Re: Bug in Python set

[Aahz]

In article 0bd314a8-db65-43f1-a999-521e2ed71...@n15g2000yqf.googlegroups.com,
dmitrey  dmitrey.kros...@scipy.org wrote:

Python 2.6.5 r265:79063
set().update(set()) is None
True
while I expect result of update to be set.
Also, result of
set().add(None)
is None while I expect it to be set with element None (or, maybe, it
should be empty set?)

Why are you assuming that your expectations are correct?  Generally
speaking, Python methods that mutate do *not* return the original object,
precisely to make sure you don't make stupid mistakes.

You should probably read this:

http://www.catb.org/~esr/faqs/smart-questions.html
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Aahz (a...@pythoncraft.com)   * http://www.pythoncraft.com/

It is easier to optimize correct code than to correct optimized code.
--Bill Harlan
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Re: Bug in Python set

[Steven D'Aprano]

On Sun, 02 May 2010 05:11:40 -0700, dmitrey wrote:

 Python 2.6.5 r265:79063
set().update(set()) is None
 True
 while I expect result of update to be set.

Change your expectations. Generally, methods which modify the object 
rather than creating a new one return None.

 s = set([1,2,3])
 s.update(set([3, 4, 5]))
 s
{1, 2, 3, 4, 5}


 Also, result of set().add(None) is None while I expect it to be set
 with element None (or, maybe, it should be empty set?)

 s = set()
 s.add(None)
 s
{None}


Python sets have been used by tens of thousands of programmers for many 
years now. Which do you think is more likely?

(1) Not one person before you noticed that something as fundamental as 
adding an item to a set is buggy;

or

(2) You have misunderstood what is happening?



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Steven
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Re: Bug in Python set

[Terry Reedy]

On 5/2/2010 8:11 AM, dmitrey wrote:

Python 2.6.5 r265:79063

set().update(set()) is None

True
while I expect result of update to be set.
Also, result of
set().add(None)
is None while I expect it to be set with element None (or, maybe, it
should be empty set?)


'Expect' has two different meanings in this context.
1. The empirical behavior surprised me (because I did not bother to read 
the manual, which clearly says what the returns are).
2. The documented behavior, which I read, surprises me, because I would 
have designed things differently, perhaps because I have used other 
languages designed differently.


I am not sure which you meant.



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