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0 pointstayo421mo ago0 comments

I know how I would do it in python. This is built into the stdlibs testing library, with mocks.

Maybe dependency injection and function pointers for the copy if function. Then you can check the call counts in your tests. But idk the cpp eco system and what's available there to do it.

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shagie1mo ago

The python code would be

    def some_call():
        return [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
    
    def print_evens(nums):
        for n in filter(lambda n: n % 2 == 0, nums):
            print(n)
    
    def func():
        filtered = list(filter(lambda n: n % 2 == 0, some_call()))
        print_evens(filtered)
    
    if __name__ == "__main__":
        func()

How would you write a failing test that prevents the list from some_call() from having the same filter applied to it twice?

tayo42OP1mo ago

You would use something like this

https://docs.python.org/3/library/unittest.mock.html#unittes...

Then you would make the mock filter with patch and test the `func` function

psuedo python code would be

   @patch(builtins.filter)
   def test_func_filter_calls(mock_filter):
     mock_filter.return_value = [2,4,6]
     func()
     mock_filter.assert_called_once_with([1, 2, 3, 4, 5, 6, 7, 8, 9, 10])

shagie1mo ago

That wouldn't fail though. It was called only once with [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]. The second time it was called with [2, 4, 6, 8, 10].

Likewise, if some_call returned [2, 4, 6, 8, 10] instead, it should only be called with [2, 4, 6, 8, 10] once then.

However, the purpose of this test then becomes questionable. Why are you testing implementation details rather than observable? Is there anything that you could observe that depended on the filter being called once or twice with the same filter function?

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shagie1mo ago

The python code would be

    def some_call():
        return [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
    
    def print_evens(nums):
        for n in filter(lambda n: n % 2 == 0, nums):
            print(n)
    
    def func():
        filtered = list(filter(lambda n: n % 2 == 0, some_call()))
        print_evens(filtered)
    
    if __name__ == "__main__":
        func()

How would you write a failing test that prevents the list from some_call() from having the same filter applied to it twice?

tayo42OP1mo ago

You would use something like this

https://docs.python.org/3/library/unittest.mock.html#unittes...

Then you would make the mock filter with patch and test the `func` function

psuedo python code would be

   @patch(builtins.filter)
   def test_func_filter_calls(mock_filter):
     mock_filter.return_value = [2,4,6]
     func()
     mock_filter.assert_called_once_with([1, 2, 3, 4, 5, 6, 7, 8, 9, 10])

shagie1mo ago

That wouldn't fail though. It was called only once with [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]. The second time it was called with [2, 4, 6, 8, 10].

Likewise, if some_call returned [2, 4, 6, 8, 10] instead, it should only be called with [2, 4, 6, 8, 10] once then.

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