class Var:
    def __init__(self, obj=None):
        self.obj=obj

    def __getattr__(self, attr):
        return getattr(self.obj, attr)

v1 = Var([1,2,3])

print(v1.__len__())
# output: 3

print(len(v1))
# output: TypeError: object of type 'Var' has no len()

class Var:
    def __init__(self, obj=None):
        self.obj=obj

    def __getattr__(self, attr):
        return getattr(self.obj, attr)

    def __len__(self):
        return len(self.obj)

v1 = Var([1,2,3])

print(v1.__len__())
print(len(v1))

pawel@pawel-XPS-15-9570:~/test$ python len.py
3
3