Flask: Decorator to verify JSON and JSON Schema

Question

I have a flask application with calls expecting JSON payload. Before each call is processed, I have a 2-step error checking process:

• Assert that the payload is a valid JSON
• Assert that the JSON payload complies with a specific schema

Which is implemented in the following fashion:

@app.route('/activate', methods=['POST'])
def activate():
    request_id = request.__hash__()

    # Assert that the payload is a valid JSON
    try:
        input = request.json
    except BadRequest, e:
        msg = "payload must be a valid json"
        return jsonify({"error": msg}), 400

    # JSON Schema Validation
    try:
        validate(request.json, app.config['activate_schema'])
    except ValidationError, e:
        return jsonify({"error": e.message}), 400

Since this code is duplicated over many calls, I wonder If I can elegantly move it to a decorator, something in the formof:

@validate_json
@validate_schema(schema=app.config['activate_schema'])
@app.route('/activate', methods=['POST'])
def activate():
    ....

The problem is that the request argument is implicit: I can refer to it within the function, but it is not a parameter to it. Therefore, I am not sure how to use it within the decorator.

How can I implement the validation checks using Python decorators?

Answer 1

Just use the request context global in your decorator. It is available during any request.

from functools import wraps
from flask import (
    current_app,
    jsonify,
    request,
)


def validate_json(f):
    @wraps(f)
    def wrapper(*args, **kw):
        try:
            request.json
        except BadRequest, e:
            msg = "payload must be a valid json"
            return jsonify({"error": msg}), 400
        return f(*args, **kw)
    return wrapper


def validate_schema(schema_name):
    def decorator(f):
        @wraps(f)
        def wrapper(*args, **kw):
            try:
                validate(request.json, current_app.config[schema_name])
            except ValidationError, e:
                return jsonify({"error": e.message}), 400
            return f(*args, **kw)
        return wrapper
    return decorator

Apply these decorators before applying the @route decorator; you want to register the wrapped function, not the original function for the route:

@app.route('/activate', methods=['POST'])
@validate_json
@validate_schema('activate_schema')
def activate():
    input = request.json

Answer 2

A late answer, but you're probably looking for something like marshmallow (flask-marshmallow) or toastedmarshmallow.