How to use spec when mocking data classes in Python

Question

I'm trying to port our namedtuple classes into dataclass in Python 3.6 using the backport package. However, I noticed when mocking dataclass classes, you cannot use the "spec" keyword anymore. I assume it's because the dataclass code is auto generated.

from dataclasses import dataclass
import mock

@dataclass
class A:
    aaa: str
    bbb: int


m = mock.Mock(spec=A)

m.aaa

And this is the error I get:

AttributeError: Mock object has no attribute 'aaa'

Any idea if there's any way to automatically set all the attributes from original object to the mock object? I have lots of data classes with a lot of data. It's going to be really tedious if I try to manually set the values one by one.

Answer 1

I ended up using this generic helper function to achieve what spec does with regular classes:

import mock
from dataclasses import fields


def create_dataclass_mock(obj):
    return mock.Mock(spec=[field.name for field in fields(obj)])