Implement custom assert for string matching

Question

I'm implementing a unittest to check if certain strings in an application start with a legal prefix. For example as a test function body I now have:

strings_to_check = ['ID_PRIMARY','ID_FOREIGN','OBJ_NAME', 'SOMETHING_ELSE']
for s in strings_to_check:
    assert s.startswith('ID_') or\
           s.startswith('OBJ_')

But the AssertionError that is returned is quite verbose (the real code has more legal prefix option). I found this in the documentation, but that focusses on assertion between (custom) objects. Is there a way to write you own custom assertion function that returns a easier to read message?

Just writing assert s.startswith(...)... , f"{s} shall start with valid prefix" is not sufficient? What is the output you want to see? — MrBean Bremen
– MrBean Bremen, Commented Feb 10, 2020 at 16:17

Jack Thomson · Accepted Answer · 2020-02-10 20:36:06Z

2

You could do something like this:

def test_myex1(myfixture):
    myfixture.append(1)
    strings_to_check = ['ID_PRIMARY','ID_FOREIGN','OBJ_NAME', 'SOMETHING_ELSE']
    for s in strings_to_check:
        failing_string = f'variable s: {s} does not start with valid string'
        assert s.startswith('ID_') or s.startswith('OBJ_'), failing_string

Which produces a traceback like:

>           assert s.startswith('ID_') or s.startswith('OBJ_'), failing_string
E           AssertionError: variable s: SOMETHING_ELSE does not start with valid string

raisetest.py:6: AssertionError

answered Feb 10, 2020 at 20:36

Jack Thomson

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1 Comment

Peter Smit Over a year ago

Ah nice, I didn't know you could add a custom text to the assert statement. I also found this question where a custom function is written that raises AssertionError: stackoverflow.com/questions/6655724/…

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