Interfaces in Python

January 29, 2014

There’s been a long history of proposals and disagreement over interfaces in Python. I’m going to ignore all of that and show one way to utilize interfaces.

An interface can be a class from which implementing classes inherit:

# interfaces.py:
class CountFishInterface(object):
    def one(self, *args, **kwargs):
        raise NotImplementedError

    def two(self, *args, **kwargs):
        raise NotImplementedError


class ColorFishInterface(object):
    def red(self, *args, **kwargs):
        raise NotImplementedError

    def blue(self, *args, **kwargs):
        raise NotImplementedError


# models.py:
from interfaces import CountFishInterface, ColorFishInterface


class OurFish(CountFishInterface, ColorFishInterface):
    pass

Now, OurFish doesn’t yet implement the interface. Before we do that, let’s add some tests. Note that since multiple classes may implement our interfaces, we make abstract tests for each interface.

# tests.py:
from unittest import TestCase
from interfaces import CountFishInterface, ColorFishInterface
from models import OurFish


class AbstractTestCountFishInterface(object):
    def test_one(self):
        try:
            self.obj.one()
        except NotImplementedError:
            self.fail(str(type(self.obj)) + "does not implement one")

    def test_two(self):
        try:
            self.obj.two()
        except NotImplementedError:
            self.fail(str(type(self.obj)) + "does not implement two")


class AbstractTestColorFishInterface(object):
    def test_red(self):
        try:
            self.obj.red()
        except NotImplementedError:
            self.fail(str(type(self.obj)) + "does not implement red")

    def test_blue(self):
        try:
            self.obj.blue()
        except NotImplementedError:
            self.fail(str(type(self.obj)) + "does not implement blue")


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

Now our tests should fail! Let’s implement the interface in OurFish:

# models.py:
from interfaces import CountFishInterface, ColorFishInterface


class OurFish(CountFishInterface, ColorFishInterface):
    def one(self):
        return 1

    def two(self):
        return 2

    def red(self):
        return "#FF0000"

    def blue(self):
        return "#0000FF"

Now our tests should pass!

Declaring new interfaces and writing the tests is quite verbose. Here’s a simpler way of declaring new interfaces:

# 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)


ColorFishInterface = Interface("ColorFishInterface", ["red", "blue"])

That’s not that pythonic looking, but here’s another way to do it:¹

# interfaces.py
interfaces = {
    "CountFishInterface": ["one", "two"],
    "ColorFishInterface": ["red", "blue"],
}

for interface_name, methods in interfaces.iteritems():
    globals()[interface_name] = Interface(interface_name, methods)

Still messy, but it makes it easy to add more interfaces.

I’ll leave refactoring the test cases as an exercise to the reader. Beyond moving the try-block into a helper method, the best solution I can presently come up with is code-generation.

Update: No one noticed, but I was originally missing the call to iteritems!↩︎

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