    update_data: PostUpdate,  
    post: Mapping = Depends(valid_post_id), 
):
    updated_post: Mapping = await service.update(id=post["id"], data=update_data)
    return updated_post


@router.get("/posts/{post_id}/reviews", response_model=list[ReviewsResponse])
async def get_post_reviews(post: Mapping = Depends(valid_post_id)):
    post_reviews: list[Mapping] = await reviews_service.get_by_post_id(post["id"])
    return post_reviews

If we didn't put data validation to dependency, we would have to add post_id validation for every endpoint and write the same tests for each of them.

5. Chain dependencies

Dependencies can use other dependencies and avoid code repetition for similar logic.

# dependencies.py
from fastapi.security import OAuth2PasswordBearer
from jose import JWTError, jwt

async def valid_post_id(post_id: UUID4) -> Mapping:
    post = await service.get_by_id(post_id)
    if not post:
        raise PostNotFound()

    return post


async def parse_jwt_data(
    token: str = Depends(OAuth2PasswordBearer(tokenUrl="/auth/token"))
) -> dict:
    try:
        payload = jwt.decode(token, "JWT_SECRET", algorithms=["HS256"])
    except JWTError:
        raise InvalidCredentials()

    return {"user_id": payload["id"]}


async def valid_owned_post(
    post: Mapping = Depends(valid_post_id), 
    token_data: dict = Depends(parse_jwt_data),
) -> Mapping:
    if post["creator_id"] != token_data["user_id"]:
        raise UserNotOwner()

    return post

# router.py
@router.get("/users/{user_id}/posts/{post_id}", response_model=PostResponse)
async def get_user_post(post: Mapping = Depends(valid_owned_post)):
    """Get post that belong the user."""
    return post

6. Decouple & Reuse dependencies. Dependency calls are cached.

Dependencies can be reused multiple times, and they won't be recalculated - FastAPI caches their result by default, e.g. if we have a dependency which calls service get_post_by_id, we won't be visiting DB each time we call this dependency - only the first function call.

Knowing this, we can easily decouple dependencies onto multiple smaller functions that operate on a smaller scope and are easier to reuse in other routes. For example, in the code below we are using parse_jwt_data three times:

  1. valid_owned_post
  2. valid_active_creator
  3. get_user_post,

but parse_jwt_data is called only once, in the very first call.

# dependencies.py
from fastapi import BackgroundTasks
from fastapi.security import OAuth2PasswordBearer
from jose import JWTError, jwt

async def valid_post_id(post_id: UUID4) -> Mapping:
    post = await service.get_by_id(post_id)
    if not post:
        raise PostNotFound()

    return post


async def parse_jwt_data(
    token: str = Depends(OAuth2PasswordBearer(tokenUrl="/auth/token"))
) -> dict:
    try:
        payload = jwt.decode(token, "JWT_SECRET", algorithms=["HS256"])
    except JWTError:
        raise InvalidCredentials()

    return {"user_id": payload["id"]}


async def valid_owned_post(
    post: Mapping = Depends(valid_post_id), 
    token_data: dict = Depends(parse_jwt_data),
) -> Mapping:
    if post["creator_id"] != token_data["user_id"]:
        raise UserNotOwner()

    return post


async def valid_active_creator(
    token_data: dict = Depends(parse_jwt_data),
):
    user = await users_service.get_by_id(token_data["user_id"])
    if not user["is_active"]:
        raise UserIsBanned()
    
    return user
        

# router.py
@router.get("/users/{user_id}/posts/{post_id}", response_model=PostResponse)
async def get_user_post(
    worker: BackgroundTasks,
    post: Mapping = Depends(valid_owned_post),
    user: Mapping = Depends(valid_active_creator),
):
    """Get post that belong the active user."""
    worker.add_task(notifications_service.send_email, user["id"])
    return post

7. Follow REST

  1. Developing RESTfull API makes it easier to reuse dependencies Following REST leads us to reuse dependencies and
    1. GET /courses/:course_id
    2. GET /courses/:course_id/chapters/:chapter_id/lessons
    3. GET /chapters/:chapter_id

The only caveat is to use the same variable names in the path, i.e. if you have two endpoints GET /profiles/:profile_id and GET /creators/:creator_id that both validate whether the given profile exists, but GET /creators/:creator_id also checks if the profile is creator, then it's better to chain those two dependencies.

# src.profiles.dependencies
async def valid_profile_id(profile_id: UUID4) -> Mapping:
    profile = await service.get_by_id(post_id)
    if not profile:
        raise ProfileNotFound()

    return profile

# src.creators.dependencies
async def valid_creator_id(profile: Mapping = Depends(valid_profile_id)) -> Mapping:
    if not profile["is_creator"]:
       raise ProfileNotCreator()

    return profile

# src.profiles.router.py
@router.get("/profiles/{user_id}", response_model=ProfileResponse)
async def get_user_profile_by_id(profile: Mapping = Depends(valid_profile_id)):
    """Get profile by id."""
    return profile

# src.creators.router.py
@router.get("/profiles/{user_id}", response_model=ProfileResponse)
async def get_user_profile_by_id(
     creator_profile: Mapping = Depends(valid_creator_id)
):
    """Get profile by id."""
    return creator_profile
  1. Add /me endpoint for users own posts
    1. No need to validate that user id exists - it's already checked via auth method
    2. No need to check whether the user id belongs to the requester

8. Don't make your routes async, if you have only blocking I/O operations

Under the hood, FastAPI can effectively handle both async and sync I/O operations.

