Problem with Sequence command

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Computation of gcd of objects of a class a created fails. I investigated the case and traced down that the problem lies in Sequence command. Consider the following example class:

class foo(object) :
    def __init__(self, L_) :
        self.L = deepcopy(L_)

    def __getitem__(self, j) :
        return self.L[j]

    def size(self) :
        return len(self.L)

Create and example object:

a = foo( [1,2,3] )
print a.size()

3

Build a nested object:

A = foo([ foo([1,2,3]), foo([2,4,8]), foo([1,1]), foo([1]) ])
print A.size()

4

Everything works as expected. So let's iterate over A:

for a in A :
    print a.size()

3 3 2 1

So far so good. Let's use Sequence as gcd internally does:

S = Sequence(A)
for s in S :
    print s.size()

TypeError: object of type 'foo' has no len()

And boom! Something went wrong. Try to call len explicitly:

for s in S :
    print len(s.L)

TypeError: object of type 'foo' has no len()

The same error! Surprisingly the following code actually works (but IMHO it should not):

for s in S :
    print len(s.L.L)

3 3 2 1

It seems like something strange happens in initialization. When creating a sequence, all objects get initialized by themselves.

Ok. So here is my question: what's up? Do I do something wrong or is this a bug in Sequence? I know that Python does not provide copy constructors, hence how to initialize objects to avoid this kind of problems?

Thanks in advance, Przemek