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OAuth2 scopes

You can use OAuth2 scopes directly with FastAPI, they are integrated to work seamlessly.

This would allow you to have a more fine-grained permission system, following the OAuth2 standard, integrated into your OpenAPI application (and the API docs).

OAuth2 with scopes is the mechanism used by many big authentication providers, like Facebook, Google, GitHub, Microsoft, Twitter, etc. They use it to provide specific permissions to users and applications.

Every time you "log in with" Facebook, Google, GitHub, Microsoft, Twitter, that application is using OAuth2 with scopes.

In this section you will see how to manage authentication and authorization with the same OAuth2 with scopes in your FastAPI application.

Warning

This is a more or less advanced section. If you are just starting, you can skip it.

You don't necessarily need OAuth2 scopes, you can handle authentication and authorization however you want.

But OAuth2 with scopes can be nicely integrated into your API (with OpenAPI) and your API docs.

Nevertheless, you still enforce those scopes or any other security/authorization requirement however you need in your code.

In many cases, OAuth2 with scopes can be an overkill.

But if you know you need it, or you are curious, keep reading.

OAuth2 scopes and OpenAPI

The OAuth2 specification defines "scopes" as a list of strings separated by spaces.

The content of each of these strings can have any format, but should not contain spaces.

These scopes represent "permissions".

In OpenAPI (e.g. the API docs), you can define "security schemes", the same as you saw in the previous sections.

When one of these security schemes uses OAuth2, you can also declare and use scopes.

Global view

First, let's quickly see the parts that change from the previous section about OAuth2 and JWT. Now using OAuth2 scopes:

from datetime import datetime, timedelta
from typing import List

import jwt
from fastapi import Depends, FastAPI, HTTPException, Security
from fastapi.security import (
    OAuth2PasswordBearer,
    OAuth2PasswordRequestForm,
    SecurityScopes,
)
from jwt import PyJWTError
from passlib.context import CryptContext
from pydantic import BaseModel, ValidationError
from starlette.status import HTTP_403_FORBIDDEN

# to get a string like this run:
# openssl rand -hex 32
SECRET_KEY = "09d25e094faa6ca2556c818166b7a9563b93f7099f6f0f4caa6cf63b88e8d3e7"
ALGORITHM = "HS256"
ACCESS_TOKEN_EXPIRE_MINUTES = 30


fake_users_db = {
    "johndoe": {
        "username": "johndoe",
        "full_name": "John Doe",
        "email": "johndoe@example.com",
        "hashed_password": "$2b$12$EixZaYVK1fsbw1ZfbX3OXePaWxn96p36WQoeG6Lruj3vjPGga31lW",
        "disabled": False,
    },
    "alice": {
        "username": "alice",
        "full_name": "Alice Chains",
        "email": "alicechains@example.com",
        "hashed_password": "$2b$12$gSvqqUPvlXP2tfVFaWK1Be7DlH.PKZbv5H8KnzzVgXXbVxpva.pFm",
        "disabled": True,
    },
}


class Token(BaseModel):
    access_token: str
    token_type: str


class TokenData(BaseModel):
    username: str = None
    scopes: List[str] = []


class User(BaseModel):
    username: str
    email: str = None
    full_name: str = None
    disabled: bool = None


class UserInDB(User):
    hashed_password: str


pwd_context = CryptContext(schemes=["bcrypt"], deprecated="auto")

oauth2_scheme = OAuth2PasswordBearer(
    tokenUrl="/token",
    scopes={"me": "Read information about the current user.", "items": "Read items."},
)

app = FastAPI()


def verify_password(plain_password, hashed_password):
    return pwd_context.verify(plain_password, hashed_password)


def get_password_hash(password):
    return pwd_context.hash(password)


def get_user(db, username: str):
    if username in db:
        user_dict = db[username]
        return UserInDB(**user_dict)


