Unittest.mock - how to mock a call to cursor.execute() from connection object?

Question

I am trying to stub out cursor.execute() in the following code with mock such that I can test execute is called with a correctly formatted query:

// Module ABC

def buildAndExecuteQuery( tablesAndColumnsToSelect ):
   '''Build and execute a query.

   Args: tablesAndColumnsToSelect (dict)
         - keys are table names, values are columns

   '''
   query = ..... < logic to build query > ....

   from django.db import connections
   cursor = connections[ 'mydb' ].cursor()
   cursor.execute( query )

How can I accomplish this type of mock in python2.7 with the mock library?

Answer 1

Since I don't know what your query logic is, I modified the query to just accept a sentinel value directly through the tables_and_columns_to_select argument.

# b_and_ex_q.py


def build_and_execute_query(tables_and_columns_to_select):
    """Build and execute a query.

    Args: tablesAndColumnsToSelect (dict)
          - keys are table names, values are columns

    """

    query = tables_and_columns_to_select  # ..... < logic to build query > ....

    from django.db import connections

    cursor = connections['mydb'].cursor()
    cursor.execute(query)

---

import unittest

from mock import patch, sentinel

from b_and_ex_q import build_and_execute_query


class TestCursorExecute(unittest.TestCase):
    @patch('django.db.connections')
    def test_build_and_execute_query(self, mock_connections):
        # Arrange
        mock_cursor = mock_connections.__getitem__('mydb').cursor.return_value

        # Act
        build_and_execute_query(sentinel.t_and_c)

        # Assert
        mock_cursor.execute.assert_called_once_with(sentinel.t_and_c)


if __name__ == '__main__':
    unittest.main()