Comment by shagie

20 days 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?

6 comments

shagie

tayo42 20 days 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])

shagie 20 days 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?
- tayo42 20 days ago
  
  Did you try it? If it doesn't work there's also called once if you scroll up on the doc
  And as far as whether it's a good idea or not, I generally wouldn't, but was saying when it is important then you do have these tools available,llms aren't the first thing to check for these mistakes. It's up to the engineer to choose between trade offs for your scenario.
  
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