def authenticate_user(fake_db, username: str, password: str):
    user = get_user(fake_db, username)
    if not user:
        return False
    if not verify_password(password, user.hashed_password):
        return False
    return user


def create_access_token(*, data: dict, expires_delta: timedelta = None):
    to_encode = data.copy()
    if expires_delta:
        expire = datetime.utcnow() + expires_delta
    else:
        expire = datetime.utcnow() + timedelta(minutes=15)
    to_encode.update({"exp": expire})
    encoded_jwt = jwt.encode(to_encode, SECRET_KEY, algorithm=ALGORITHM)
    return encoded_jwt


async def get_current_user(
    security_scopes: SecurityScopes, token: str = Depends(oauth2_scheme)
):
    credentials_exception = HTTPException(
        status_code=HTTP_403_FORBIDDEN, detail="Could not validate credentials"
    )
    try:
        payload = jwt.decode(token, SECRET_KEY, algorithms=[ALGORITHM])
        username: str = payload.get("sub")
        if username is None:
            raise credentials_exception
        token_scopes = payload.get("scopes", [])
        token_data = TokenData(scopes=token_scopes, username=username)
    except (PyJWTError, ValidationError):
        raise credentials_exception
    user = get_user(fake_users_db, username=token_data.username)
    if user is None:
        raise credentials_exception
    for scope in security_scopes.scopes:
        if scope not in token_data.scopes:
            raise HTTPException(
                status_code=HTTP_403_FORBIDDEN, detail="Not enough permissions"
            )
    return user


async def get_current_active_user(
    current_user: User = Security(get_current_user, scopes=["me"])
):
    if current_user.disabled:
        raise HTTPException(status_code=400, detail="Inactive user")
    return current_user


@app.post("/token", response_model=Token)
async def login_for_access_token(form_data: OAuth2PasswordRequestForm = Depends()):
    user = authenticate_user(fake_users_db, form_data.username, form_data.password)
    if not user:
        raise HTTPException(status_code=400, detail="Incorrect username or password")
    access_token_expires = timedelta(minutes=ACCESS_TOKEN_EXPIRE_MINUTES)
    access_token = create_access_token(
        data={"sub": user.username, "scopes": form_data.scopes},
        expires_delta=access_token_expires,
    )
    return {"access_token": access_token, "token_type": "bearer"}


@app.get("/users/me/", response_model=User)
async def read_users_me(current_user: User = Depends(get_current_active_user)):
    return current_user


@app.get("/users/me/items/")
async def read_own_items(
    current_user: User = Security(get_current_active_user, scopes=["items"])
):
    return [{"item_id": "Foo", "owner": current_user.username}]

Now let's review those changes step by step.

OAuth2 Security scheme

The first change is that now we are declaring the OAuth2 security scheme with two available scopes, me and items.

The scopes parameter receives a dict with each scope as a key and the description as the value:

from datetime import datetime, timedelta
from typing import List

import jwt
from fastapi import Depends, FastAPI, HTTPException, Security
from fastapi.security import (
    OAuth2PasswordBearer,
    OAuth2PasswordRequestForm,
    SecurityScopes,
)
from jwt import PyJWTError
from passlib.context import CryptContext
from pydantic import BaseModel, ValidationError
from starlette.status import HTTP_403_FORBIDDEN

# to get a string like this run:
# openssl rand -hex 32
SECRET_KEY = "09d25e094faa6ca2556c818166b7a9563b93f7099f6f0f4caa6cf63b88e8d3e7"
ALGORITHM = "HS256"
ACCESS_TOKEN_EXPIRE_MINUTES = 30


fake_users_db = {
    "johndoe": {
        "username": "johndoe",
        "full_name": "John Doe",
        "email": "johndoe@example.com",
        "hashed_password": "$2b$12$EixZaYVK1fsbw1ZfbX3OXePaWxn96p36WQoeG6Lruj3vjPGga31lW",
        "disabled": False,
    },
    "alice": {
        "username": "alice",
        "full_name": "Alice Chains",
        "email": "alicechains@example.com",
        "hashed_password": "$2b$12$gSvqqUPvlXP2tfVFaWK1Be7DlH.PKZbv5H8KnzzVgXXbVxpva.pFm",
        "disabled": True,
    },
}


