Flask

The Oso Flask integration provides a more convenient interface to Oso for usage with Flask.

Installation

The Oso Flask integration is available on PyPI and can be installed using pip:

$ pip install flask-oso==0.22.0

Usage

Initialization

The FlaskOso class is the entrypoint to the integration. It must be initialized with the Flask app and Oso:

from flask import Flask
from oso import Oso

app = Flask("app")
oso = Oso()
flask_oso = FlaskOso(app=app, oso=oso)

Alternatively, to support the Flask factory pattern, the init_app() method can be used:

from flask import Flask

from oso import Oso
from flask_oso import FlaskOso

oso = Oso()
flask_oso = FlaskOso(oso=oso)

def create_app():
    app = Flask("app")

    # Initialize Oso for this application
    flask_oso.init_app(app)

    return app

app = create_app()

This factory function can be a useful place for loading policy files, and calling configuration functions on FlaskOso like flask_oso.FlaskOso.require_authorization():

def create_app():
    app = Flask("app")

    flask_oso.init_app(app)
    flask_oso.require_authorization(app)

    oso.load_file("authorization.polar")
    oso.register_class(Expense)

Performing authorization

When using the flask-oso integration, the primary authorization function is flask_oso.FlaskOso.authorize(). It accepts the same arguments as is_allowed(), but provides sensible defaults for working with Flask. The actor defaults to flask.g.current_user (this can be customized, see set_get_actor()). The action defaults to the method of the current request flask.request.method. resource must be provided.

flask_oso.FlaskOso.authorize() can be used within route handlers, or in the data access layer, depending upon how you want to express authorization.

Here’s a basic example in a route:

@app.route("/<int:id>", methods=["GET"])
def get_expense(id):
    expense = Expense.query.get(id)
    if expense is None:
        raise NotFound()

    flask_oso.authorize(action="read", resource=expense)
    return expense.json()

Notice we didn’t need to check the return value of authorize. By default, a failed authorization will return a ``403 Forbidden`` response for the current request. This can be controlled with set_unauthorized_action().

Working with LocalProxy objects

When using a library that exposes the current user (or similar authorization data) through LocalProxy objects, such as Flask-Login’s current_user, you might need to explicitly dereference the proxy to pass the underlying object to Oso:

from flask_login import current_user

def create_app():
    app = Flask("app")

    flask_oso.init_app(app)
    flask_oso.require_authorization(app)
    # Dereference the current_user LocalProxy
    flask_oso.set_get_actor(lambda: current_user._get_current_object())

    oso.load_file("authorization.polar")
    oso.register_class(User)
    return app

By dereferencing the proxy, Oso will use the underlying object when determining authorization instead of the proxy object.

Requiring authorization

One downside to calling flask_oso.FlaskOso.authorize() explicitly within route handlers is that the check might be forgotten. To help detect this, the flask_oso.FlaskOso.require_authorization() option can be enabled during initialization. This will cause an oso.OsoError to be raised if a call to flask_oso.FlaskOso.authorize() is not made during the processing of a request.

Sometimes a route will not need authorization. To prevent this route from causing an authorization error, call flask_oso.FlaskOso.skip_authorization() during request processing:

oso = Oso()
flask_oso = FlaskOso()

def create_app():
    app = Flask("app")

    flask_oso.init_app(app)
    flask_oso.require_authorization(app)

    oso.load_file("authorization.polar")

    return app

app = create_app()

@app.route("/about")
def about():
    flask_oso.skip_authorization()
    return "about us"

Using decorators

Some developers may prefer a decorator-based API for performing authorization. flask_oso provides two decorators:

flask_oso.skip_authorization() marks a route as not requiring authorization. It is the decorator version of flask_oso.FlaskOso.skip_authorization().

flask_oso.authorize() decorates a route and calls flask_oso.FlaskOso.authorize() before the route body is entered. For example:

from flask_oso import authorize

@authorize(resource="get_user")
@app.route("/user")
def get_user():
    return "current user"

This decorator can be used if the resource is known before entering the request body.

Route authorization

One common usage of flask_oso.authorize() is to perform authorization based on the Flask request object:

from flask import request

@flask_oso.authorize(resource=request)
@app.route("/")
def route():
    return "authorized"

A policy can then be written controlling authorization based on request attributes, like the path:

# Allow any actor to make a GET request to "/".
allow(_actor, "GET", _resource: Request{path: "/"});

To enforce route authorization on all requests (the equivalent of decorating every route as we did above), use the perform_route_authorization() method during initialization.

Example

Check out the Flask integration example app on GitHub: osohq/oso-flask-integration.

API Reference

The Flask API reference is automatically generated from the Oso Flask library source files.

Set up a 1x1 with an Oso Engineer

Our team is happy to help you get started with Oso. If you'd like to learn more about using Oso in your app or have any questions about this guide, schedule a 1x1 with an Oso engineer.


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