fastapi-has-permissions

Introduction

Declarative permissions system for FastAPI. Define permission checks as classes or functions, compose them with &, |, ~ operators, and plug them into FastAPI's dependency injection.

Installation

pip install fastapi-has-permissions

Usage

Setup

Call add_permissions() on your app — it binds an inject scope to every request, which is what lets permissions and Evaluate resolve their dependencies at check time:

from fastapi import FastAPI

from fastapi_has_permissions import add_permissions

app = FastAPI()
add_permissions(app)

add_permissions() is idempotent, but it has to be called before the routes are declared — FastAPI copies the router dependencies into every route at registration time.

Class-Based Permissions

Subclass Permission and implement check_permissions():

from fastapi import Depends, FastAPI, Request

from fastapi_has_permissions import Permission, add_permissions


class HasAuthorizationHeader(Permission):
    async def check_permissions(self, request: Request) -> bool:
        return "Authorization" in request.headers


app = FastAPI()
add_permissions(app)


@app.get(
    "/protected",
    dependencies=[Depends(HasAuthorizationHeader())],
)
async def protected():
    return {"message": "You have access!"}

Permissions with parameters are automatically dataclasses:

class HasRole(Permission):
    role: str

    async def check_permissions(self, request: Request) -> bool:
        return request.headers.get("role") == self.role

Boolean Composition

Combine permissions with & (AND), | (OR), and ~ (NOT):

# All must pass
Depends(HasAuthorizationHeader() & HasRole("admin"))

# Any must pass
Depends(HasAuthorizationHeader() | HasRole("admin"))

# Negated
Depends(~HasAuthorizationHeader())

All three operators are lazy: each branch's dependencies are resolved at request time, one branch at a time, and evaluation short-circuits as soon as the outcome is decided — a failing or expensive dependency in a losing branch is never resolved. Permissions still document themselves: add_permissions() teaches app.openapi() what each route's permission tree requires, so headers, query params and OAuth2 scopes appear in the schema without ever being resolved to produce it.

When a composed check fails, the failing permission's message, status code, error code, and headers are propagated: & reports the first failing permission, and | combines the failure reasons of all branches when none of them passed.

Skipped permissions (see skip()) are ignored by & and | — the remaining permissions decide the outcome. A composite where every permission skipped is itself skipped, and ~ passes a skip through unchanged:

A skip is an abstention, not an approval: if it reaches the root of the tree — the permission you passed to Depends() or to evaluator.require() — the request is denied. Wrap the permission into AllowSkipped to state explicitly that an abstention should grant access:

from fastapi_has_permissions import AllowSkipped

# skipped -> 403 Forbidden
Depends(IsArticleAuthor())

# skipped -> allowed
Depends(AllowSkipped(IsArticleAuthor()))

Note: every permission instance is a distinct FastAPI dependency (identity-based hashing), so two equal instances are resolved and checked independently within one request. Reuse the same instance when you want FastAPI's per-request dependency cache to apply.

Function-Based Permissions

Use the @permission decorator for a lightweight alternative:

from typing import Annotated

from fastapi import Header

from fastapi_has_permissions import permission


@permission
async def has_admin_role(role: Annotated[str, Header()]) -> bool:
    return role == "admin"


@app.get("/admin", dependencies=[Depends(has_admin_role())])
async def admin_endpoint():
    return {"message": "Admin access granted"}

Function-based permissions also support dependency arguments -- annotate them with Resolved[T], which declares the resolved value the check receives:

from fastapi_has_permissions import DepFactory, Resolved, permission


async def get_admin_role() -> str:
    return "admin"


AdminRoleDep = DepFactory[str, get_admin_role]  # == Annotated[str, Depends(get_admin_role)]


@permission
async def has_role(admin_role: Resolved[str], /, role: Annotated[str, Header()]) -> bool:
    return role == admin_role


@app.get("/admin", dependencies=[Depends(has_role(AdminRoleDep))])
async def admin_endpoint():
    return {"message": "Admin access granted"}

Function-based permissions support the same &, |, ~ composition.

Deferred Resolution

Every permission resolves its dependencies at check time, so a dependency is only paid for if its permission is actually reached. When a dependency may not resolve at all, wrap the permission to decide what that means:

from fastapi.exceptions import RequestValidationError

from fastapi_has_permissions import SkipOnExc

# Skip the check instead of failing if the "age" header is missing
Depends(SkipOnExc(AgeIsMoreThan(age=18), (RequestValidationError,)))

This works at any depth -- inside a composition, or nested in another wrapper.