class Token(BaseModel):
    access_token: str
    token_type: str


class TokenData(BaseModel):
    username: str = None
    scopes: List[str] = []


class User(BaseModel):
    username: str
    email: str = None
    full_name: str = None
    disabled: bool = None


class UserInDB(User):
    hashed_password: str


pwd_context = CryptContext(schemes=["bcrypt"], deprecated="auto")

oauth2_scheme = OAuth2PasswordBearer(
    tokenUrl="/token",
    scopes={"me": "Read information about the current user.", "items": "Read items."},
)

app = FastAPI()


def verify_password(plain_password, hashed_password):
    return pwd_context.verify(plain_password, hashed_password)


def get_password_hash(password):
    return pwd_context.hash(password)


def get_user(db, username: str):
    if username in db:
        user_dict = db[username]
        return UserInDB(**user_dict)


def authenticate_user(fake_db, username: str, password: str):
    user = get_user(fake_db, username)
    if not user:
        return False
    if not verify_password(password, user.hashed_password):
        return False
    return user


def create_access_token(*, data: dict, expires_delta: timedelta = None):
    to_encode = data.copy()
    if expires_delta:
        expire = datetime.utcnow() + expires_delta
    else:
        expire = datetime.utcnow() + timedelta(minutes=15)
    to_encode.update({"exp": expire})
    encoded_jwt = jwt.encode(to_encode, SECRET_KEY, algorithm=ALGORITHM)
    return encoded_jwt


async def get_current_user(
    security_scopes: SecurityScopes, token: str = Depends(oauth2_scheme)
):
    credentials_exception = HTTPException(
        status_code=HTTP_403_FORBIDDEN, detail="Could not validate credentials"
    )
    try:
        payload = jwt.decode(token, SECRET_KEY, algorithms=[ALGORITHM])
        username: str = payload.get("sub")
        if username is None:
            raise credentials_exception
        token_scopes = payload.get("scopes", [])
        token_data = TokenData(scopes=token_scopes, username=username)
    except (PyJWTError, ValidationError):
        raise credentials_exception
    user = get_user(fake_users_db, username=token_data.username)
    if user is None:
        raise credentials_exception
    for scope in security_scopes.scopes:
        if scope not in token_data.scopes:
            raise HTTPException(
                status_code=HTTP_403_FORBIDDEN, detail="Not enough permissions"
            )
    return user


async def get_current_active_user(
    current_user: User = Security(get_current_user, scopes=["me"])
):
    if current_user.disabled:
        raise HTTPException(status_code=400, detail="Inactive user")
    return current_user


@app.post("/token", response_model=Token)
async def login_for_access_token(form_data: OAuth2PasswordRequestForm = Depends()):
    user = authenticate_user(fake_users_db, form_data.username, form_data.password)
    if not user:
        raise HTTPException(status_code=400, detail="Incorrect username or password")
    access_token_expires = timedelta(minutes=ACCESS_TOKEN_EXPIRE_MINUTES)
    access_token = create_access_token(
        data={"sub": user.username, "scopes": form_data.scopes},
        expires_delta=access_token_expires,
    )
    return {"access_token": access_token, "token_type": "bearer"}


@app.get("/users/me/", response_model=User)
async def read_users_me(current_user: User = Depends(get_current_active_user)):
    return current_user


@app.get("/users/me/items/")
async def read_own_items(
    current_user: User = Security(get_current_active_user, scopes=["items"])
):
    return [{"item_id": "Foo", "owner": current_user.username}]

Because we are now declaring those scopes,they will show up in the API docs when you log-in/authorize.

And you will be able to select which scopes you want to give access to: me and items.

