class Metaclass(type):
    def __new__(metacls, name, bases, attrs):                                                                     
        try:                                                                   
            return attrs['length']()                                                                                                                                              
        except KeyError:                                                                          
            pass                                                               
 
        return type.__new__(metacls, name, bases, attrs)
 
def a(array):
    class MyClass(metaclass=Metaclass):
        def length():
            return len(array)
    return MyClass
 
print(a([1,2,3,4]).length()) #   4                                                  
print(a([1,2,3,4]))          #   4
 

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