In this article, I will show you how you can test a Python web service that was built using Connexion (a wrapper library around Flask). We will go over how you can mock functions and how you can test your endpoints. There are two related articles I have written in the past listed below. In the first one we go over how to create a web service using Connexions, the same web service we will in this article. In the second article I introduce how you can use pytest-mock and pytest-flask to test a Flask web service.

The example app we will be writing tests for is a very simple CRUD API managing a pet store. It allows us to add pets, remove pets, update pets and query pets we have in the store.


You can find the source code here. Our project structure looks like this:

├── openapi
│   └── specification.yml
├── requirements.txt
├── test_api
│   ├── core
│   │   ├──
│   │   ├── pets.json
│   │   └──
│   ├──
│   ├──
│   └── web
│       ├── controllers
│       │   ├──
│       │   └──
│       ├──
│       ├──
│       ├── models
│       │   ├──
│       │   ├──
│       │   ├──
│       │   └──
│       └──
└── tests


Here is our controller module called web/controller/ This is where connexion routes are requests to:

import connexion
import six

from import Pet  # noqa: E501
from ..models.pets import Pets  # noqa: E501
from .. import util

from test_api.core import pets

def get_pet(pet_id):  # noqa: E501
    """Get a pet in the store

     # noqa: E501

    :param pet_id: The id of the pet to retrieve
    :type pet_id: str

    :rtype: Pet
        pet = pets.get_pet(pet_id)
        response = Pet(, breed=pet.breed,, price=pet.price), 200
    except KeyError:
        response = {}, 404

    return response

Connexion uses the open API specification openapi/specification.yml, to work out which function to route requests for the path /pet/{pet_id}. It uses the operationId alongside the x-swagger-router-controller to determine the function to call in the module.

      - "pet"
    summary: "Get a pet in the store"
    operationId: "get_pet"
      - name: "pet_id"
        in: "path"
        description: "The id of the pet to retrieve"
        required: true
        type: "string"
        description: "Successfully retrived pet"
          $ref: "#/definitions/Pet"
        description: "Pet doesn't exist"
    x-swagger-router-controller: "test_api.web.controllers.pets_controller"


Now onto our tests!


pytest-flask allows us to specify an app fixture and then send API requests with this app. Usage is similar to the requests library when sending HTTP requests to our app.

pytest-mock is a simple wrapper around the unit test mock library, so anything you can do using unittest.mock you can do with pytest-mock. The main difference in usage is you can access it using a fixture mocker, also the mock ends at the end of the test. Whereas with the normal mock library if you say mock the open() function, it will be mocked for the remaining duration of that test module, i.e. it will affect other tests.

The file is automatically run by pytest and allows our test modules to access fixtures defined in this file. One of the best features of Pytest is fixtures. Fixture are functions that have re-usable bits of code we can run in our unit tests, such as static data used by tests.

import os
import json

import pytest

from import create_app

def app():
    abs_file_path = os.path.abspath(os.path.dirname(__file__))
    openapi_path = os.path.join(abs_file_path, "../", "openapi")
    os.environ["SPEC_PATH"] = openapi_path

    app = create_app()
    return app

@pytest.fixture(scope="session", autouse=True)
def clean_up():
    default_pets = {
        "1": {"name": "ginger", "breed": "bengal", "price": 100},
        "2": {"name": "sam", "breed": "husky", "price": 10},
        "3": {"name": "guido", "breed": "python", "price": 518},

    abs_file_path = os.path.abspath(os.path.dirname(__file__))
    json_path = os.path.join(abs_file_path, "../", "test_api", "core", "pets.json")
    with open(json_path, "w") as pet_store:
        json.dump(default_pets, pet_store, indent=4)


In this file, we have two functions: the app allows users to pass the client argument to other tests and then we can test our web application. You can get more information here about how Flask apps can be tested. Essentially we don’t need to start/stop a server before/after our tests.

By giving it the scope=session the fixture will be created once before all of our tests run. Our file looks like this:

import os

import connexion

from .web import encoder

def create_app():
    if "SPEC_PATH" in os.environ:
        openapi_path = os.environ["SPEC_PATH"]
        abs_file_path = os.path.abspath(os.path.dirname(__file__))
        openapi_path = os.path.join(abs_file_path, "../", "../", "openapi")
    app = connexion.FlaskApp(
        options={"swagger_ui": False, "serve_spec": False},
    app.add_api("specification.yml", strict_validation=True)
    flask_app =
    flask_app.json_encoder = encoder.JSONEncoder

    return flask_app

The create_app function creates our web application and returns a Flask object. Remember the Connexion library is just a wrapper around Flask. Connexion just reduces the boilerplate code we wrote. Again you can have a read of the article above to get more details about how it works.