This is the same mechanism used when you give permissions while logging in with Facebook, Google, GitHub, etc:

JWT token with scopes

Now, modify the token path operation to return the scopes requested.

We are still using the same OAuth2PasswordRequestForm. It includes a property scopes with each scope it received.

And we return the scopes as part of the JWT token.

Danger

For simplicity, here we are just adding the scopes received directly to the token.

But in your application, for security, you should make sure you only add the scopes that the user is actually able to have, or the ones you have predefined.

from datetime import datetime, timedelta
from typing import List

import jwt
from fastapi import Depends, FastAPI, HTTPException, Security
from fastapi.security import (
    OAuth2PasswordBearer,
    OAuth2PasswordRequestForm,
    SecurityScopes,
)
from jwt import PyJWTError
from passlib.context import CryptContext
from pydantic import BaseModel, ValidationError
from starlette.status import HTTP_403_FORBIDDEN

# to get a string like this run:
# openssl rand -hex 32
SECRET_KEY = "09d25e094faa6ca2556c818166b7a9563b93f7099f6f0f4caa6cf63b88e8d3e7"
ALGORITHM = "HS256"
ACCESS_TOKEN_EXPIRE_MINUTES = 30


fake_users_db = {
    "johndoe": {
        "username": "johndoe",
        "full_name": "John Doe",
        "email": "johndoe@example.com",
        "hashed_password": "$2b$12$EixZaYVK1fsbw1ZfbX3OXePaWxn96p36WQoeG6Lruj3vjPGga31lW",
        "disabled": False,
    },
    "alice": {
        "username": "alice",
        "full_name": "Alice Chains",
        "email": "alicechains@example.com",
        "hashed_password": "$2b$12$gSvqqUPvlXP2tfVFaWK1Be7DlH.PKZbv5H8KnzzVgXXbVxpva.pFm",
        "disabled": True,
    },
}


class Token(BaseModel):
    access_token: str
    token_type: str


class TokenData(BaseModel):
    username: str = None
    scopes: List[str] = []


class User(BaseModel):
    username: str
    email: str = None
    full_name: str = None
    disabled: bool = None


class UserInDB(User):
    hashed_password: str


pwd_context = CryptContext(schemes=["bcrypt"], deprecated="auto")

oauth2_scheme = OAuth2PasswordBearer(
    tokenUrl="/token",
    scopes={"me": "Read information about the current user.", "items": "Read items."},
)

app = FastAPI()


def verify_password(plain_password, hashed_password):
    return pwd_context.verify(plain_password, hashed_password)


def get_password_hash(password):
    return pwd_context.hash(password)


def get_user(db, username: str):
    if username in db:
        user_dict = db[username]
        return UserInDB(**user_dict)


def authenticate_user(fake_db, username: str, password: str):
    user = get_user(fake_db, username)
    if not user:
        return False
    if not verify_password(password, user.hashed_password):
        return False
    return user


def create_access_token(*, data: dict, expires_delta: timedelta = None):
    to_encode = data.copy()
    if expires_delta:
        expire = datetime.utcnow() + expires_delta
    else:
        expire = datetime.utcnow() + timedelta(minutes=15)
    to_encode.update({"exp": expire})
    encoded_jwt = jwt.encode(to_encode, SECRET_KEY, algorithm=ALGORITHM)
    return encoded_jwt


async def get_current_user(
    security_scopes: SecurityScopes, token: str = Depends(oauth2_scheme)
):
    credentials_exception = HTTPException(
        status_code=HTTP_403_FORBIDDEN, detail="Could not validate credentials"
    )
    try:
        payload = jwt.decode(token, SECRET_KEY, algorithms=[ALGORITHM])
        username: str = payload.get("sub")
        if username is None:
            raise credentials_exception
        token_scopes = payload.get("scopes", [])
        token_data = TokenData(scopes=token_scopes, username=username)
    except (PyJWTError, ValidationError):
        raise credentials_exception
    user = get_user(fake_users_db, username=token_data.username)
    if user is None:
        raise credentials_exception
    for scope in security_scopes.scopes:
        if scope not in token_data.scopes:
            raise HTTPException(
                status_code=HTTP_403_FORBIDDEN, detail="Not enough permissions"
            )
    return user


async def get_current_active_user(
    current_user: User = Security(get_current_user, scopes=["me"])
):
    if current_user.disabled:
        raise HTTPException(status_code=400, detail="Inactive user")
    return current_user


