How to mock a imported object with pytest-mock or magicmock

Question

I am trying to understand the mock/monkeypatch/pytest-mock capabilities.

Let me know if this is possible. If not could you please suggest how I can test this code.

My code structure:

/
./app
../__init__.py
../some_module1
.../__init__.py
../some_module2
.../__init__.py
./tests
../test_db.py

The /app/__init__.py is where my application (a Flask application if it helps) is started along with initializing a database connection object to a MongoDB database:

# ...

def create_app():
  # ...
  return app

db_conn = DB()

The some_module1 and some_module import the db_conn object and use it as part of their functions:

## some_module1/__init__.py
from app import db_conn

...
db = db_conn.db_name2.db_collection2

def some_func1():
    data = db.find()
    # check and do something with data
    return boolean_result

...

## some_module2/__init__.py
from app import db_conn

...
db = db_conn.db_name1.db_collection1

def some_func2():
    data = db.find()
    # check and do something with data
    return boolean_result
...

In my tests, I want to test if my code works properly based on the data received from the database. I want to mock the database, more specifically the db_conn object since I don't want to use a real database (which would be a lot of work setting up the environment and maintaining it).

Any suggestions on how I can emulate the db_conn?

I've been exploring pytest-mock and magicmock but I don't think or know how to mock the db_conn in my test.

Answer 1

I believe you are right not testing cases on a real database because it's not unit testing anymore if you are using external dependencies.

There is a possibility to specify return-value and customize it (different return values on each iteration even) for Mock or MagicMock objects.

from unittest.mock import Mock, patch 

from app import db_conn


@patch('app.db_conn.find')
def test_some_func1(db_con_mock):
    ...
    assert ...

Keep in mind that in each patch you should specify the import path of db_conn - the path where db_conn **is used (I assume it's a different path in each test), not where it is defined.