Add fine-grained authorization to a FastAPI app

Build a FastAPI app for a blogging platform that registers users in Permit.io and allows only users with the Author role to create posts. This tutorial is for Python backend developers who want to enforce Permit.io policies from FastAPI endpoints.

When you finish, your FastAPI app has two endpoints:

Endpoint	What it does
POST /register	Syncs a user to Permit.io and assigns the user the Reader role in the default tenant
POST /posts	Calls permit.check() and returns 403 unless the user has permission to create a Post

Prerequisites

• A Permit.io account. See Create a Permit.io account.
• Python 3 with FastAPI and the Uvicorn server installed.
• Node.js and npm, to install the Permit CLI.
• Docker, to run the policy decision point (PDP) container.

1. Configure the policy in Permit

Create the blogging platform policy with the Permit CLI. If your environment already has a policy with a Post resource and an Author role that can create posts, skip to 2. Get your API key.

1

Install the Permit CLI

The Permit CLI creates policies and runs the PDP from your terminal. Install the CLI with npm:

npm install -g @permitio/cli

Run permit to confirm that the CLI is installed.

2

Sign in with the Permit CLI

Authenticate the CLI with your Permit.io account:

permit login

The command opens a browser window where you sign in. After you sign in, the CLI uses your default environment. To use a different environment, run permit env select and choose the environment.

3

Apply the blogging platform template

Permit CLI templates create a policy with predefined resources, roles, and rules. To see the available templates, run permit env template list. The template source files are in the Permit CLI repository.

Apply the blogging-platform template to your environment:

permit env template apply --template blogging-platform

The CLI prints a success message when the template is applied.

4

Review the policy in the Policy Editor

In the Permit dashboard, select your project and open the Policy screen.

The blogging-platform template creates:

Policy element	What the template defines
Resources	Post (with a premium boolean attribute) and Comment, each with create, read, update, and delete actions
Roles	Admin (all actions), Author (create and read posts, read comments), Reader (create and read comments), and Premium Reader (read posts and comments)
Relationship	A Post is the parent of its Comment instances. An Author of a post instance becomes a Moderator of the comments on that post. This rule is relationship-based access control (ReBAC).
Resource set	Free Post contains posts where premium is false. Readers can read free posts. This rule is attribute-based access control (ABAC).

This tutorial uses one rule from the policy: the Author role can create a Post, and the Reader role cannot. To change which role can perform an action, check or clear the box in the Policy Editor.

2. Get your API key

Your FastAPI app and the PDP authenticate with Permit.io with your environment API key. Copy the API key of the environment where you applied the template. See Get your API key.

Keep the API key out of your code

Anyone with the environment API key can change that environment's policy through the Permit API. Load the key from an environment variable, and don't commit it.

3. Run the PDP

The PDP evaluates each permit.check() call against your policy. Start a PDP container with the Permit CLI:

permit pdp run

The command starts the PDP in Docker and prints the container ID and name. The PDP listens on port 7766, so your app connects to it at http://localhost:7766.

The Free Post resource set is an ABAC rule, and the Cloud PDP doesn't evaluate ABAC rules, so run the container PDP for this policy. To run the container with docker run instead, or to check that the PDP is healthy, see Run the PDP.

4. Build the FastAPI app

1

Install the Python SDK

In your FastAPI project, install the Permit Python SDK:

pip install permit

The permit package installs Pydantic with email validation, which the request models in this tutorial use. This tutorial uses the asyncio version of the SDK, which matches FastAPI's async endpoints. For all SDK options, see Check permissions with the Python SDK (asyncio).

2

Initialize the Permit client in main.py

Create a file named main.py with the following code:

# main.py
from fastapi import FastAPI, Request, status
from permit import Permit
from pydantic import BaseModel, EmailStr, ValidationError
import os
from fastapi.responses import JSONResponse

app = FastAPI()

class UserIn(BaseModel):
    email: EmailStr
    first_name: str
    last_name: str
permit = Permit(
    token=os.getenv("PERMIT_API_KEY"),
    pdp=os.getenv("PDP_URL"),  # Replace with your PDP URL
)

The UserIn Pydantic model validates the body of registration requests. The Permit client reads two environment variables:

Variable	Value
PERMIT_API_KEY	Your environment API key from 2. Get your API key
PDP_URL	The PDP address from 3. Run the PDP: http://localhost:7766

3

Add the /register endpoint

Add the following code to main.py. The register endpoint syncs the user to Permit.io with permit.api.users.sync(), then assigns the user the Reader role in the default tenant with permit.api.users.assign_role().