@app.post("/token", response_model=Token)
async def login_for_access_token(form_data: OAuth2PasswordRequestForm = Depends()):
    user = authenticate_user(fake_users_db, form_data.username, form_data.password)
    if not user:
        raise HTTPException(status_code=400, detail="Incorrect username or password")
    access_token_expires = timedelta(minutes=ACCESS_TOKEN_EXPIRE_MINUTES)
    access_token = create_access_token(
        data={"sub": user.username, "scopes": form_data.scopes},
        expires_delta=access_token_expires,
    )
    return {"access_token": access_token, "token_type": "bearer"}


@app.get("/users/me/", response_model=User)
async def read_users_me(current_user: User = Depends(get_current_active_user)):
    return current_user


@app.get("/users/me/items/")
async def read_own_items(
    current_user: User = Security(get_current_active_user, scopes=["items"])
):
    return [{"item_id": "Foo", "owner": current_user.username}]

Declare scopes in path operations and dependencies

Now we declare that the path operation for /users/me/items/ requires the scope items.

For this, we import and use Security from fastapi.

You can use Security to declare dependencies (just like Depends), but Security also receives a parameter scopes with a list of scopes (strings).

In this case, we pass a dependency function get_current_active_user to Security (the same way we would do with Depends).

But we also pass a list of scopes, in this case with just one scope: items (it could have more).

And the dependency function get_current_active_user can also declare sub-dependencies, not only with Depends but also with Security. Declaring its own sub-dependency function (get_current_user), and more scope requirements.

In this case, it requires the scope me (it could require more than one scope).

Note

You don't necessarily need to add different scopes in different places.

We are doing it here to demonstrate how FastAPI handles scopes declared at different levels.

from datetime import datetime, timedelta
from typing import List

import jwt
from fastapi import Depends, FastAPI, HTTPException, Security
from fastapi.security import (
    OAuth2PasswordBearer,
    OAuth2PasswordRequestForm,
    SecurityScopes,
)
from jwt import PyJWTError
from passlib.context import CryptContext
from pydantic import BaseModel, ValidationError
from starlette.status import HTTP_403_FORBIDDEN

# to get a string like this run:
# openssl rand -hex 32
SECRET_KEY = "09d25e094faa6ca2556c818166b7a9563b93f7099f6f0f4caa6cf63b88e8d3e7"
ALGORITHM = "HS256"
ACCESS_TOKEN_EXPIRE_MINUTES = 30


fake_users_db = {
    "johndoe": {
        "username": "johndoe",
        "full_name": "John Doe",
        "email": "johndoe@example.com",
        "hashed_password": "$2b$12$EixZaYVK1fsbw1ZfbX3OXePaWxn96p36WQoeG6Lruj3vjPGga31lW",
        "disabled": False,
    },
    "alice": {
        "username": "alice",
        "full_name": "Alice Chains",
        "email": "alicechains@example.com",
        "hashed_password": "$2b$12$gSvqqUPvlXP2tfVFaWK1Be7DlH.PKZbv5H8KnzzVgXXbVxpva.pFm",
        "disabled": True,
    },
}


class Token(BaseModel):
    access_token: str
    token_type: str


class TokenData(BaseModel):
    username: str = None
    scopes: List[str] = []


class User(BaseModel):
    username: str
    email: str = None
    full_name: str = None
    disabled: bool = None


