How to use assertRaisesRegex in this situation?

I'm trying to use assertRaisesRegex (from the unittest module) to ensure that an exception (from our C++ backend) is raised when performing the following assignment:

a[:, 0] = b[:, 0]

a and b here are custom types defined in our backend.

In the cases I've seen assertRaisesRegex used, a function along with its arguments is passed into assertRaisesRegex, but in this case I need to test that the exception is raised during the above assignment. How can I best do that without having to write an additional function to do the assignment?

import unittest


class SomeSpecificError(Exception):
    pass


class OtherError(Exception):
    pass


class FakeData:
    def __getitem__(self, key):
        raise SomeSpecificError(f"can't get slice {key!r}")


class MyTestCase(unittest.TestCase):
    def test(self):
        a = FakeData()
        b = FakeData()

        with self.assertRaisesRegex(SomeSpecificError, "can't get slice"):
            a[:, 0] = b[:, 0]
        # passes

        with self.assertRaisesRegex(SomeSpecificError, "something something"):  #
            a[:, 0] = b[:, 0]
        # fails:
        # AssertionError: "something something" does not match "can't get slice (slice(None, None, None), 0)"

        with self.assertRaisesRegex(OtherError, "something something"):  #
            "no exception raised in this block"
        # fails:
        # AssertionError: OtherError not raised

Obviously, substitute my FakeData with your own actual class, mine is for testing (like a mock).