Python mocking psycopg2 connection and cursor

Question

I've been unable to mock the psycopg2 database connection and cursor since I've refactored it to use a context manager for the cursor. I'm aware that with a context manager, there are additional magic methods that are called for resource setup and cleanup (__enter__, __exit__), but even adding this into the mix has not narrowed down the problem.

Here is the code:

import os
import psycopg2
import psycopg2.extras

DB_HOST = os.getenv('DB_HOST')     
DB_PORT = os.getenv('DB_PORT')
DB_NAME = os.getenv('DB_NAME')
DB_USER = os.getenv('DB_USER')
DB_PASSWORD = os.getenv('DB_PASSWORD')            
CONN = psycopg2.connect(f'dbname={DB_NAME} user={DB_USER} host={DB_HOST} port={DB_PORT} password={DB_PASSWORD}')

def my_func():
    message = None
    print(CONN)  # Added to debug - this never prints out a magic mock reference

    select_sql = 'SELECT * FROM app.users WHERE name = %s LIMIT 1;'
    with CONN.cursor(cursor_factory = psycopg2.extras.DictCursor) as cursor:
        print(cursor) # Debug - no magic mock reference
        cursor.execute(select_sql, ("Bob"))
        row = cursor.fetchone()

    if row is not None: 
        message = "name found"
    else:
        message = "name not found"

    return message

Here's the test code and my attempt to mock the connection and cursor

import pytest

from src import my_class
from unittest import mock

class TestFunction:
    @mock.patch('psycopg2.connect')
    def test_my_func(self, mock_connect):

        mock_cursor = mock.MagicMock()
        mock_cursor.__enter__.return_value.fetchone.return_value = {
            "id": 1,
            "name": "Bob",
            "age": 25
        }
        mock_connect.return_value.cursor.return_value = mock_cursor

        result = my_class.my_func()
        assert result == "found"

I'm not sure how to mock the connection or the cursor if I'm calling cursor on the global connection, CONN. Currently running pytest shows the test fails and the print statements show that the underlying object is not a magic mock. How does one mock a global db connection and a db cursor using a context manager? Any help is appreciated.

snackoverflow · Accepted Answer · 2019-10-29 18:43:16Z

9

I found a solution! I had a feeling my problems were around the global variable CONN and looked into how to mock python global variables and tried that out. My print statements finally showed that these instances were indeed Magic Mocks.

It makes sense that in my test I'm only calling the method under test, not triggering the code with the global variable - so just set it to what you want it to be. Sounds straight forward once you have the answer, right?

mock_cursor = mock.MagicMock()
mock_cursor.fetchone.return_value = {
      "id": 1,
      "name": "Bob",
      "age": 25
}
mock_connect.cursor.return_value.__enter__.return_value = mock_cursor
my_class.CONN = mock_connect

Nothing like answering your own question!

answered Oct 29, 2019 at 18:43

snackoverflow

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