class UserInDB(User):
    hashed_password: str


pwd_context = CryptContext(schemes=["bcrypt"], deprecated="auto")

oauth2_scheme = OAuth2PasswordBearer(
    tokenUrl="/token",
    scopes={"me": "Read information about the current user.", "items": "Read items."},
)

app = FastAPI()


def verify_password(plain_password, hashed_password):
    return pwd_context.verify(plain_password, hashed_password)


def get_password_hash(password):
    return pwd_context.hash(password)


def get_user(db, username: str):
    if username in db:
        user_dict = db[username]
        return UserInDB(**user_dict)


def authenticate_user(fake_db, username: str, password: str):
    user = get_user(fake_db, username)
    if not user:
        return False
    if not verify_password(password, user.hashed_password):
        return False
    return user


def create_access_token(*, data: dict, expires_delta: timedelta = None):
    to_encode = data.copy()
    if expires_delta:
        expire = datetime.utcnow() + expires_delta
    else:
        expire = datetime.utcnow() + timedelta(minutes=15)
    to_encode.update({"exp": expire})
    encoded_jwt = jwt.encode(to_encode, SECRET_KEY, algorithm=ALGORITHM)
    return encoded_jwt


async def get_current_user(
    security_scopes: SecurityScopes, token: str = Depends(oauth2_scheme)
):
    credentials_exception = HTTPException(
        status_code=HTTP_403_FORBIDDEN, detail="Could not validate credentials"
    )
    try:
        payload = jwt.decode(token, SECRET_KEY, algorithms=[ALGORITHM])
        username: str = payload.get("sub")
        if username is None:
            raise credentials_exception
        token_scopes = payload.get("scopes", [])
        token_data = TokenData(scopes=token_scopes, username=username)
    except (PyJWTError, ValidationError):
        raise credentials_exception
    user = get_user(fake_users_db, username=token_data.username)
    if user is None:
        raise credentials_exception
    for scope in security_scopes.scopes:
        if scope not in token_data.scopes:
            raise HTTPException(
                status_code=HTTP_403_FORBIDDEN, detail="Not enough permissions"
            )
    return user


async def get_current_active_user(
    current_user: User = Security(get_current_user, scopes=["me"])
):
    if current_user.disabled:
        raise HTTPException(status_code=400, detail="Inactive user")
    return current_user


@app.post("/token", response_model=Token)
async def login_for_access_token(form_data: OAuth2PasswordRequestForm = Depends()):
    user = authenticate_user(fake_users_db, form_data.username, form_data.password)
    if not user:
        raise HTTPException(status_code=400, detail="Incorrect username or password")
    access_token_expires = timedelta(minutes=ACCESS_TOKEN_EXPIRE_MINUTES)
    access_token = create_access_token(
        data={"sub": user.username, "scopes": form_data.scopes},
        expires_delta=access_token_expires,
    )
    return {"access_token": access_token, "token_type": "bearer"}


@app.get("/users/me/", response_model=User)
async def read_users_me(current_user: User = Depends(get_current_active_user)):
    return current_user


@app.get("/users/me/items/")
async def read_own_items(
    current_user: User = Security(get_current_active_user, scopes=["items"])
):
    return [{"item_id": "Foo", "owner": current_user.username}]

Use SecurityScopes

Now update the dependency get_current_user.

This is the one used by the dependencies above.

Here's were we are declaring the same OAuth2 scheme we created above as a dependency: oauth2_scheme.

Because this dependency function doesn't have any scope requirements itself, we can use Depends with oauth2_scheme, we don't have to use Security.

We also declare a special parameter of type SecurityScopes, imported from fastapi.security.

This SecurityScopes class is similar to Request (Request was used to get the request object directly).

The parameter security_scopes will be of type SecurityScopes. It will have a property scopes with a list containing all the scopes required by itself and all the dependencies that use this as a sub-dependency. That means, all the "dependants" or all the super-dependencies (the contrary of sub-dependencies).

