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Showing content from http://mail.python.org/pipermail/python-dev/attachments/20110406/79246962/attachment-0001.html below:

<div class="gmail_quote">On Tue, Apr 5, 2011 at 05:01, Nick Coghlan <<a href="mailto:ncoghlan@gmail.com">ncoghlan@gmail.com</a>> wrote: <blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;">

<div class="im">On Tue, Apr 5, 2011 at 9:46 AM, Brett Cannon <<a href="mailto:brett@python.org">brett@python.org</a>> wrote:

> Â Â Â try: 
> Â Â Â Â Â Â Â c_heapq.heappop(Spam()) 
> Â Â Â except TypeError: 
> Â Â Â Â Â Â Â # "heap argument must be a list" 
> Â Â Â Â Â Â Â pass 
> 
> Â Â Â try: 
> Â Â Â Â Â Â Â py_heapq.heappop(Spam()) 
> Â Â Â except AttributeError: 
> Â Â Â Â Â Â Â # "'Foo' object has no attribute 'pop'" 
> Â Â Â Â Â Â Â 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. 
 
</div>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. 
 
However, there actually *is* a significant semantic discrepancy in the 
heapq case, which is that py_heapq is duck-typed, while c_heapq is 
<div class="im">not: </div></blockquote><div> That's true. I will re-word it to point that out. The example code still shows it, I just didn't explicitly state that in the example. -Brett Â </div><blockquote class="gmail_quote" style="margin: 0pt 0pt 0pt 0.8ex; border-left: 1px solid rgb(204, 204, 204); padding-left: 1ex;">

>>> from test.support import import_fresh_module 
>>> c_heapq = import_fresh_module('heapq', fresh=['_heapq']) 
>>> py_heapq = import_fresh_module('heapq', blocked=['_heapq']) 
</div>>>> from collections import UserList 
>>> class Seq(UserList): pass 
... 
>>> c_heapq.heappop(UserList()) 
Traceback (most recent call last): 
Â File "<stdin>", line 1, in <module> 
<div class="im">TypeError: heap argument must be a list 
</div>>>> py_heapq.heappop(UserList()) 
Traceback (most recent call last): 
Â File "<stdin>", line 1, in <module> 
Â File "/home/ncoghlan/devel/py3k/Lib/heapq.py", line 140, in heappop 
Â Â lastelt = heap.pop() Â Â # raises appropriate IndexError if heap is empty 
Â File "/home/ncoghlan/devel/py3k/Lib/collections/__init__.py", line 848, in pop 
Â Â 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): 
Â File "<stdin>", line 1, in <module> 
TypeError: 'int' object is not callable 
>>> with 1: pass 
... 
Traceback (most recent call last): 
Â File "<stdin>", line 1, in <module> 
AttributeError: 'int' object has no attribute '__exit__' 
 
 
 
Cheers, 
Nick. 
 
-- 
Nick CoghlanÂ Â |Â Â <a href="mailto:ncoghlan@gmail.com">ncoghlan@gmail.com</a>Â Â |Â Â Brisbane, Australia 
</blockquote></div>

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