Descriptor puzzlement

[John Roth]

Using Python 2.2.3, I create this script:

[beginning of script]

class AnObject(object ):
"This might be a descriptor, but so far it doesn't seem like it"
def __init__(self, somedata):
self.it = somedata
def __get__(self, obj, type=None):
print "in get method"
return self.it
def __set__(self, obj, it):
print "in set method"
self.somedata = it
return None
## def foo(self):
## return 1

class AnotherObject(o bject):
def __init__(self):
self.prop = AnObject("snafu ")

myClass = AnotherObject()
print myClass.prop
myClass.prop = "foobar"
print myClass.prop

[end of script]

Then I execute it:

C:\Documents and Settings\John\M y Documents\Proje cts\AstroPy4>b

C:\Documents and Settings\John\M y Documents\Proje cts\AstroPy4>py thon b.py
<__main__.AnObj ect object at 0x0086D248>
foobar

It doesn't look like the descriptor protocol is getting
invoked at all.

What's happening here?

John Roth

[Alexander Schmolck]

Re: Descriptor puzzlement

"John Roth" <newsgroups@jhr othjr.com> writes:[color=blue]
> What's happening here?[/color]

cf.

class AnotherObject(o bject):
prop = AnObject("snafu ")

'as

[Mirko Zeibig]

Re: Descriptor puzzlement

John Roth said the following on 01/08/2004 01:34 PM:[color=blue]
> Using Python 2.2.3, I create this script:
>
> [beginning of script]
>
> class AnObject(object ):
> "This might be a descriptor, but so far it doesn't seem like it"
> def __init__(self, somedata):
> self.it = somedata
> def __get__(self, obj, type=None):
> print "in get method"
> return self.it
> def __set__(self, obj, it):
> print "in set method"
> self.somedata = it
> return None
> ## def foo(self):
> ## return 1[/color]
Hm, I don't know __set__ and __get__, there are __getattr__ (or
__getattribute_ _) and __setattr__ for dynamically assigning attributes.
Or take a look at properties
(http://www.python.org/2.2/descrintro.html#property)
[color=blue]
> class AnotherObject(o bject):
> def __init__(self):
> self.prop = AnObject("snafu ")
>
> myClass = AnotherObject()
> print myClass.prop[/color]
Now just do:
print id(myClass.prop ) to see the internal reference.
[color=blue]
> myClass.prop = "foobar"[/color]
Here you bind an immutable string-object to myClass.prop, the object of
type AnObject you have bound before has no further references in the
code and will be garbage collected.

If you create a destructor for AnObject:

def __del__(self):
print "%s.__del__ " % self

you will see that this happens immediately.
[color=blue]
> print myClass.prop[/color]

Regards
Mirko

[Peter Otten]

Re: Descriptor puzzlement

John Roth wrote:
[color=blue]
> Using Python 2.2.3, I create this script:
>
> [beginning of script]
>
> class AnObject(object ):
> "This might be a descriptor, but so far it doesn't seem like it"
> def __init__(self, somedata):
> self.it = somedata
> def __get__(self, obj, type=None):
> print "in get method"
> return self.it
> def __set__(self, obj, it):
> print "in set method"
> self.somedata = it
> return None
> ## def foo(self):
> ## return 1
>
> class AnotherObject(o bject):
> def __init__(self):
> self.prop = AnObject("snafu ")
>
> myClass = AnotherObject()
> print myClass.prop
> myClass.prop = "foobar"
> print myClass.prop
>
> [end of script]
>
> Then I execute it:
>
> C:\Documents and Settings\John\M y Documents\Proje cts\AstroPy4>b
>
> C:\Documents and Settings\John\M y Documents\Proje cts\AstroPy4>py thon b.py
> <__main__.AnObj ect object at 0x0086D248>
> foobar
>
> It doesn't look like the descriptor protocol is getting
> invoked at all.
>
> What's happening here?[/color]

The descriptor protocol works on the class, not the instance, so

class AnotherObject(o bject):
prop = AnObject("snafu ")

or something similar should work. This means in particular that you have to
store the property's state in the AnotherObject rather than the AnObject
instance.