We verify that all the scopes required, by this dependency and all the dependants (including path operations), are included in the scopes provided in the token received, otherwise raise an HTTPException.

We also check that the token data is validated with the Pydantic model (catching the ValidationError exception), and if we get an error reading the JWT token or validating the data with Pydantic, we also raise an HTTPException.

By validating the data with Pydantic we can make sure that we have, for example, exactly a list of str with the scopes and a str with the username. Instead of, for example, a dict, or something else, as it could break the application at some point later.

from datetime import datetime, timedelta
from typing import List

import jwt
from fastapi import Depends, FastAPI, HTTPException, Security
from fastapi.security import (
    OAuth2PasswordBearer,
    OAuth2PasswordRequestForm,
    SecurityScopes,
)
from jwt import PyJWTError
from passlib.context import CryptContext
from pydantic import BaseModel, ValidationError
from starlette.status import HTTP_403_FORBIDDEN

# to get a string like this run:
# openssl rand -hex 32
SECRET_KEY = "09d25e094faa6ca2556c818166b7a9563b93f7099f6f0f4caa6cf63b88e8d3e7"
ALGORITHM = "HS256"
ACCESS_TOKEN_EXPIRE_MINUTES = 30


fake_users_db = {
    "johndoe": {
        "username": "johndoe",
        "full_name": "John Doe",
        "email": "johndoe@example.com",
        "hashed_password": "$2b$12$EixZaYVK1fsbw1ZfbX3OXePaWxn96p36WQoeG6Lruj3vjPGga31lW",
        "disabled": False,
    },
    "alice": {
        "username": "alice",
        "full_name": "Alice Chains",
        "email": "alicechains@example.com",
        "hashed_password": "$2b$12$gSvqqUPvlXP2tfVFaWK1Be7DlH.PKZbv5H8KnzzVgXXbVxpva.pFm",
        "disabled": True,
    },
}


class Token(BaseModel):
    access_token: str
    token_type: str


class TokenData(BaseModel):
    username: str = None
    scopes: List[str] = []


class User(BaseModel):
    username: str
    email: str = None
    full_name: str = None
    disabled: bool = None


class UserInDB(User):
    hashed_password: str


pwd_context = CryptContext(schemes=["bcrypt"], deprecated="auto")

oauth2_scheme = OAuth2PasswordBearer(
    tokenUrl="/token",
    scopes={"me": "Read information about the current user.", "items": "Read items."},
)

app = FastAPI()


def verify_password(plain_password, hashed_password):
    return pwd_context.verify(plain_password, hashed_password)


def get_password_hash(password):
    return pwd_context.hash(password)


def get_user(db, username: str):
    if username in db:
        user_dict = db[username]
        return UserInDB(**user_dict)


def authenticate_user(fake_db, username: str, password: str):
    user = get_user(fake_db, username)
    if not user:
        return False
    if not verify_password(password, user.hashed_password):
        return False
    return user


def create_access_token(*, data: dict, expires_delta: timedelta = None):
    to_encode = data.copy()
    if expires_delta:
        expire = datetime.utcnow() + expires_delta
    else:
        expire = datetime.utcnow() + timedelta(minutes=15)
    to_encode.update({"exp": expire})
    encoded_jwt = jwt.encode(to_encode, SECRET_KEY, algorithm=ALGORITHM)
    return encoded_jwt


async def get_current_user(
    security_scopes: SecurityScopes, token: str = Depends(oauth2_scheme)
):
    credentials_exception = HTTPException(
        status_code=HTTP_403_FORBIDDEN, detail="Could not validate credentials"
    )
    try:
        payload = jwt.decode(token, SECRET_KEY, algorithms=[ALGORITHM])
        username: str = payload.get("sub")
        if username is None:
            raise credentials_exception
        token_scopes = payload.get("scopes", [])
        token_data = TokenData(scopes=token_scopes, username=username)
    except (PyJWTError, ValidationError):
        raise credentials_exception
    user = get_user(fake_users_db, username=token_data.username)
    if user is None:
        raise credentials_exception
    for scope in security_scopes.scopes:
        if scope not in token_data.scopes:
            raise HTTPException(
                status_code=HTTP_403_FORBIDDEN, detail="Not enough permissions"
            )
    return user


async def get_current_active_user(
    current_user: User = Security(get_current_user, scopes=["me"])
):
    if current_user.disabled:
        raise HTTPException(status_code=400, detail="Inactive user")
    return current_user


