Python: Can a subclass of float take extra arguments in its constructor?

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As float is immutable you have to overwrite __new__ as well. The following should do what you want:

class Foo(float):
    def __new__(self, value, extra):
        return float.__new__(self, value)
    def __init__(self, value, extra):
        float.__init__(value)
        self.extra = extra

foo = Foo(1,2)
print(str(foo))
1.0
print(str(foo.extra))
2

See also Sub-classing float type in Python, fails to catch exception in __init__()

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