How to send requests with JSON in unit tests

Changing the post to

response=self.app.post('/test_function',                        data=json.dumps(dict(foo='bar')),                       content_type='application/json')

fixed it.

Thanks to user3012759.

Since Flask 1.0 release flask.testing.FlaskClient methods accepts json argument and Response.get_json method added, see pull request

    with app.test_client() as c:        rv = c.post('/api/auth', json={            'username': 'flask', 'password': 'secret'        })        json_data = rv.get_json()

For Flask 0.x compatibility you may use receipt below:

    from flask import Flask, Response as BaseResponse, json    from flask.testing import FlaskClient            class Response(BaseResponse):        def get_json(self):            return json.loads(self.data)            class TestClient(FlaskClient):        def open(self, *args, **kwargs):            if 'json' in kwargs:                kwargs['data'] = json.dumps(kwargs.pop('json'))                kwargs['content_type'] = 'application/json'            return super(TestClient, self).open(*args, **kwargs)        app = Flask(__name__)    app.response_class = Response    app.test_client_class = TestClient    app.testing = True