Steven D'Aprano wrote: > I was writing some tests for a mapping class I have made, and I decided > to run those same tests over dict and UserDict. The built-in dict passed > all the tests, but UserDict failed one: > > class SimpleMappingTest(unittest.TestCase): > type2test = UserDict.UserDict > def test_iter(self): > k, v = [0, 1, 2, 3], 'abcd' > m = self.type2test(zip(k, v)) > it = iter(m) > self.assert_(iter(it) is it) > self.assertEquals(sorted(it), k) # This line fails. > # many more tests > > To cut a long story short, the problem is that UserDict doesn't support > the modern iteration protocol. Python falls back on the old-fashioned > __getitem__ iteration protocol, but since it expects IndexError rather > than KeyError, iteration fails for UserDict once it hits key=4. > > If I look at the source code for the UserDict module, I discover that > there's a second mapping class, IterableUserDict, specifically to make > UserDict iterable. To do this requires a single one-line method: > > class IterableUserDict(UserDict): > def __iter__(self): > return iter(self.data) > > This class was apparently added to the module in 2.2 -- it doesn't exist > in 2.1. > > Now that boggles my brain. Why was support for iteration added as a > subclass, instead of simply adding the __iter__ method to UserDict? > UserDict is supposed to be a drop-in replacement for dict (although the > use-cases for it are much fewer now that we can inherit from dict), so it > doesn't make sense to me to have a non-iterable UserDict plus a subclass > which is iterable. > > Can anyone shed any light on this apparently bizarre design decision?
I looked it up for you: http://svn.python.org/view/python/trunk/Lib/UserDict.py?view=log """ Revision 22248 - (view) (download) (as text) (annotate) - [select for diffs] Modified Tue Aug 7 17:40:42 2001 UTC (9 years, 1 month ago) by gvanrossum File length: 1913 byte(s) Diff to previous 21167 Remove the __iter__ method from the UserDict class -- it can silently break old code (in extreme cases). See SF bug #448153. Add a new subclass IterableUserDict that has the __iter__ method. Note that for new projects, unless backwards compatibility with pre-2.2 Python is required, subclassing 'dictionary' is recommended; UserDict might become deprecated. """ The bug report is also interesting: http://bugs.python.org/issue448153 Peter -- http://mail.python.org/mailman/listinfo/python-list