Imperative Checks

Use the Evaluate dependency to check permissions inside a handler body without raising — for branching logic, partial responses, or explicit control:

from fastapi_has_permissions import Evaluate


@app.get("/posts")
async def list_posts(evaluate: Evaluate):
    if await evaluate.check(IsAdmin()):
        return all_posts()

    return public_posts()

evaluate(perm) returns the raw CheckResult, evaluate.check(perm) returns a bool, and evaluate.require(perm) raises the permission's HTTP error on failure.

Outside a request (e.g. in unit tests), the module-level evaluate() resolves dependencies against a fresh scope — combine it with fastapi_injected.push_overrides to stub them:

from fastapi_has_permissions import evaluate
from fastapi_injected import push_overrides

with push_overrides({get_role: "admin"}):
    assert await evaluate(HasAdminRole())

Error Model

• Permissions raise 403 by default; authentication-style permissions can set default_exc_status_code = 401 and default_exc_headers = {"WWW-Authenticate": "Bearer"} (the built-in IsAuthenticated does exactly this)
• Set a machine-readable error code (code=... / default_exc_code) to get a structured body: {"detail": {"code": "not_admin", "message": "Admin role required"}}; without a code the body stays a plain string
• All of message, status code, code, and headers propagate through &, |, ~
• A permission's own error config applies when it is the root of the tree; use WithError(...) to give a nested subtree one error (e.g. a 404 that does not admit the resource exists)

Policies

A Policy groups the rules for one resource, and the request method picks which one runs:

from fastapi_has_permissions import DepFactory, Policy, Requires
from fastapi_has_permissions.common import Allow, HasRole

PostDep = DepFactory[Post, get_post]


class PostPolicy(Policy[Post]):
    read = Allow()
    create = HasRole(RoleDep, "author")
    update = HasRole(RoleDep, "author")
    delete = HasRole(RoleDep, "admin")

    __resource__ = PostDep

GET/QUERY/HEAD map to read, POST to create, PUT/PATCH to update, DELETE to delete, and anything else to default. All five default to Deny(), so a policy opens only what it declares.

Attach it once to a router and every route under it is covered by the action matching its verb -- which is what the method mapping is for:

posts = APIRouter(prefix="/posts", dependencies=[Depends(PostPolicy())])


@posts.get("")  # -> read
async def list_posts() -> list[Post]: ...


@posts.post("")  # -> create
async def create_post(body: PostIn) -> Post: ...


@posts.delete("/{post_id}")  # -> delete
async def delete_post(post_id: int) -> None: ...

A route added later is covered the moment it exists, and a verb with no action falls to default, which denies -- so you cannot leave a route on this router unprotected by forgetting something. A router-level policy never resolves __resource__, so the collection routes are never asked for a post_id they do not have.

or with Requires, which checks and injects the loaded resource:

@app.get("/posts/{post_id}")
async def read(post: Annotated[Post, Requires(PostPolicy())]) -> Post:
    return post

The check runs before the resource is resolved and before the handler body, so it cannot be skipped. Actions are ordinary permissions, so they compose with &/|/~, take the wrappers, and propagate their own error config.

Actions are not limited to CRUD -- declare one and name it at the call site:

class PostPolicy(Policy[Post]):
    publish = HasRole(RoleDep, "editor")

    __resource__ = PostDep


@app.post("/posts/{post_id}/publish")
async def publish(post: Annotated[Post, Requires(PostPolicy(), PostPolicy.publish)]) -> Post:
    return post

PostPolicy.bind(OtherDep) reuses the same rules against a different loader.

Other Features

• Custom error responses -- set default_exc_message / default_exc_status_code / default_exc_code / default_exc_headers class variables or the corresponding message / status_code / code / headers init parameters
• Skip / Fail helpers -- call skip() or fail() inside check_permissions() for explicit control flow
• Wrappers -- AllowSkipped / DenySkipped / Advisory rewrite a check's result, WithError gives a whole subtree one error config, FailOnExc / SkipOnExc keep a broken check from turning into a 500, Undocumented keeps a check out of the OpenAPI schema
• Built-in common permissions -- IsAuthenticated, HasScope, HasRole ready to use with your auth dependencies
• Policies -- Policy groups a resource's rules and dispatches on the request method; Requires checks and injects the loaded object for object-level permissions
• Full FastAPI DI support -- check_permissions() accepts any FastAPI-injectable parameters
• Built on fastapi-injected -- Dep / DepFactory are re-exported from it, and permissions resolve through its inject scope, so they share the request's dependency cache with the route