Python unit test that nothing was executed

Question

In a method that looks like:

def my_method(p):
    if p == 5:
        return
    elif p == 6:
        do_something()
        return
    else:
        do_something_different()
        return

How do I use Python unit test to check that when I call my_method(5) nothing extra is executed or changed outside my_method? I know that I can mock do_something and do_something_different and use assert_not_called but, is there a way to do it that only considers the if p == 5: part?

How could you ever tell without checking all memory that the method could potentially affect? If you're testing it as a black box, you can't. If you have a white box, use your common sense. — Mad Physicist
– Mad Physicist, Commented Jan 20, 2022 at 15:53
If you restrict the question to something more sensible, like do*() all affect some limited portion of the state, you can verify that that portion does not change. But "no side effects at all" is a tall order to prove satisfactorily. — Mad Physicist
– Mad Physicist, Commented Jan 20, 2022 at 15:55
Any chance that you actually want to test if everything got executed? In that case take a look at coverage. In any case: With code-coverage you'll see what wasn't executed. — Stefan Falk
– Stefan Falk, Commented Jan 20, 2022 at 15:55
There's a joke in the Haskell community that a function that returns an IO value could do anything, including launch nuclear missiles. Here you have a function which is called only for its side effect; there are too many things that my_method could leave undone to check for all of them. — chepner
– chepner, Commented Jan 20, 2022 at 16:25

Stefan Falk · Accepted Answer · 2022-01-20 16:15:05Z

"nothing extra is executed .."

Using coverage you can see what has been executed and therefore also see what has not been executed.

You'll get a report and see what your tests didn't cover. For example:

$ coverage report -m
Name                      Stmts   Miss  Cover   Missing
-------------------------------------------------------
my_program.py                20      4    80%   33-35, 39
my_other_module.py           56      6    89%   17-23
-------------------------------------------------------
TOTAL                        76     10    87%

It also supports the generation of HTML files so you get a visualization too if needed. In the docs find this example.

If you run my_method(5), coverage will therefore show you what parts of your program weren't executed.

".. or changed outside my_method"

I have never done something like this and I am not sure if it's possible in a sensible way but I found this question which refers to tracemalloc. Maybe this module can help you.

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