How to write python polyglot unittest for method using type function?

Question

I am trying to write python2 and python3 compatible code which uses type function. Both return different strings in 2 v/s 3, which I have to assert in my unit tests.

Python 3.7.4 (default, Oct 18 2019, 15:58:40)
>>> type(2)
<class 'int'>

Python 2.7.16 (default, Jul 24 2019, 16:45:12)
>>> type(2)
<type 'int'>

The intention here is to validate the exception messages raised from my function. For example,

def func(arg1):
    if not isinstance(arg1, int):
        raise TypeError('Expected type %s but got %s' % (type(int), type(arg1)))
        
    print("Function executed")
    
try:
    func('str')
except TypeError as e:
    assert e.args[0]== "Expected type <type 'type'> but got <type 'str'>"
    print("Correct exception was raised")

Assertion here passes in python2 but fails in python3.

@Davis I have updated my question for better clarity. Please have a re-look. Thanks for the help :) — Abhinav Gupta
– Abhinav Gupta, Commented Jun 24, 2020 at 14:06
I ran the code and got this error message AttributeError: 'TypeError' object has no attribute 'message' in python 3. This is because the attribute message from errors has been deprecated since python 2.6 according to: github.com/google/yapf/issues/564 It has been deprecated since python 2.6 and to look to: python.org/dev/peps/pep-0352 for further information. It seems like using e.message will not allow for the code to work in both versions. — Eno Gerguri
– Eno Gerguri, Commented Jun 24, 2020 at 14:18
@Eno Thanks for pointing that out. I've updated the line to be compatible with both versions. — Abhinav Gupta
– Abhinav Gupta, Commented Jun 24, 2020 at 22:43

Davis Herring · Accepted Answer · 2020-06-24 14:27:05Z

2

Variations like this are the motivation for the assertRaisesRegexp function from the standard unittest module (which you should probably be using). (Note the spelling: Python 3 supports the only spelling from 2 (with the p), so that’s what you’d want here.)

answered Jun 24, 2020 at 14:27

Davis Herring

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1 Comment

Abhinav Gupta Over a year ago

Thanks @Davis, this is what I was looking for. However, I've decided to wrap the error and override the message string for now. It would require simple changes in the unit tests.

Eno Gerguri · Accepted Answer · 2020-06-24 14:23:31Z

0

Using type() in python for almost anything is not flexible. Instead you should use isinstance(var, type). It is almost the same in both versions of python 2.7 and 3:

isinstance(2, int)

This returns True in both versions every time.

Make sure you do not accidentally put the type in a string:

INCORRECT: 'int'

CORRECT: int

OR:

print(type(2) is int)

will output:

True

edited Jun 24, 2020 at 14:23

answered Jun 24, 2020 at 14:00

Eno Gerguri

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2 Comments

Abhinav Gupta Over a year ago

Hi @Eno, I have updated my question. Can you please have a re-look? Thanks for helping out :)

Eno Gerguri Over a year ago

Yes sure I will update my answer as soon as I find out a solution with this new information you have provided.

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