How to mock the service layer in a python (flask) webapp for unit testing?

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I am working on a webapp in flask and using a services layer to abstract database querying and manipulation away from the views and api routes. Its been suggested that this makes testing easier because you can mock out the services layer, but I am having trouble figuring out a good way to do this. As a simple example, imagine that I have three SQLAlchemy models:

class User(db.Model):
    id = db.Column(db.Integer, primary_key = True)
    email = db.Column(db.String)

class Group(db.Model):
    id = db.Column(db.Integer, primary_key = True)
    name = db.Column

class Transaction(db.Model):
    id = db.Column(db.Integer, primary_key = True)
    from_id = db.Column(db.Integer, db.ForeignKey(''))
    to_id = db.Column(db.Integer, db.ForeignKey(''))
    group_id = db.Column(db.Integer, db.ForeignKey(''))
    amount = db.Column(db.Numeric(precision = 2))

There are users and groups, and transactions (which represent money changing hands) between users. Now I have a that has a bunch of functions for things like checking if certain users or groups exist, checking if a user is a member of a particular group, etc. I use these services in an api route which is sent JSON in a request and uses it to add transactions to the db, something similar to this:

import services

def addtrans():
    # get the values out of the json in the request
    args = request.get_json()
    group_id = args['group_id']
    from_id = args['from']
    to_id = args['to'] 
    amount = args['amount']

    # check that both users exist
    if not services.user_exists(to_id) or not services.user_exists(from_id):
        return "no such users"

    # check that the group exists
    if not services.group_exists(to_id):
        return "no such group"

    # add the transaction to the db
    return "success"

The problem comes when I try to mock out these services for testing. I've been using the mock library, and I'm having to patch the functions from the services module in order to get them to be redirected to mocks, something like this:

mock = Mock()
mock.user_exists.return_value = True
mock.group_exists.return_value = True

def test_addtrans_route(self):
    assert "success" in routes.addtrans()

This feels bad for any number of reasons. One, patching feels dirty; two, I don't like having to patch every service method I'm using individually (as far as I can tell there's no way to patch out a whole module).

I've thought of a few ways around this.

  1. Reassign so that it refers to my mock rather than the actual services module, something like: = mymock
  2. Have the services be methods of a class which is passed as a keyword argument to each route and simply pass in my mock in the test.
  3. Same as (2), but with a singleton object.

I'm having trouble evaluating these options and thinking of others. How do people who do python web development usually mock services when testing routes that make use of them?

Using Mocks to Test External Dependencies or Reduce Duplication, This isn't even something that only works inside unit tests. You can do this kind of "monkeypatching" in any kind of Python code! That may take a  The solution is simple - we can create a bare-bones local implementation of the service we are calling (which is called a mock in testing parlance) and run our unit tests using this mock. We can separate out our integration tests into a separate file and run those before we commit changes to our code.

You can patch out the entire services module at the class level of your tests. The mock will then be passed into every method for you to modify.

class MyTestCase(unittest.TestCase):

    def test_my_code_when_services_returns_true(self, mock_services):
        mock_services.user_exists.return_value = True

        self.assertIn('success', routes.addtrans())

    def test_my_code_when_services_returns_false(self, mock_services):
        mock_services.user_exists.return_value = False

        self.assertNotIn('success', routes.addtrans())

Any access of an attribute on a mock gives you a mock object. You can do things like assert that a function was called with the mock_services.return_value.some_method.return_value. It can get kind of ugly so use with caution.

Guide to Python Flask Unit Testing, Even if you don't use Flask, the unit-testing concepts illustrated are generally applicable. We will use a pytest feature called “fixtures” to turn our web app into a Python is a layer of abstraction that handles all of the app's interactions with the server. from test.unit.webapp import client import mock from mock import call  This package provides some classes to build a Service layer and expose an API that interacts with the model. The first idea is to remove all logic of the routes and model of the Flask application, and put it in the service layer. The second goal is to provide a common API that can be use to manipulate a model regardless of its storage backend.

I would also raise a hand for using dependency injection for such needs. You can use Dependency Injector to describe structure of your application using inversion of control container(s) to make it look like this:

"""Example of dependency injection in Python."""

import logging
import sqlite3

import boto3

import example.main

import dependency_injector.containers as containers
import dependency_injector.providers as providers

class Core(containers.DeclarativeContainer):
    """IoC container of core component providers."""

    config = providers.Configuration('config')

    logger = providers.Singleton(logging.Logger, name='example')

class Gateways(containers.DeclarativeContainer):
    """IoC container of gateway (API clients to remote services) providers."""

    database = providers.Singleton(sqlite3.connect, Core.config.database.dsn)

    s3 = providers.Singleton(
        boto3.client, 's3',,

class Services(containers.DeclarativeContainer):
    """IoC container of business service providers."""

    users = providers.Factory(,

    auth = providers.Factory(,

    photos = providers.Factory(,

class Application(containers.DeclarativeContainer):
    """IoC container of application component providers."""

    main = providers.Callable(example.main.main,

Having this will give your a chance to override particular implementations later:


Hope it helps.

Python Web Applications With Flask – Part III – Real Python, a Web Application with Flask we'll explore unit and integration testing. Integration Tests; Mocking Free GeoIP; Set up the test data and mocks; Patch the mock This ensures that our unit tests will run quickly - otherwise, every time we the database and the user - it also interacts with the third-party service Free GeoIP. Such a layer is commonly called a REST layer, because usually the semantic of the addresses comes from the REST recommendations. Flask is a lightweight web server with a modular structure that provides just the parts that the user needs. In particular, we will not use any database/ORM, since we already implemented our own repository layer.

def test_lambda_handler(): 
lambda_handler(event, None)

I used mocking seeing your example and my method expects to return 20 whenever in lambda function testing locally get_max_qualcomm_id us made .but on reaching the above method i get a exception int type object is not Callable. Please let me know what is the problem here .

This is actual method being call which i am trying to mock :

 last_max_id = QualcommTransactionService().get_max_qualcomm_id(self.subscriber_id)

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