python - How to mock calls to function that receives mutable object as parameter? -

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consider example:

def func_b(a):     print  def func_a():     = [-1]     in xrange(0, 2):         a[0] =         func_b(a)

and test function tries test func_a , mocks func_b:

import mock mock import call  def test_a():     datatransform.test import func_a     mock.patch('datatransform.test.func_b', autospec=true) func_b_mock:         func_a()         func_b_mock.assert_has_calls([call(0), call(1)])

after func_a has executed try test if func_a made correct calls func_b, since in loop mutating list in end get:

assertionerror: calls not found. expected: [call(0), call(1)] actual: [call([1]), call([1])]

the following works (the importing mock unittest python 3 thing, , module func_a , func_b are):

import mock mock import call import copy  class modifiedmagicmock(mock.magicmock):     def _mock_call(_mock_self, *args, **kwargs):         return super(modifiedmagicmock, _mock_self)._mock_call(*copy.deepcopy(args), **copy.deepcopy(kwargs))

this inherits magicmock, , redefines call behaviour deepcopy arguments , keyword arguments.

def test_a():     module import func_a     mock.patch('module.func_b', new_callable=modifiedmagicmock) func_b_mock:         func_a()         func_b_mock.assert_has_calls([call([0]), call([1])])

you can pass new class patch using new_callable parameter, cannot co-exist autospec. note function calls func_b list, call(0), call(1) has changed call([0]), call([1]). when run calling test_a, nothing (passes).

now cannot use both new_callable , autospec because new_callable generic factory in our case magicmock override. autospeccing cool mock's feature, don't want lose it.

what need replace magicmock modifiedmagicmock our test: want avoid change magicmock behavior tests... dangerous. have tool , patch, used new argument replace destination.

in case use decorators avoid indentation , make more readable: 

@mock.patch('module.func_b', autospec=true) @mock.patch("mock.magicmock", new=modifiedmagicmock) def test_a(func_b_mock):     module import func_a     func_a()     func_b_mock.assert_has_calls([call([0]), call([1])])

or:

@mock.patch("mock.magicmock", new=modifiedmagicmock) def test_a():     mock.patch('module.func_b') func_b_mock:         module import func_a         func_a()         func_b_mock.assert_has_calls([call([0]), call([1])])

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