Protect endpoints by requiring a JWT

Now that our users can sign up and log in, that means we can start requiring login for certain endpoints.

All this means in practice is that the client making the request must send a valid JWT.

Remember, we can tell if a JWT is valid because it is signed by our app. If the user changes the JWT at all, the signature will be invalid, and we'll know it has been tampered with. Flask-JWT-Extended takes care of all that for us.

Protecting routes in the `Item` resource

resources/item.py
from flask.views import MethodView
from flask_smorest import Blueprint, abort
from flask_jwt_extended import jwt_required
from sqlalchemy.exc import SQLAlchemyError

from db import db
from models import ItemModel
from schemas import ItemSchema, ItemUpdateSchema

blp = Blueprint("Items", "items", description="Operations on items")


@blp.route("/item/<string:item_id>")
class Item(MethodView):
    @jwt_required()
    @blp.response(200, ItemSchema)
    def get(self, item_id):
        item = ItemModel.query.get_or_404(item_id)
        return item

    @jwt_required()
    def delete(self, item_id):
        item = ItemModel.query.get_or_404(item_id)
        db.session.delete(item)
        db.session.commit()
        return {"message": "Item deleted."}

    @blp.arguments(ItemUpdateSchema)
    @blp.response(200, ItemSchema)
    def put(self, item_data, item_id):
        item = ItemModel.query.get_or_404(item_id)

        if item:
            item.price = item_data["price"]
            item.name = item_data["name"]
        else:
            item = ItemModel(**item_data)

        db.session.add(item)
        db.session.commit()

        return item


@blp.route("/item")
class ItemList(MethodView):
    @jwt_required()
    @blp.response(200, ItemSchema(many=True))
    def get(self):
        return ItemModel.query.all()

    @jwt_required()
    @blp.arguments(ItemSchema)
    @blp.response(201, ItemSchema)
    def post(self, item_data):
        item = ItemModel(**item_data)

        try:
            db.session.add(item)
            db.session.commit()
        except SQLAlchemyError:
            abort(500, message="An error occurred while inserting the item.")

        return item

Error handling with Flask-JWT-Extended

There are many things that could go wrong with JWTs:

The JWT may be expired (they don't last forever!)
The JWT may be invalid, such as if the client makes changes to it
A JWT may be required, but none was provided
There's more (we'll look at them in coming lectures!)

Let's go to app.py and add some configuration to tell Flask-JWT-Extended what to do in each of these cases.

At the top, let's import jsonify:

app.py
from flask import Flask, jsonify

Then, after we define the jwt = JWTManager(app) variable, we can write some functions, each of which can run in different problem scenarios.

app.py
...

app.config["JWT_SECRET_KEY"] = "jose"
jwt = JWTManager(app)

@jwt.expired_token_loader
def expired_token_callback(jwt_header, jwt_payload):
    return (
        jsonify({"message": "The token has expired.", "error": "token_expired"}),
        401,
    )

@jwt.invalid_token_loader
def invalid_token_callback(error):
    return (
        jsonify(
            {"message": "Signature verification failed.", "error": "invalid_token"}
        ),
        401,
    )

@jwt.unauthorized_loader
def missing_token_callback(error):
    return (
        jsonify(
            {
                "description": "Request does not contain an access token.",
                "error": "authorization_required",
            }
        ),
        401,
    )

...

tip

Note that some Flask-JWT-Extended error functions take two arguments: jwt_header and jwt_payload. Others take a single argument, error.

The ones that don't take JWT information are those that would be called when a JWT is not present (above, when the JWT is invalid or required but not received).

Protect endpoints by requiring a JWT

Protecting routes in the Item resource​

Error handling with Flask-JWT-Extended​

Protecting routes in the `Item` resource

Error handling with Flask-JWT-Extended