Row Level Permissions for FastAPI

While trying out the excellent FastApi framework there was one peace missing for me: an easy, declarative way to define permissions of users (and roles/groups) on resources. Since I reall love the way Pyramid handles this, I re-implemented and adapted the system for FastApi (well, you might call it a blatant rip-off).

An extremely simple and incomplete example:

from fastapi import Depends, FastAPI
from fastapi.security import OAuth2PasswordBearer
from fastapi_permissions import configure_permissions, Allow, Deny
from pydantic import BaseModel

app = FastAPI()
oauth2_scheme = OAuth2PasswordBearer(tokenUrl="/token")

class Item(BaseModel):
    name: str
    owner: str

    def __acl__(self):
        return [
            (Allow, Authenticated, "view"),
            (Allow, "role:admin", "edit"),
            (Allow, f"user:{self.owner}", "delete"),
        ]

class User(BaseModel):
    name: str

    def principals(self):
        return [f"user:{self.name}"]

def get_current_user(token: str = Depends(oauth2_scheme)):
    ...

def get_active_user_principals(user:User = Depends(get_current_user)):
    ...

def get_item(item_identifier):
    ...

# Permission is already wrapped in Depends()
Permission = configure_permissions(get_active_user_principals)

@app.get("/item/{item_identifier}")
async def show_item(item: Item=Permission("view", get_item)):
    return [{"item": item}]

For a better example install fastapi_permissions source in an virtual environment (see further below), and start a test server:

(permissions) $ uvicorn fastapi_permissions.example:app --reload

Visit http://127.0.0.1:8000/docs to try it out. There are two users available: "bob" and "alice", both have the password "secret".

The example is derived from the FastApi examples, so it should be familiar. New / added stuff is marked with comments in the source file fastapi_permissions/example.py

Why not use Scopes?

For most applications the use of scopes to determine the rights of a user is sufficient enough. So if scopes fit your application, please use them - they are already a part of the FastAPI framework.

While scopes are tied only to the state of the user, fastapi_permissions also take the state of the requested resource into account.

Let's take an scientific paper as an example: depending on the state of the submission process (like "draft", "submitted", "peer review" or "published") different users should have different permissions on viewing, editing or retracting. This could be acomplished with custom code in the path definition functions, but fastapi_permissions offers a method to define these constraints in a single place.

There is a second case, where fastapi_permissions might be the right addition to your app: If your brain is wired / preconditioned like mine to such a permission model - e.g. exposed for a long time to Pyramid...

Long Story Short: Use scopes until you need something different.

Concepts

Since fastapi_permissions heavely derived from the Pyramid framework, I strongly suggest to take a look at its security documentation if anything is unclear to you.

The system depends on a couple of concepts not found in FastAPI:

• resources: objects that provide an access controll list
• access controll lists: a list of rules defining which principal has what permission
• principal: an identifier of a user or his/her associated groups/roles
• permission: an identifier (string) for an action on an object

resources & access controll lists

A resource provides an access controll list via it's __acl__ attribute. It can either be an property of an object or a callable. Each entry in the list is a tuple containing three values:

1. an action: fastapi_permissions.Allow or fastapi_permissions.Deny
2. a principal: e.g. "role:admin" or "user:bob"
3. a permission or a tuple thereof: e.g. "edit" or ("view", "delete")

Examples:

from fastapi_permissions import Allow, Deny, Authenticated, Everyone

class StaticAclResource:
    __acl__ =  [
        (Allow, Everyone, "view"),
        (Allow, "role:user", "share")
    ]

class DynamicAclResource:
    def __acl__(self):
        return [
        (Allow, Authenticated, "view"),
        (Allow, "role:user", "share"),
        (Allow, f"user:{self.owner}", "edit"),
    ]

# in contrast to pyramid, resources might be access conroll list themselves
# this can save some typing:

AclResourceAsList = [(Allow, Everyone, "view"), (Deny, "role:troll", "edit")]

You don't need to add any "deny-all-clause" at the end of the access controll list, this is automagically implied. All entries in a ACL are checked in the order provided in the list. This makes some complex configurations simple, but can sometimes be a pain in the lower back…

The two principals Everyone and Authenticated will be discussed in short time.

users & principal identifiers

You must provide a function that returns the principals of the current active user. The principals is just a list of strings, identifying the user and groups/roles the user belongs to:

Example:

def get_active_principals(user: User = Depends(get_current_user)):
    if user:
        # user is logged in
        principals = [Everyone, Authenticated]
        principals.extend(getattr(user, "principals", []))
    else:
        # user is not logged in
        principals = [Everyone]
    return principals

special principals

There are two special principals that also help providing access controll lists: Everyone and Authenticated.

The Everyone principal should be added regardless of any other defined principals or login status, Authenticated should only be added for a user that is logged in.

permissions

A permission is just a string that represents an action to be performed on a resource. Just make something up.

As with the special principals, there is a special permission that is usable as a wildcard: fastapi_permisssions.All.

