[Python-bugs-list] [ python-Bugs-448153 ] Dictionary iterators break old code

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Bugs item #448153, was opened at 2001-08-05 06:27
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>Category: Python Library
>Group: Python 2.2
Status: Open
Resolution: None
>Priority: 5
Submitted By: Martin v. Löwis (loewis)
>Assigned to: Martin v. Löwis (loewis)
Summary: Dictionary iterators break old code

Initial Comment:
With the introduction of dictionary iterators, 
iteration over UserDict specializations may silently 
break existing code. The attached code is a 
simplification of the xml.dom.NamedNodeMap class, 
which provides iteration over the values using a 
well-defined order, which is implemented in 
__getitem__. In Python 2.2a1, __getitem__ is not 
called anymore, thus the code breaks.


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>Comment By: Guido van Rossum (gvanrossum)
Date: 2001-08-06 13:59

Message:
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Aaaaaaargh!!!!!!!

You mean somebody wrote a subclass of UserDict that attempts
to behave like a sequence by checking the type of the
__getitem__ argument??????????

Yuck!!!!!!!!!!!!!!!

You XML weenies are sickos. :-)

Now that I've got that out of the way :-),

Short of totally dropping dictionary iterators, the only
solution I can see it to remove the __iter__ method of
UserDict. That is backwards compatible, but it removes the
ability to iterate over UserDict instances (except for the
sick ones :-), thus making UserDict less like a dictionary.

But since UserDict is likely to become deprecated anyway in
favor of subclassing from the 'dictionary' built-in type,
this solution seems acceptable -- but only if this trick is
also in use outside the XML code.  I would prefer to see the
XML code fixed, since it can now properly define __iter__ to
iterate over whatever it wants to in any sequence it would
like. :-)


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