Python mock object instantiation

Using Python 2.7, and mock library

How can I test that certain patched object has been initialized with some specific arguments using mock?

Here some sample code and pseudo-code:

unittest.py :

import mock
@mock.patch('mylib.SomeObject')
def test_mytest(self, mock_someobject):
  test1 = mock_someobject.return_value
  test1 = method_inside_someobject.side_effect = ['something']

  mylib.method_to_test()

  # How can I assert that method_to_test instanced SomeObject with certain arguments?
  # I further test things with that method_inside_someobject call, no problems there...

mylib.py :

from someobjectmodule import SomeObject
def method_to_test():
  obj = SomeObject(arg1=val1, arg2=val2, arg3=val3)
  obj.method_inside_someobject()

So, how can I test SomeObject was instanced with arg1=val1, arg2=val2, arg3=val3?

Solution

If you replaced a class with a mock, creating an instance is just another call. Assert that the right parameters have been passed to that call, for example, with mock.assert_called_with():

mock_someobject.assert_called_with(arg1=val1, arg2=val2, arg3=val3)

To illustrate, I've updated your MCVE to a working example:

test.py:

import mock
import unittest

import mylib


class TestMyLib(unittest.TestCase):
    @mock.patch('mylib.SomeObject')
    def test_mytest(self, mock_someobject):
        mock_instance = mock_someobject.return_value
        mock_instance.method_inside_someobject.side_effect = ['something']

        retval = mylib.method_to_test()

        mock_someobject.assert_called_with(arg1='foo', arg2='bar', arg3='baz')
        self.assertEqual(retval, 'something')


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

mylib.py:

from someobjectmodule import SomeObject

def method_to_test():
    obj = SomeObject(arg1='foo', arg2='bar', arg3='baz')
    return obj.method_inside_someobject()

someobjectmodule.py:

class SomeObject(object):
    def method_inside_someobject(self):
        return 'The real thing'

and running the test:

$ python test.py
.
----------------------------------------------------------------------
Ran 1 test in 0.001s

OK