Usage

There are some things you must provide before using the permissions system:

• a callable (FastApi dependency) that returns the principal of the logged in (active) user
• a resource with an access controll list

Configuring the permissions system

Simple configuration with some defaults:

from fastapi_permissions import configure_permissions

# must be provided
def get_active_principals(...):
    """ returns the principals of the current logged in user"""
    ...

# Permission is already wrapped in Depends()
Permission = configure_permissions(get_active_principals)

One configuration option is available:

• permission_exception:
  • this exception will be raised if a permission is denied
  • defaults to fastapi_permissions.permission_exception

from fastapi_permissions import configure_permissions

# must be provided
def get_active_principals(...):
    """ returns the principals of the current logged in user"""
    ...

# Permission is already wrapped in Depends()
Permission = configure_permissions(
    get_active_principals,
    permission_exception

)

using permissions in path operation

To use access controll in a path operation, you call the perviously configured function with a permission and the resource. If the permission is granted, the requested resource the permission is checked on will be returned, or in this case, the acl list

from fastapi_permissions import configure_permissions, Allow

# must be provided
def get_active_principals(...):
    """ returns the principals of the current logged in user"""
    ...

example_acl = [(Allow, "role:user", "view")]

# Permission is already wrapped in Depends()
Permission = configure_permissions(get_active_principals)

@app.get("/")
async def root(acls:list=Permission("view", example_acl)):
    return {"OK"}

Instead of using an access controll list directly, you can also provide a dependency function that might fetch a resource from a database, the resouce should provide its access controll list via the __acl__ attribute:

from fastapi_permissions import configure_permissions, Allow

# must be provided
def get_active_principals(...):
    """ returns the principals of the current logged in user"""
    ...

# fetches a resource from the database
def get_item(item_id: int):
    """ returns a resource from the database

    The resource provides an access controll list via its "__acl__" attribute.
    """
    ...

# Permission is alredy wrapped in Depends()
Permission = configure_permissions(get_active_principals)

@app.get("/item/{item_id}")
async def show_item(item:Item=Permission("view", get_item)):
    return {"item": item}

helper functions

Sometimes you might want to check permissions inside a function and not as the definition of a path operation:

With has_permission(user_principals, permission, resource) you can preform the permission check programatically. The function signature can easily be remebered with something like "John eat apple?". The result will be either True or False, so no need for try/except blocks \o/.

from fastapi_permissions import (
    has_permission, Allow, All, Everyone, Authenticated
)

user_principals == [Everyone, Authenticated, "role:owner", "user:bob"]
apple_acl == [(Allow, "role:owner", All)]

if has_permission(user_principals, "eat", apple_acl):
    print "Yum!"

The other function provided is list_permissions(user_principals, resource) this will return a dict of all available permissions and a boolean value if the permission is granted or denied:

from fastapi_permissions import list_permissions, Allow, All

user_principals == [Everyone, Authenticated, "role:owner", "user:bob"]
apple_acl == [(Allow, "role:owner", All)]

print(list_permissions(user_principals, apple_acl))
{"permissions:*": True}

Please note, that "permissions:*" is the string representation of fastapi_permissions.All.

How it works

The main work is done in the has_permissions() function, but the most interesting ones (at least for me) are the configure_permissions() and permission_dependency_factory() functions.

Wait. I didn't tell you about the latter one?

The permission() thingy used in the path operation definition before is actually the mentioned permission_dependency_factory(). The configure_permissions() function just provisiones it with some default values using functools.partial. This reduces the function signature from permission_dependency_factory(permission, resource, active_principals_func, permission_exception) down to partial_function(permission, resource).

The permission_dependency_factory returns another function with the signature permission_dependency(Depends(resource), Depends(active_principals_func)). This is the acutal signature, that Depends() uses in the path operation definition to search and inject the dependencies. The rest is just some closure magic ;-).

Or in other words: to have a nice API, the Depends() in the path operation function should only have a function signature for retrieving the active user and the resource. On the other side, when writing the code, I wanted to only specifiy the parts relevant to the path operation function: the resource and the permission. The rest is just on how to make it work.

(F.)A.Q.

Permission check on collection of resources

How to use the library with something like this: List[Item]=Permission("edit", get_items). The question was actually issue #3 and I have written a longer answer in the issue, please have a look there.

Dev & Test virtual environment

There is an easy to use make command for setting up a virtual environment, installing the required packages and installing the project in an editable way.

$ git clone https://github.com/holgi/fastapi-permissions.git
$ cd fastapi-permissions
$ make devenv
$ source .venv/bin/activate

Then you can test any changes locally with make test. This will stop on the first error and not report coverage.

(permissions) $ make test

If you can also run all tests and get a coverage report with

(permissions) $ make coverage

And when ready to test everything as an installed package (bonus point if using make clean before)

(permissions) $ make tox

Thanks

• Sebastián Ramírez, for creating FastAPI
• William, for fixing my stupid bug