# main.py
@app.post("/register")
async def register(user_in: UserIn):
    try:
        user = await permit.api.users.sync(
            {
                "key": user_in.email,
                "email": user_in.email,
                "first_name": user_in.first_name,
                "last_name": user_in.last_name,
            }
        )
        # Assign role as part of registration
        role_assignment = await permit.api.users.assign_role({
            "user": user_in.email,
            "role": "Reader",
            "tenant": "default",
        })
        return {
            "message": "User registered and role assigned",
            "user": user,
            "role_assignment": role_assignment
        }
    except ValidationError as e:
        return JSONResponse(content=e.errors(), status_code=422)
    except Exception as e:
        return JSONResponse(content={"error": str(e)}, status_code=500)

The user's email address is the user key in Permit.io. Your app passes the same key to permit.check().

4

Protect the /posts endpoint with permit.check()

Add the following code to main.py. The posts endpoint asks the PDP whether the user in the request body can create a Post, and returns 403 when the PDP denies the request.

# main.py
# Posts model
class Posts(BaseModel):
    user: str

# post creation endpoint
@app.post("/posts")
async def posts(request: Request):
    action = 'create'
    resource = 'Post'
    try:
        data = await request.json()
        check = Posts(**data)
    except ValidationError as e:
        return JSONResponse(content=e.errors(), status_code=422)
    except Exception as e:
        print("Error in posts:", str(e))
        return JSONResponse(content={"error": str(e)}, status_code=500)

    try:
        permitted = await permit.check(check.user, action, resource)
    except Exception as e:
        print("Error during permission check:", str(e))
        return JSONResponse(content={"error": "Permission check failed"}, status_code=500)

    if permitted:
        return {"message": "User is permitted"}
    else:
        return JSONResponse(content={"message": "User is not permitted"}, status_code=403)

permit.check() takes the user key, the action, and the resource type, and returns True or False. To protect other endpoints, such as commenting or editing, change the action and resource values.

note

In a production app, take the user key from your authenticated session. This example reads the user key from the request body so that you can test the endpoint with curl.

5

Start the FastAPI app

Set the environment variables and start the app with Uvicorn, replacing <YOUR_API_KEY> with your API key:

export PERMIT_API_KEY=<YOUR_API_KEY>
export PDP_URL=http://localhost:7766
uvicorn main:app --port 8000

The app listens on http://localhost:8000.

5. Test the permission check

Register two users, give one of them the Author role, and confirm that the PDP allows only that user to create a post.

1

Register two users

In a second terminal, register John and Emma:

curl -X POST http://localhost:8000/register \
  -H "Content-Type: application/json" \
  -d '{"email": "john@example.com", "first_name": "John", "last_name": "Doe"}'

curl -X POST http://localhost:8000/register \
  -H "Content-Type: application/json" \
  -d '{"email": "emma@example.com", "first_name": "Emma", "last_name": "Den"}'

Each request returns "message": "User registered and role assigned", the synced user under user, and the role assignment under role_assignment, with "role": "Reader" and "tenant": "default". Both users appear in the Directory screen of the Permit dashboard.

2

Assign John the Author role

Both users have the Reader role, which can't create posts. Give John the Author role in the Permit dashboard:

1. Open the Directory screen and select john@example.com to open the Edit User panel.
2. Under Permissions Per Tenant, select the Default Tenant.
3. In Top Level Access, add the Author role.
4. Click Save.

For other ways to assign roles, including the API and SDK, see Sync users.

3

Check that John can create a post and Emma can't

Send a POST /posts request for John:

curl -X POST http://localhost:8000/posts \
  -H "Content-Type: application/json" \
  -d '{"user": "john@example.com"}'

The PDP allows the request because John has the Author role. The app returns HTTP 200 with "message": "User is permitted" in the JSON body.

Send the same request for Emma:

curl -X POST http://localhost:8000/posts \
  -H "Content-Type: application/json" \
  -d '{"user": "emma@example.com"}'

The PDP denies the request because Emma has only the Reader role. The app returns HTTP 403 with "message": "User is not permitted" in the JSON body.

Each check also appears in the Audit Log screen of the Permit dashboard, with the user, action, resource, and decision.

Next steps

• Check permissions with the Python SDK (asyncio): SDK installation, configuration, and more permit.check() examples.
• Check permissions with permit.check(): check against tenants, resource instances, and attributes.
• Build RBAC policies: create roles and permissions for your own resources.
• Build ABAC policies: write rules based on user and resource attributes, like the Free Post resource set.

Next step

Going to production? Review the PDP deployment checklist, or talk to an engineer about scale and latency.