Python recursive dict merge

dictmerge.py

def merge(d1: dict, d2: dict) -> dict:
    """Return a new dict instance containing the recursive
    merge of d2 into d1. Neither input is mutated.
    """
    d1 = deepcopy(d1)
    d2 = deepcopy(d2)
    for k2, v2 in d2.items():
        if isinstance(v2, dict) and isinstance(v1 := d1.get(k2), dict):
            v2 = merge(v1, v2)
        d1[k2] = v2
    return d1
  
def tests():
    cases = [
        ({}, {}, {}),
        ({0:0}, {}, {0:0}),
        ({0:0}, {0: 1}, {0: 1}),
        ({0:0, 1:1}, {0: 1}, {0: 1, 1:1}),
        ({'a':0}, {'a': {'b': 0}}, {'a': {'b': 0}}),
        ({'a':{'b': None, 'c': 0}}, {'a': {'b': 0}}, {'a': {'b': 0, 'c': 0}}),
    ]
    for i, (d1, d2, expected) in enumerate(cases):
        try:
            actual = merge(d1, d2)
            assert(actual == expected)
        except AssertionError:
            print(f"test case {i} failed")
            print("Expected:")
            print(expected)
            print("Actual:")
            print(actual)
        else:
            print(f"test case {i} passed")