Peter

[Michael Hudson]

Re: Descriptor puzzlement

"John Roth" <newsgroups@jhr othjr.com> writes:

[snippety]
[color=blue]
> It doesn't look like the descriptor protocol is getting
> invoked at all.
>
> What's happening here?[/color]

Descriptors need to be attached to classes.

Cheers,
mwh

--
6. Symmetry is a complexity-reducing concept (co-routines include
subroutines); seek it everywhere.
-- Alan Perlis, http://www.cs.yale.edu/homes/perlis-alan/quotes.html

[John Roth]

Re: Descriptor puzzlement


"Michael Hudson" <mwh@python.net > wrote in message
news:m34qv6388c .fsf@pc150.math s.bris.ac.uk...[color=blue]
> "John Roth" <newsgroups@jhr othjr.com> writes:
>
> [snippety]
>[color=green]
> > It doesn't look like the descriptor protocol is getting
> > invoked at all.
> >
> > What's happening here?[/color]
>
> Descriptors need to be attached to classes.[/color]

Arrrgggh! Of course!

John Roth[color=blue]
>
> Cheers,
> mwh
>[/color]

[Jacek Generowicz]

Re: Descriptor puzzlement

"John Roth" <newsgroups@jhr othjr.com> writes:
[color=blue]
> It doesn't look like the descriptor protocol is getting
> invoked at all.
>
> What's happening here?[/color]

Try changing the following
[color=blue]
> class AnotherObject(o bject):
> def __init__(self):
> self.prop = AnObject("snafu ")[/color]

to

class AnotherObject(o bject):
prop = AnObject("snafu ")

[Francis Avila]

Re: Descriptor puzzlement

Mirko Zeibig wrote in message ...[color=blue]
>John Roth said the following on 01/08/2004 01:34 PM:
>Hm, I don't know __set__ and __get__, there are __getattr__ (or
>__getattribute __) and __setattr__ for dynamically assigning attributes.
>Or take a look at properties
>(http://www.python.org/2.2/descrintro.html#property)[/color]

Properties are just a wrapper/interface/application of/to descriptors (whose
protocol involves __set__ and __get__).
http://users.rcn.com/python/download/Descriptor.htm for details.
--
Francis Avila

[Terry Reedy]

Re: Descriptor puzzlement


"John Roth" <newsgroups@jhr othjr.com> wrote in message
news:vvqjj92il4 o1d0@news.super news.com...[color=blue]
> Using Python 2.2.3, I create this script:
>
> [beginning of script]
>
> class AnObject(object ):
> "This might be a descriptor, but so far it doesn't seem like it"
> def __init__(self, somedata):
> self.it = somedata
> def __get__(self, obj, type=None):
> print "in get method"
> return self.it
> def __set__(self, obj, it):
> print "in set method"
> self.somedata = it[/color]

Did you mean to set self.it to match the __init__ and __get__ methods?
Or am I missing something about the esoterics of properties?

Terry J. Reedy

[John Roth]

Re: Descriptor puzzlement


"Terry Reedy" <tjreedy@udel.e du> wrote in message
news:NaudnQjKFt 8IOmCiRVn-tw@comcast.com. ..[color=blue]
>
> "John Roth" <newsgroups@jhr othjr.com> wrote in message
> news:vvqjj92il4 o1d0@news.super news.com...[color=green]
> > Using Python 2.2.3, I create this script:
> >
> > [beginning of script]
> >
> > class AnObject(object ):
> > "This might be a descriptor, but so far it doesn't seem like it"
> > def __init__(self, somedata):
> > self.it = somedata
> > def __get__(self, obj, type=None):
> > print "in get method"
> > return self.it
> > def __set__(self, obj, it):
> > print "in set method"
> > self.somedata = it[/color]
>
> Did you mean to set self.it to match the __init__ and __get__ methods?
> Or am I missing something about the esoterics of properties?[/color]

No, you're not missing anything. This was simply the result
of futzing around trying to get it to work, and has no other
conceptual value at all. The problem turned out to be that
I was putting it in the instance instead of the class. [sigh.]

John Roth[color=blue]
>
> Terry J. Reedy
>
>[/color]