CODEWITHSUNDEEP
pythonpython-2.7frozenset
xupeng
xupeng

Reputation: 67

In python, frozenset's subclass's __init__ method throw TypeError of arguments number

The class's __init__ method has 3 arguments, but when I instantiate it with 3 arguments, it throw an error that it expected 1 arguments. I cannot undrestand. 

class ArrObj(frozenset):
    def __init__(self, elem_list, elem_count, self_count):
        super(ArrObj, self).__init__(elem_list)  # Enums, ArrObj, race_id
        self.elem_count = elem_count
        self.self_count = self_count
        assert self_count > 0


if __name__ == '__main__':
    a = ArrObj(['a', 'b', 'c'], {'a':1, 'b':2, 'c':3}, 8)

Traceback (most recent call last):
  File "G:/pycharm-projects/new_keyinfo/verify_treekeys.py", line 34, in <module>
    a = ArrObj(['a', 'b', 'c'], {'a':1, 'b':2, 'c':3}, 8)
TypeError: ArrObj expected at most 1 arguments, got 3

Upvotes: 2

Views: 155

Answers (1)

chepner
chepner

Reputation: 531235

frozenset.__init__ doesn't take additional arguments, because you can't modify a frozenset after it has been created. (In fact, frozenset doesn't define __init__ at all; it just uses the __init__ it inherits from object.) The iterable you pass to frozenset is consumed by frozenset.__new__ instead.

class ArrObj(frozenset):
    def __new__(cls, elem_list, elem_count, self_count):
        # May as well assert this before you do any more work
        assert self_count > 0

        obj = super().__new__(cls, elem_list)
        obj.elem_count = elem_count
        obj.self_count = self_count
        return obj

Upvotes: 2

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