Why does defining __getitem__ on a class make it iterable?

For instance if I write:

class B:
    def __getitem__(self, k):
        return k
    
cb = B()
        
for k in cb:
    print k

I get the output:

0
1
2
3
4
5
...

I would really expect to see an error returned from for k in cb:.

python iterator overloading
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