python - Mocking __init__() for unittesting

Question

I have a class:

class DatabaseThing():
     def __init__(self, dbName, user, password):
          self.connection = ibm_db_dbi.connect(dbName, user, password)

I want to test this class but with a test database. So in my test class I am doing something like this:

import sqlite3 as lite
import unittest
from DatabaseThing import *

class DatabaseThingTestCase(unittest.TestCase):

    def setUp(self):
        self.connection = lite.connect(":memory:")
        self.cur = self.connection.cursor()
        self.cur.executescript ('''CREATE TABLE APPLE (VERSION INT, AMNT SMALLINT);
            INSERT INTO APPLE VALUES(16,0);
            INSERT INTO APPLE VALUES(17,5);
            INSERT INTO APPLE VALUES(18,1);
            INSERT INTO APPLE VALUES(19,15);
            INSERT INTO APPLE VALUES(20,20);
            INSERT INTO APPLE VALUES(21,25);''')

How would I go about using this connection than the connection from the class I want to test? Meaning using the connection from setUp(self) instead of the connection from DatabaseThing. I cannot test the functions without instantiating the class. I want to mock the __init__ method somehow in the Test Class, but I didn't find anything that seemed useful in the documentation.

Answer 1

Instead of mocking, you could simply subclass the database class and test against that:

class TestingDatabaseThing(DatabaseThing):
     def __init__(self, connection):
          self.connection = connection

and instantiate that class instead of DatabaseThing for your tests. The methods are still the same, the behaviour will still be the same, but now all methods using self.connection use your test-supplied connection instead.