# A class that breaks singledispatch

class MetaA(type):
    def __len__(self):
        return 0

class A(metaclass=MetaA):
    pass

class AA(A):
    pass

from functools import singledispatch

@singledispatch
def fun(a):
    print('base A')

@fun.register(A)
def _(a):
    print('fun A')

aa = AA()

# This should print
# fun A
fun(aa)