fix: keepdims passed through 'min' correctly

[allenlawrence94]

Description

The min atom ignores the keepdims flag.

Type of change

• New feature (backwards compatible)
• New feature (breaking API changes)
• Bug fix
• Other (Documentation, CI, ...)

Contribution checklist

• Add our license to new files.
• Check that your code adheres to our coding style.
• Write unittests.
• Run the unittests and check that they’re passing.
• Run the benchmarks to make sure your change doesn’t introduce a regression.

[SteveDiamond]

Good catch!

[SteveDiamond]

We should add a test that would fail without this fix.

[phschiele]

Thanks @allenlawrence94! Would you mind adding a simple unit test as well?

[allenlawrence94]

@phschiele - sure! Are there unit tests for the canonicalizers? Or would we rather just have a test on the min atom keepdims=True?

[SteveDiamond]

@phschiele - sure! Are there unit tests for the canonicalizers? Or would we rather just have a test on the min atom keepdims=True?

Do a test on the min atom. i.e. construct a problem then solve it that requires passing through keepdims=True

[SteveDiamond]

@allenlawrence94 please update the test_min function in test_atoms to the following code. We are trying to merge all the outstanding bugfix MRs.

    def test_min(self) -> None:
        """Test min.
        """
        # One arg, test sign.
        self.assertEqual(cp.min(1).sign, s.NONNEG)
        self.assertEqual(cp.min(-2).sign, s.NONPOS)
        self.assertEqual(cp.min(Variable()).sign, s.UNKNOWN)
        self.assertEqual(cp.min(0).sign, s.ZERO)

        # Test with axis argument.
        self.assertEqual(cp.min(Variable(2), axis=0).shape, tuple())
        self.assertEqual(cp.min(Variable(2), axis=1).shape, (2,))
        self.assertEqual(cp.min(Variable((2, 3)), axis=0).shape, (3,))
        self.assertEqual(cp.min(Variable((2, 3)), axis=1).shape, (2,))

        # Invalid axis.
        with self.assertRaises(Exception) as cm:
            cp.min(self.x, axis=4)
        self.assertEqual(str(cm.exception), "Invalid argument for axis.")
        with self.assertRaises(ValueError) as cm:
            cp.min(self.x, self.x)  # a common erroneous use-case
        self.assertEqual(str(cm.exception), cp.min.__EXPR_AXIS_ERROR__)

        # Test canonicalization with keepdims=True
        # https://github.com/cvxpy/cvxpy/pull/2419
        X = cp.Variable((2, 3))
        X_val = np.arange(6).reshape((2, 3))
        c = np.ones((1, 3))
        expr = cp.min(X, axis=0, keepdims=True)
        obj = cp.Maximize(cp.sum(expr + c))
        prob = cp.Problem(obj, [X == X_val])
        prob.solve()

[SteveDiamond]

I can't push to your fork for some reason, and we like to keep track of contributors so would rather not make a new PR.

[SteveDiamond]

Picked up in #2431