  • FastAPI calls sync routes in the threadpool and blocking I/O operations won't stop event loop from executing the tasks.
  • Otherwise, if the route is defined as async then it's called regularly via await and FastAPI trusts you to do only non-blocking I/O operations.

The caveat is if you fail that trust and execute blocking operations within async routes, event loop will not be able to run the next tasks until that blocking operation is done.

import asyncio
import time

@router.get("/terrible-ping")
async def terrible_catastrophic_ping():
    time.sleep(10) # I/O blocking operation for 10 seconds
    pong = service.get_pong()  # I/O blocking operation to get pong from DB
    
    return {"pong": pong}

@router.get("/good-ping")
def good_ping():
    time.sleep(10) # I/O blocking operation for 10 seconds, but in another thread
    pong = service.get_pong()  # I/O blocking operation to get pong from DB, but in another thread
    
    return {"pong": pong}

@router.get("/perfect-ping")
async def perfect_ping():
    await asyncio.sleep(10) # not I/O blocking operation
    pong = await service.async_get_pong()  # non I/O blocking db call

    return {"pong": pong}

What happens when we call:

  1. GET /terrible-ping
    1. FastAPI server receives a request and starts handling it
    2. Server's event loop and all the tasks in the queue will be waiting until time.sleep() is finished
      1. Server thinks time.sleep() is not an I/O task, so it waits until it is finished
      2. Server won't accept any new requests while waiting
    3. Then, event loop and all the tasks in the queue will be waiting until service.get_pong is finished
      1. Server thinks service.get_pong() is not an I/O task, so it waits until it is finished
      2. Server won't accept any new requests while waiting
    4. Server returns the response.
      1. After a response, server starts accepting new requests
  2. GET /good-ping
    1. FastAPI server receives a request and starts handling it
    2. FastAPI sends the whole route good_ping to the threadpool, where a worker thread will run the function
    3. While good_ping is being executed, event loop selects next tasks from the queue and works on them (e.g. accept new request, call db)
      • Independently of main thread (i.e. our FastAPI app), worker thread will be waiting for time.sleep to finish and then for service.get_pong to finish
    4. When good_ping finishes its work, server returns a response to the client
  3. GET /perfect-ping
    1. FastAPI server receives a request and starts handling it
    2. FastAPI awaits asyncio.sleep(10)
    3. Event loop selects next tasks from the queue and works on them (e.g. accept new request, call db)
    4. When asyncio.sleep(10) is done, servers goes to the next lines and awaits service.async_get_pong
    5. Event loop selects next tasks from the queue and works on them (e.g. accept new request, call db)
    6. When service.async_get_pong is done, server returns a response to the client

The caveat is that operations that are sent to thread pool or are non-blocking awaitables should be I/O intensive tasks (e.g. open file, db call, external API call).

  • Awaiting CPU intensive tasks (e.g. heavy calculations, data processing, video transcoding) is worthless, since CPU has to work to finish the tasks, while I/O operations are external and server does nothing while waiting for that operations to finish, thus it can go to the next tasks.
  • Running CPU intensive tasks in other threads also isn't effective, because of GIL. In short, GIL allows only one thread to work at a time, which makes it useless for CPU tasks.
  • If you want to optimize CPU intensive tasks you should send them to workers in another process.

9. Custom base model model from day 0,

convert datetime to common standard

10. Hide docs by default. Show it explicitly on the selected envs

11. Use Starlette's Config object, instead of 3rd party ones - it's decent enough

12. Set DB keys naming convention immediately, from day 0

13. Set DB table naming convention immediately, from day 0

14. Set uuids within the app

it's easier to test

15. Set tests client async from day 0

  1. Unless you aren't planning to add integrational tests with db
  2. If you do, then do it. Problems with event loop will appear once you want to prepare objects

16. Set postgres identity from day 0

17. take use of background workers - they are stable enough

good for both async and sync routes

18. take use of response model, response status, responses

19. typing is important - use it everywhere

20. save files in chunk

21. use smart union or add explicit invalidation

22. do a lot of logic in db, use pydantic to parse it (show the way we evolved creator field)

23. validate file formats

24. validate url source (if users are able to send files)

25. root_validator if multiple columns

26. pre if data need to be pre-handled before validation

27. you can just raise a ValueError in pydantic schemas, if that's it faces user request.

it will return a nice response

28. don't forget that fastapi converts response Model to Dict then to Model then to JSON

29. if no async lib, and poor documentation, then use asgiref

30. use linters (black, isort, autoflake)

31. set logs from day 0