@app.post("/token", response_model=Token)
async def login_for_access_token(form_data: OAuth2PasswordRequestForm = Depends()):
    user = authenticate_user(fake_users_db, form_data.username, form_data.password)
    if not user:
        raise HTTPException(status_code=400, detail="Incorrect username or password")
    access_token_expires = timedelta(minutes=ACCESS_TOKEN_EXPIRE_MINUTES)
    access_token = create_access_token(
        data={"sub": user.username, "scopes": form_data.scopes},
        expires_delta=access_token_expires,
    )
    return {"access_token": access_token, "token_type": "bearer"}


@app.get("/users/me/", response_model=User)
async def read_users_me(current_user: User = Depends(get_current_active_user)):
    return current_user


@app.get("/users/me/items/")
async def read_own_items(
    current_user: User = Security(get_current_active_user, scopes=["items"])
):
    return [{"item_id": "Foo", "owner": current_user.username}]

So, as the other dependency get_current_active_user has as a sub-dependency this get_current_user, the scope "me" declared at get_current_active_user will be included in the security_scopes.scopes list inside of get_current_user.

And as the path operation itself also declares a scope "items", it will also be part of this list security_scopes.scopes in get_current_user.

Here's how the hierarchy of dependencies and scopes looks like:

  • The path operation read_own_items has:
    • Required scopes ["items"] with the dependency:
    • get_current_active_user:
      • The dependency function get_current_active_user has:
        • Required scopes ["me"] with the dependency:
        • get_current_user:
          • The dependency function get_current_user has:
            • No scopes required by itself.
            • A dependency using oauth2_scheme.
            • A security_scopes parameter of type SecurityScopes:
              • This security_scopes parameter has a property scopes with a list containing all these scopes declared above, so:
                • security_scopes.scopes will contain ["me", "items"]

More details about SecurityScopes

You can use SecurityScopes at any point, and in multiple places, it doesn't have to be at the "root" dependency.

It will always have the security scopes declared in the current Security dependencies and all the super-dependencies/dependants.

Because the SecurityScopes will have all the scopes declared by super-dependencies/dependants, you can use it to verify that a token has the required scopes in a central dependency function, and then declare different scope requirements in different path operations.

Check it

If you open the API docs, you can authenticate and specify which scopes you want to authorize.

If you don't select any scope, you will be "authenticated", but when you try to access /users/me/ or /users/me/items/ you will get an error saying that you don't have enough permissions.

And if you select the scope me but not the scope items, you will be able to access /users/me/ but not /users/me/items/.

That's what would happen to a third party application that tried to access one of these path operations with a token provided by a user, depending on how many permissions the user gave the application.

About third party integrations

In this example we are using the OAuth2 "password" flow.

This is appropriate when we are logging in to our own application, probably with our own frontend.

Because we can trust it to receive the username and password, as we control it.

But if you are building an OAuth2 application that others would connect to (i.e., if you are building an authentication provider equivalent to Facebook, Google, GitHub, etc.) you should use one of the other flows.

The most common is the implicit flow.

The most secure is the code flow, but is more complex to implement as it requires more steps. As it is more cumbersome, many providers end up suggesting the implicit flow.

Note

It's common that each authentication provider names their flows in a different way, to make it part of their brand.

But in the end, they are implementing the same OAuth2 standard.

FastAPI includes utilities for all these OAuth2 authentication flows in fastapi.security.oauth2.