@pytest.fixture(scope="session", autouse=True)
def clean_up():
    default_pets = {
        "1": {"name": "ginger", "breed": "bengal", "price": 100},
        "2": {"name": "sam", "breed": "husky", "price": 10},
        "3": {"name": "guido", "breed": "python", "price": 518},

    abs_file_path = os.path.abspath(os.path.dirname(__file__))
    json_path = os.path.join(abs_file_path, "../", "test_api", "core", "pets.json")
    with open(json_path, "w") as pet_store:
        json.dump(default_pets, pet_store, indent=4)

The second fixture we define is called clean_up, because of the yield line, this function will run after all of our tests have completed. The yield command is related to generators, you can read more here. In our case, it’s used in Pytest fixtures so that we can run some cleanup jobs after our test is completed. In this example, I am simply replacing the contents of the JSON file which acts as a data store (like a database), to its default values before the test was run.

Since pytest-3.0, fixtures using the normal fixture decorator can use a yield statement to provide fixture values and execute teardown code - Pytest Docs

Now we have gone over the setup required for our tests, let’s take a look at how we can test our code. So our first test looks like:

def test_get_all_pets(client):
    url = "/api/v1/pet"
    expected_json = [
        {"id": "1", "name": "ginger", "breed": "bengal", "price": 100},
        {"id": "2", "name": "sam", "breed": "husky", "price": 10},
        {"id": "3", "name": "guido", "breed": "python", "price": 518},
    response = client.get(url)
    assert response.json == expected_json

It’s a very simple test, here we use the app fixture we defined above. This client fixture can be used because we are using the pytest-flask library. As you can see it looks very similar to requests, where we give it a path /API/v1/pet and then tell it what kind of request to make client.get. Whilst the syntax between the requests library and the client fixture is almost identical. One big difference that always seems to trip me up is, in requests to get the JSON data from the response object would be response.json() i.e it is a function. However in client (pytest-flask) fixture do get the JSON data we do response.json which is just an attribute of the object not a function.

The test itself is very simple, it’s making a request to get all pets in the pet store. We then compare that with= what we expect to be in the pet store assert response.json == expected_json.

The next test we have looks like this:

    "pet_data, expected_status, expected_data",
        ({"name": "Yolo", "breed": "shorthair", "price": 100}, 201, {"id": 4}),
        ({}, 400, {}),
        ({"a": "b"}, 400, {}),

def test_add_a_pet(client, pet_data, expected_status, expected_data):
    url = "/api/v1/pet"
    response =, json=pet_data)
    assert response.status_code == expected_status
    if response.status_code == 200:
        assert response.json == expected_data

This test is attempting to add a new pet to the store. It’s similar to the other test we still use the client fixture to make the request. This time we also give it some json data hence we provide the json argument json=pet_data this automatically sets the headers correctly so the server knows it’s receiving JSON data.

We also use a decorate called @pytest.mark.parametrize. This allows us to run our tests against a list of data. So we don’t have to write the same test x number of times. We just pass the test different arguments. Pytest will run this test x number of times once for each item in the list. So, for example, the first time the test runs:

pet_data = {"name": "Yolo", "breed": "shorthair", "price": 100}
expected_status = 200
expected_data = {"id": 4}

The second like this:

pet_data = {}
expected_status = 200
expected_data = {}

And so on and so on. This helps keep our test file smaller and keeps the DRY (do not repeat yourself). A very nice feature of Pytest and one I use heavily.

The final test we have in this file looks like:

def test_add_pet_fail_json(client, mocker):
    pet_data = {"name": "Yolo", "breed": "shorthair", "price": 100}
    url = "/api/v1/pet"
    mock = mocker.patch("connexion.request")
    mock.is_json = False
    response =, json=pet_data)
    assert response.status_code == 400

At last, we see pytest-mock being used via the mocker fixture we automatically get access to. The mocker is just a simple wrapper around the unittest.mock module. The main difference being the mock on exists for the duration of that test. Mocking is often used when unit testing and we cannot rely on external dependencies such as database connections or another web service.

def add_pet(body):  # noqa: E501
    # ...
    if connexion.request.is_json:
        body = Pet.from_dict(connexion.request.get_json())  # noqa: E501
    # ...

In this example, we want to mock the part of connexion that checks if the data being sent is valid JSON. We want the connexion.request.is_json to return False, we can do this like so:

mock = mocker.patch("connexion.request")
mock.is_json = False

Since is_json is an attribute of the connexion.request module and not a function we need to set it false on another line. If is_json was a function that we wanted to return False we could’ve done mocker.patch("connexion.request.is_json") instead.

You can run the tests locally by running the pytest command or if you want to run the code in this article, you can by doing the following:

cd 27.\ Mocking\ in\ Flask\ with\ Pytest/source_code
virtualenv .venv
source .venv/bin/activate
pip install -r requirements.txt

That’s it, the examples above cover most of the things you’ll need to mock and test your connexion web service.

INFO: pytest-flask provides a whole bunch of other features that may be useful, you can find the full list here