overwrite set behavior

Michele Petrazzo

Hi all, I want to modify the method that set use for see if there is
already an object inside its obj-list. Something like this:

class foo: pass

bar1 = foo()
bar1.attr = 1

bar2 = foo()
bar2.attr = 1

set( (bar1, bar2), key=lambda o: o.attr)

and, of course, set has only one value.

It's possible?

Thanks,
Michele

Sep 4 '08 #1

Wojtek Walczak

On Thu, 04 Sep 2008 11:48:18 +0200, Michele Petrazzo wrote:

Hi all, I want to modify the method that set use for see if there is
already an object inside its obj-list. Something like this:

....

It's possible?

As far as I understand you, you need descriptors:
http://users.rcn.com/python/download/Descriptor.htm
--
Regards,
Wojtek Walczak,
http://tosh.pl/gminick/

Sep 4 '08 #2

Marco Bizzarri

Ciao, Michele:

On Thu, Sep 4, 2008 at 11:48 AM, Michele Petrazzo
<mi**************@togliunipex.itwrote:

Hi all, I want to modify the method that set use for see if there is
already an object inside its obj-list. Something like this:

class foo: pass

bar1 = foo()
bar1.attr = 1

bar2 = foo()
bar2.attr = 1

set( (bar1, bar2), key=lambda o: o.attr)

and, of course, set has only one value.

It's possible?

Thanks,
Michele
--
http://mail.python.org/mailman/listinfo/python-list

looking at the source, maybe you could create a subclass of Set
redefining the __contains__ method?

Regards
Marco
--
Marco Bizzarri
http://notenotturne.blogspot.com/
http://iliveinpisa.blogspot.com/

Sep 4 '08 #3

Marco Bizzarri

On Thu, Sep 4, 2008 at 11:58 AM, Wojtek Walczak <gm*****@bzt.bztwrote:

On Thu, 04 Sep 2008 11:48:18 +0200, Michele Petrazzo wrote:

>Hi all, I want to modify the method that set use for see if there is
already an object inside its obj-list. Something like this:

...

>It's possible?

As far as I understand you, you need descriptors:
http://users.rcn.com/python/download/Descriptor.htm

I know descriptors a litte, Wojtek, but didn't use them often; can you
elaborate a little more on your idea?

--
Regards,
Wojtek Walczak,
http://tosh.pl/gminick/
--
http://mail.python.org/mailman/listinfo/python-list

Saluti
Marco
--
Marco Bizzarri
http://notenotturne.blogspot.com/
http://iliveinpisa.blogspot.com/

Sep 4 '08 #4

Diez B. Roggisch

Michele Petrazzo wrote:

Hi all, I want to modify the method that set use for see if there is
already an object inside its obj-list. Something like this:

class foo: pass

bar1 = foo()
bar1.attr = 1

bar2 = foo()
bar2.attr = 1

set( (bar1, bar2), key=lambda o: o.attr)

and, of course, set has only one value.

It's possible?

Using a decorator/delegate-pattern, yes:

class Foo(object):

def __init__(self, delegate):
self.delegate = delegate

def __hash__(self):
return hash(self.delegate.attr)

def __cmp__(self, other):
return cmp(self.delegate.attr, other.delegate.attr)
set(Foo(a) for a in bar1, bar2)

Diez

Sep 4 '08 #5

Michele Petrazzo

Marco Bizzarri wrote:

looking at the source, maybe you could create a subclass of Set
redefining the __contains__ method?

Made some tries, but __contains__ are never called

>>class foo(set):

.... def __contains__(self, value):
.... print value
....

>>a = foo((1,2))

Thanks,
Michele

Sep 4 '08 #6

Maric Michaud

Le Thursday 04 September 2008 14:31:23 Michele Petrazzo, vous avez écrit*:

Marco Bizzarri wrote:

looking at the source, maybe you could create a subclass of Set
redefining the __contains__ method?

Made some tries, but __contains__ are never called

No, __contains__ is only called with "in" operator, not for internal hashing.
Anyway this solution is bad, you'll need to compare the new element with all
the set contain, which would result in a O(n) algorithm for adding elements
to the set in place of the O(1) it use.

The right way to go is one like Diez show you in a previous post.

*>>class foo(set):
... *def __contains__(self, value):
... * print value
...
*>>a = foo((1,2))
*>>>

--
_____________

Maric Michaud

Sep 4 '08 #7

Marco Bizzarri

On Thu, Sep 4, 2008 at 3:07 PM, Maric Michaud <ma***@aristote.infowrote:

Le Thursday 04 September 2008 14:31:23 Michele Petrazzo, vous avez écrit :

>Marco Bizzarri wrote:

looking at the source, maybe you could create a subclass of Set
redefining the __contains__ method?

Made some tries, but __contains__ are never called

No, __contains__ is only called with "in" operator, not for internal hashing.
Anyway this solution is bad, you'll need to compare the new element with all
the set contain, which would result in a O(n) algorithm for adding elements
to the set in place of the O(1) it use.

Thanks for the clarification, Maric; I take notices to watch source
more closely next time (( hopefully, before writing a wrong answer )).

Regards
Marco

_____________

Maric Michaud
--
http://mail.python.org/mailman/listinfo/python-list

--
Marco Bizzarri
http://notenotturne.blogspot.com/
http://iliveinpisa.blogspot.com/

Sep 4 '08 #8

Wojtek Walczak

On Thu, 4 Sep 2008 12:06:14 +0200, Marco Bizzarri wrote:

>As far as I understand you, you need descriptors:
http://users.rcn.com/python/download/Descriptor.htm

I know descriptors a litte, Wojtek, but didn't use them often; can you
elaborate a little more on your idea?

Marco, I think that I misunderstood the OP, but I was thinking
about immutable (or set-once) attributes. Something like this:

---
class SetOnce(object):
def __init__(self):
self._attr1 = ""
self._attr1flag = False

def setatt(self, x):
if self._attr1flag:
raise ValueError("attribute already set")
self._attr1flag = True
self._attr1 = x

def getatt(self):
return self._attr1

attr1 = property(getatt, setatt)

a = SetOnce()
a.attr1 = 1
print a.attr1
try:
a.attr1 = 2
except ValueError:
print a.attr1
---

$ python immutattr.py
1
1
$

--
Regards,
Wojtek Walczak,
http://tosh.pl/gminick/

Sep 4 '08 #9