PropertyMock with changing values after every call

Question

I'm trying to mock psycopg2 and need to handle cursor.description

  # run query 1
  column_names = [desc[0] for desc in cursor.description]
  # run query 2
  # NB cursor.description now returns different value
  column_names = [desc[0] for desc in cursor.description]

I can do this via a PropertyMock:

  type(fake_cursor).description = PropertyMock(return_value=descriptions[0])

but it doesn't solve my problem because the system under test does two queries and the description will be different for the second time.

How can I change the return value of the mock on the second call?

I tried:

cursor_execute_call_count = 0

def handle_cursor_execute(arg1, arg2):
    cursor_execute_call_count = cursor_execute_call_count + 1
    type(fake_cursor).description = PropertyMock(return_value=descriptions[cursor_execute_call_count])

fake_cursor.execute = Mock(side_effect=handle_cursor_execute)

But I get

E     UnboundLocalError: local variable 'cursor_execute_call_count' referenced before assignment

(That doesn't make sense to me. There must be a weird scoping issue going on.)

I did eventually get it working with this code but it seems like there must be a better way:

def handle_cursor_execute(arg1, arg2):
  description = descriptions.pop()
  type(fake_cursor).description = PropertyMock(return_value=description)

Can you clarify your question? If what's your code is doing is that `column_names = [desc[0] for desc in cursor.description]` so it's really should be as simple as puting a list in return_value of your mock. — Javed, Mar 05 '20 at 03:59
The code does two queries. The output of description changes for every query. — jeznag, Mar 05 '20 at 04:23
That's better, I updated my answer based on your clarifications. — Javed, Mar 05 '20 at 07:41

Javed · Accepted Answer · 2020-03-05T07:38:57.407

Edit based on clarification:

I Guess you can implement an iterator:

class MyDescription():

    descriptions = iter([[1, 2, 3], [4, 5, 6], [7, 8, 9]])

    def __iter__(self):
        for description_list in self.descriptions:
            for description in description_list:
                yield description
            raise StopIteration
type(fake_cursor).description = PropertyMock(return_value=MyDescription())

So it will look like this:

>>> z = MyDescription()
>>> [i for i in z]
[1, 2, 3]
>>> [i for i in z]
[4, 5, 6]
>>> [i for i in z]
[7, 8, 9]
>>>

PropertyMock with changing values after every call

1 Answers1