On Tue, Apr 5, 2011 at 05:01, Nick Coghlan <ncogh...@gmail.com> wrote:
> On Tue, Apr 5, 2011 at 9:46 AM, Brett Cannon <br...@python.org> wrote:
> > try:
> > c_heapq.heappop(Spam())
> > except TypeError:
> > # "heap argument must be a list"
> > pass
> >
> > try:
> > py_heapq.heappop(Spam())
> > except AttributeError:
> > # "'Foo' object has no attribute 'pop'"
> > pass
> >
> > This kind of divergence is a problem for users as they unwittingly
> > write code that is CPython-specific. This is also an issue for other
> > VM teams as they have to deal with bug reports from users thinking
> > that they incorrectly implemented the module when in fact it was
> > caused by an untested case.
>
> While I agree with the PEP in principle, I disagree with the way this
> example is written. Guido has stated in the past that code simply
> *cannot* rely on TypeError being consistently thrown instead of
> AttributeError (or vice-versa) when it comes to duck-typing. Code that
> cares which of the two is thrown is wrong.
>
Which is unfortunate since least common base class is Exception. But I can
add a note to the PEP saying that this is the case and change the example.
>
> However, there actually *is* a significant semantic discrepancy in the
> heapq case, which is that py_heapq is duck-typed, while c_heapq is
> not:
>
> >>> from test.support import import_fresh_module
> >>> c_heapq = import_fresh_module('heapq', fresh=['_heapq'])
> >>> py_heapq = import_fresh_module('heapq', blocked=['_heapq'])
> >>> from collections import UserList
> >>> class Seq(UserList): pass
> ...
> >>> c_heapq.heappop(UserList())
> Traceback (most recent call last):
> File "<stdin>", line 1, in <module>
> TypeError: heap argument must be a list
> >>> py_heapq.heappop(UserList())
> Traceback (most recent call last):
> File "<stdin>", line 1, in <module>
> File "/home/ncoghlan/devel/py3k/Lib/heapq.py", line 140, in heappop
> lastelt = heap.pop() # raises appropriate IndexError if heap is empty
> File "/home/ncoghlan/devel/py3k/Lib/collections/__init__.py", line 848, in
> pop
> def pop(self, i=-1): return self.data.pop(i)
> IndexError: pop from empty list
>
> Cheers,
> Nick.
>
> P.S. The reason I was bugging Guido to answer the TypeError vs
> AttributeError question in the first place was to find out whether or
> not I needed to get rid of the following gross inconsistency in the
> behaviour of the with statement relative to other language constructs:
>
> >>> 1()
> Traceback (most recent call last):
> File "<stdin>", line 1, in <module>
> TypeError: 'int' object is not callable
> >>> with 1: pass
> ...
> Traceback (most recent call last):
> File "<stdin>", line 1, in <module>
> AttributeError: 'int' object has no attribute '__exit__'
>
>
>
> Cheers,
> Nick.
>
> --
> Nick Coghlan | ncogh...@gmail.com | Brisbane, Australia
>
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