Python interpreter bug

**Robert Kern** · Oct 7 '05, 03:25 PM

Re: Python interpreter bug

alainpoint@yaho o.fr wrote:[color=blue]
> In fact, i want to sort the list based on the 'allocated attribute' and
> at the same time, test membership based on the id attribute.
> __cmp__ logically implies an ordering test, not an identity test. These
> two notions seems to be confounded in python which is unfortunate. Two
> objects could have the same rank while still being different.[/color]

In that case, define __lt__ and __eq__ separately instead of __cmp__.
list.sort() will use __lt__ if it's provided rather than __cmp__.

--
Robert Kern
rkern@ucsd.edu

"In the fields of hell where the grass grows high
Are the graves of dreams allowed to die."
-- Richard Harter

**bruno modulix** · Oct 7 '05, 03:25 PM

Re: Python interpreter bug

alainpoint@yaho o.fr wrote:[color=blue]
> Sorry Fredrik but I don't understand. Just comment out the assert and
> you have different results depending on whether an unrelated sort
> function is defined.
> This seems weird to me ![/color]

code snippet:
[color=blue]
> from random import choice
> class OBJ:
> def __init__(self,i dentifier):
> self.id=identif ier
> self.allocated= 0 # Your problem begins here...[/color]
[color=blue]
> def __cmp__(self,ot her):[/color]
# it continues here[color=blue]
> return cmp(other.alloc ated,self.alloc ated)
> mylist=[OBJ(i) for i in range(20)]
> excluded=[obj for obj in mylist if obj.id>choice(r ange(20))]
> for obj in mylist:
> if obj in excluded: # and you see it in action here
> assert obj.id in [objt.id for objt in excluded]
> continue[/color]

How do you think the "membership " test works ? Could it be possible that
it uses the __cmp__ method ? And if so, how do you think your instances
will compare knowing that your __cmp__ method will return equality for
all the instances in this snippet ?

Just change your __init__ method to:

def __init__(self,i dentifier):
self.id=identif ier
self.allocated= identifier

and rerun the test.

I don't think this is a bug...

--
bruno desthuilliers
python -c "print '@'.join(['.'.join([w[::-1] for w in p.split('.')]) for
p in 'onurb@xiludom. gro'.split('@')])"

**Fredrik Lundh** · Oct 7 '05, 03:35 PM

Re: Python interpreter bug

alainpoint@yaho o.fr wrote:[color=blue]
> In fact, i want to sort the list based on the 'allocated attribute' and
> at the same time, test membership based on the id attribute.
> __cmp__ logically implies an ordering test[/color]

really?

http://dictionary.reference.com/search?q=compare

com·pare
v. com·pared, com·par·ing, com·pares
v. tr.
1. To consider or describe as similar, equal, or analogous; liken.
2. To examine in order to note the similarities or differences of.
/.../
[color=blue]
> not an identity test.[/color]

it's not an identity test. it's a comparision, and the "in" operator
uses it to determine *equality*, not identity. you need to read
up on object comparisions in the reference manual.

</F>

**alainpoint@yahoo.fr** · Oct 7 '05, 03:45 PM

Re: Python interpreter bug

No doubt you're right but common sense dictates that membership testing
would test identity not equality.
This is one of the rare occasions where Python defeats my common sense
;-(

Alain

**Christopher Subich** · Oct 7 '05, 03:55 PM

Re: Python interpreter bug

alainpoint@yaho o.fr wrote:[color=blue]
> No doubt you're right but common sense dictates that membership testing
> would test identity not equality.
> This is one of the rare occasions where Python defeats my common sense[/color]

But object identity is almost always a fairly ill-defined concept.
Consider this (Python 2.2, 'cuz that's what I have right now):

Python 2.2.3 (#1, Nov 12 2004, 13:02:04)
[GCC 3.2.3 20030502 (Red Hat Linux 3.2.3-42)] on linux2
Type "help", "copyright" , "credits" or "license" for more information.[color=blue][color=green][color=darkred]
>>> al = "ab"
>>> alp = al+"c"
>>> alphabet = "abc"
>>> al[/color][/color][/color]
'ab'[color=blue][color=green][color=darkred]
>>> alp[/color][/color][/color]
'abc'[color=blue][color=green][color=darkred]
>>> alphabet[/color][/color][/color]
'abc'[color=blue][color=green][color=darkred]
>>> alp is alphabet[/color][/color][/color]
0[color=blue][color=green][color=darkred]
>>> alp == alphabet[/color][/color][/color]
1 # True on Py2.4

The only reliable thing that object identity tells you is "these two
foos occupy the same memory location." Even for identical, immutable
objects, this may not be true (see case above) -- some immutables do end
up occupying the same memory location (try i=1;j=2;k=j-1;i is k), but
this is by no means guraranteed, and indeed only happens sometimes
because of optimizations.

