Deep merge two Python dicts with dict comprehensions

merge.py

def merge(original: dict, update: dict):
    return {
        k: (merge(original[k], v) if M and isinstance(v, dict) and k in original else v)
        for d, M in ((original, False), (update, True))
        for k, v in d.items()
    }


if __name__ == "__main__":
    base = {"foo": 1, "bar": {"abc": 123, "def": 456}, "baz": 2}
    ext = {"foo": 11, "bar": {"abc": 321}}
    assert merge(base, ext) == {
        "foo": 11,
        "bar": {"abc": 321, "def": 456},
        "baz": 2,
    }, f"merge({base}, {ext})"