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

[noreply@sourceforge.net]

Bugs item #448153, was opened at 2001-08-05 06:27
You can respond by visiting: 
http://sourceforge.net/tracker/?func=detail&atid=105470&aid=448153&group_id=5470

Category: Python Library
Group: Python 2.2
>Status: Closed
>Resolution: Fixed
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.


----------------------------------------------------------------------

>Comment By: Guido van Rossum (gvanrossum)
Date: 2001-08-07 10:53

Message:
Logged In: YES 
user_id=6380

Closed by adding an IterableUserDict to UserDict.py.


----------------------------------------------------------------------

Comment By: Martin v. Löwis (loewis)
Date: 2001-08-06 23:16

Message:
Logged In: YES 
user_id=21627

I've already fixed it in PyXML, although the only way I 
could find to fix it was to add an __iter__; I cannot take 
away the __getitem__ since the code has to work in older 
Python versions.

The solution I would prefer to see is that UserDict offers 
a second class, say IterableUserDict, which would be for 
educational purposes only; anybody who really wants a 
dictionary that you can iterate over should inherit from 
dictionary.

That also raises the question whether UserDict 
applications in the standard library should be changed to 
inherit from dictionary. The major incompatibility I can 
see with doing so is that the .data member goes away.


----------------------------------------------------------------------

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

Message:
Logged In: YES 
user_id=6380

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. :-)


----------------------------------------------------------------------

You can respond by visiting: 
http://sourceforge.net/tracker/?func=detail&atid=105470&aid=448153&group_id=5470