**Fredrik Lundh** · Oct 7 '05, 04:25 PM

Re: Python interpreter bug

alainpoint@yaho o.fr wrote:
[color=blue]
> No doubt you're right but common sense dictates that membership testing
> would test identity not equality.[/color]

what does "common sense" have to say about this case:
[color=blue][color=green][color=darkred]
>>> L = ("aa", "bb", "cc", "dd")
>>> S = "a" + "a"
>>> L[/color][/color][/color]
('aa', 'bb', 'cc', 'dd')[color=blue][color=green][color=darkred]
>>> S[/color][/color][/color]
'aa'[color=blue][color=green][color=darkred]
>>> S in L[/color][/color][/color]
# True or False ?

</F>

**Steve Holden** · Oct 7 '05, 04:25 PM

Re: Python interpreter bug

alainpoint@yaho o.fr wrote:[color=blue]
> I understand this, Steve.
> I thought the _cmp_ method was a helper for sorting purposes. Why is it
> that a membership test needs to call the __cmp__ method?[/color]

Can you suggest another way to test for set membership, given that
instances aren't singletons? The only way to do so is to iterate over
the list, asking "are these the same instance", followed (in the case of
a "no" answer) by "are these two instances equal?".

Consider:
[color=blue][color=green][color=darkred]
>>> a = {1:'one'}
>>> b = {2:'two'}
>>> c = {1:'one'}
>>> a is c[/color][/color][/color]
False[color=blue][color=green][color=darkred]
>>> a in [b, c][/color][/color][/color]
True[color=blue][color=green][color=darkred]
>>>[/color][/color][/color]

What would you have Python do differently in these circumstances?
[color=blue]
> If this isn't a bug, it is at least unexpected in my eyes.[/color]

Ah, right. So it's your eyes that need fixing! :-)
[color=blue]
> Maybe a candidate for inclusion in the FAQ?
> Thank you for answering[/color]

A pleasure.

regards
Steve
--
Steve Holden +44 150 684 7255 +1 800 494 3119
Holden Web LLC www.holdenweb.com
PyCon TX 2006 www.python.org/pycon/

**alainpoint@yahoo.fr** · Oct 7 '05, 07:15 PM

Re: Python interpreter bug

> Steve Holden wrote:[color=blue]
>Consider:[/color]
[color=blue][color=green][color=darkred]
>>> a = {1:'one'}
>>> b = {2:'two'}
>>> c = {1:'one'}
>>> a is c[/color][/color][/color]
False[color=blue][color=green][color=darkred]
>>> a in [b, c][/color][/color][/color]
True[color=blue][color=green][color=darkred]
>>>[/color][/color][/color]

[color=blue]
>What would you have Python do differently in these circumstances?[/color]

You mean: What i would do i if i was the benevolent dictator ?
I would make a distinction between mutables and immutables. Immutables
would test for equality and mutables would test for identity.
Membership testing for objects is a very common use case which is
totally unrelated to their being sorted according to a key.
I am no expert on languages so i could be wrong. Don't hesitate to
correct me.
Alain

**Steve Holden** · Oct 7 '05, 08:05 PM

Re: Python interpreter bug

alainpoint@yaho o.fr wrote:[color=blue][color=green]
>>Steve Holden wrote:
>>Consider:[/color]
>
>[color=green][color=darkred]
> >>> a = {1:'one'}
> >>> b = {2:'two'}
> >>> c = {1:'one'}
> >>> a is c[/color][/color]
> False[color=green][color=darkred]
> >>> a in [b, c][/color][/color]
> True[color=green][color=darkred]
> >>>[/color][/color]
>
>[color=green]
>>What would you have Python do differently in these circumstances?[/color]
>
>
> You mean: What i would do i if i was the benevolent dictator ?
> I would make a distinction between mutables and immutables. Immutables
> would test for equality and mutables would test for identity.[/color]

Which is exactly the wrong way round.
[color=blue]
> Membership testing for objects is a very common use case which is
> totally unrelated to their being sorted according to a key.
> I am no expert on languages so i could be wrong. Don't hesitate to
> correct me.
> Alain
>[/color]
It's not worth bothering - just work with Python how it is, and enjoy
the language!

regards
Steve
--
Steve Holden +44 150 684 7255 +1 800 494 3119
Holden Web LLC www.holdenweb.com
PyCon TX 2006 www.python.org/pycon/

Python interpreter bug

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