switchy.py

from contextlib import contextmanager

@contextmanager
def switch(value):
    yield Case(value)

class Case(object):
    def __init__(self, value):
        self.value = value
    
    def __call__(self, value):
        return self.value == value

test.py

import unittest

from switchy import switch


class TestSwitch(unittest.TestCase):
    def test(self):
        with switch('a') as case:
            self.assertFalse(case('b'))
            self.assertTrue(case('a'))

        with switch(None) as case:
            self.assertTrue(case(None))

    def test_nested(self):
        with switch('a') as case1:
            with switch('b') as case2:
                self.assertTrue(case1('a'))
                self.assertTrue(case2('b'))
                self.assertFalse(case2('a'))
                self.assertFalse(case1('b'))

if __name__ == '__main__':
    unittest.main()