[ python-Bugs-1522016 ] filter() doesn't use __len__ of str/unicode/tuple subclasses

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Bugs item #1522016, was opened at 2006-07-13 15:25
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Category: Python Interpreter Core
Group: Python 2.5
Status: Open
Resolution: None
Priority: 5
Submitted By: Collin Winter (collinwinter)
Assigned to: Nobody/Anonymous (nobody)
Summary: filter() doesn't use __len__ of str/unicode/tuple subclasses

Initial Comment:
Consider the following code:

>>> class badstr(str):
...     def __len__(self):
...         return 6
...     def __getitem__(self, index):
...         return "a"
...
>>> filter(None, badstr("bbb"))
'aaa'

I would have expected the answer to be 'aaaaaa'.

The cause for this is that
Python/bltinmodule.c:filter{string,unicode,tuple} all
call PyString_Size (or the appropriate equivalent),
even if the sequence is a subclass of one of these
types. This bypasses any overloading of __len__ done by
the subclass, as demonstrated above.

If filter() is going to respect the subclass's
__getitem__ overload, it should also respect
overloading of __len__. The attached patch corrects this.


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