Unexpected behavior when calling super().__repr__() on a subclass of set?

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See edit below for an explanation of this behavior (in CPython)

In Python 3.9.5, I'm creating two custom collection types, one inheriting from list and the other from set. I want them to have custom __repr__ methods. Simple example:

class MyList(list):

    def __repr__(self) -> str:
        return f'MyList({super().__repr__()})'


class MySet(set):

    def __repr__(self) -> str:
        return f'MySet({super().__repr__()})'

Note the __repr__() definitions are identical except for the initial string prefix. But when I print them, I get different results:

ml = MyList('abc')
print(ml) # MyList(['a', 'b', 'c'])

ms = MySet('abc')
print(ms) # MySet(MySet({'c', 'a', 'b'}))

The display of MyList is what I would expect. But for some reason MySet is printed twice, as if the __repr__ is being called recursively or something. Does anyone know what's going on, or how best to get it to show MySet({'c', 'a', 'b'}), instead of MySet(MySet({'c', 'a', 'b'}))?

[Edit] As of 3.9.5, in https://github.com/python/cpython/blob/main/Objects/setobject.c function set_repr has the code:

    if (!PySet_CheckExact(so))
        result = PyUnicode_FromFormat("%s({%U})",
                                      Py_TYPE(so)->tp_name,
                                      listrepr);
    else
        result = PyUnicode_FromFormat("{%U}", listrepr);

In other words, set is hardcoded so that its __repr__ omits the class name if the type is exactly a set and the set is non-empty, but shows the class name otherwise. But dict and list don't do this, for some reason.

repr python
1 Answers

That is the default print of set, it prints the class name. If you remove the format you added it comes out exactly as you wanted.

class MySet(set):

    def __repr__(self) -> str:
        return super().__repr__()

if __name__ == '__main__':

    ms = MySet('abc')
    print(ms)
    MySet.__name__ = 'New_name'
    print(ms)

Oututput :

MySet({'b', 'c', 'a'})
New_name({'b', 'c', 'a'})
user_114206300
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