How to mock psycopg2 cursor object?

Question

I have this code segment in Python2:

def super_cool_method():
    con = psycopg2.connect(**connection_stuff)
    cur = con.cursor(cursor_factory=DictCursor)
         


        

Answer 1

You have a series of chained calls, each returning a new object. If you mock just the psycopg2.connect() call, you can follow that chain of calls (each producing mock objects) via .return_value attributes, which reference the returned mock for such calls:

@mock.patch("psycopg2.connect")
def test_super_awesome_stuff(self, mock_connect):
    expected = [['fake', 'row', 1], ['fake', 'row', 2]]

    mock_con = mock_connect.return_value  # result of psycopg2.connect(**connection_stuff)
    mock_cur = mock_con.cursor.return_value  # result of con.cursor(cursor_factory=DictCursor)
    mock_cur.fetchall.return_value = expected  # return this when calling cur.fetchall()

    result = super_cool_method()
    self.assertEqual(result, expected)

Because you hold onto references for the mock connect function, as well as the mock connection and cursor objects you can then also assert if they were called correctly:

mock_connect.assert_called_with(**connection_stuff)
mock_con.cursor.called_with(cursor_factory=DictCursor)
mock_cur.execute.called_with("Super duper SQL query")

If you don't need to test these, you could just chain up the return_value references to go straight to the result of cursor() call on the connection object:

@mock.patch("psycopg2.connect")
def test_super_awesome_stuff(self, mock_connect):
    expected = [['fake', 'row', 1], ['fake', 'row' 2]]
    mock_connect.return_value.cursor.return_value.fetchall.return_value = expected

    result = super_cool_method()
    self.assertEqual(result, expected)

Note that if you are using the connection as a context manager to automatically commit the transaction and you use as to bind the object returned by __enter__() to a new name (so with psycopg2.connect(...) as conn: # ...) then you'll need to inject an additional __enter__.return_value in the call chain:

mock_con_cm = mock_connect.return_value  # result of psycopg2.connect(**connection_stuff)
mock_con = mock_con_cm.__enter__.return_value  # object assigned to con in with ... as con    
mock_cur = mock_con.cursor.return_value  # result of con.cursor(cursor_factory=DictCursor)
mock_cur.fetchall.return_value = expected  # return this when calling cur.fetchall()

The same applies to the result of with conn.cursor() as cursor:, the conn.cursor.return_value.__enter__.return_value object is assigned to the as target.

Answer 2

Since the cursor is the return value of con.cursor, you only need to mock the connection, then configure it properly. For example,

query_result = [("field1a", "field2a"), ("field1b", "field2b")]
with mock.patch('psycopg2.connect') as mock_connect:
    mock_connect.cursor.return_value.fetchall.return_value = query_result
    super_cool_method()

Answer 3

The following answer is the variation of above answers. I was using django.db.connections cursor object.

So following code worked for me

@patch('django.db.connections')
def test_supercool_method(self, mock_connections):
    query_result = [("field1a", "field2a"), ("field1b", "field2b")]
    mock_connections.__getitem__.return_value.cursor.return_value.__enter__.return_value.fetchall.return_value = query_result

    result = supercool_method()
    self.assertIsInstance(result, list)