Testing Interfaces in Python

January 30, 2014

In yesterday’s post, I proposed a way to write and test interfaces in Python. Testing these interfaces was quite verbose. I left refactoring that testing as an exercise to the reader. Then I decided to do that exercise. Here’s a neat way to generate interfaces and abstract tests given a dictionary mapping interface names to a list of method names:

# interfaces.py
def Interface(interface_name, method_names):
    def interface_helper(*args, **kwargs):
        raise NotImplementedError

    methods = {method_name: interface_helper for method_name in method_names}
    return type(interface_name, (object,), methods)


def AbstractInterfaceTest(test_name, method_names):
    def abstract_interface_test_helper(method_name):
        def test_method(self):
            try:
                getattr(self.obj, method_name)()
            except NotImplementedError:
                self.fail(
                    type(self.obj).__name__ + " does not implement " + method_name
                )

        return test_method

    methods = {
        "test_" + method_name: abstract_interface_test_helper(method_name)
        for method_name in method_names
    }
    return type(test_name, (object,), methods)


interfaces = {"CountFish": ["one", "two"], "ColorFish": ["red", "blue"]}

for interface_name, methods in interfaces.iteritems():
    interface_name += "Interface"
    globals()[interface_name] = Interface(interface_name, methods)
    test_name = "AbstractTest" + interface_name
    globals()[test_name] = AbstractInterfaceTest(test_name, methods)

In order to use this with the other code from yesterday, we’d have to update tests.py as well:

# tests.py
from unittest import TestCase, main
from interfaces import (
    CountFishInterface,
    ColorFishInterface,
    AbstractTestColorFishInterface,
    AbstractTestCountFishInterface,
)
from models import OurFish


class TestOurFish(
    AbstractTestCountFishInterface, AbstractTestColorFishInterface, TestCase
):
    def setUp(self):
        self.obj = OurFish()


if __name__ == "__main__":
    main()

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