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The problem isn't with filter() which correctly calls iter() in 
both cases.

Tuple object have their own iterator which loops over 
elements directly and has no intervening calls to 
__getitem__().

String objects do not define a custom iterator, so iter() 
wraps itself around consecutive calls to __getitem__().

The resolution is to provide string objects with their own 
iterator. As a side benefit, iteration will run just a tiny bit 
faster.  The same applies to unicode objects.

Guido, do you care about this and want me to fix it or 
would you like to close it as "won't fix".