Re: different instances with different data descriptors with the same name

[Peter Otten]

Fabrizio Pollastri wrote:

 Data descriptors are set as attributes of object types. So if one has many
 instances of the same class and wants each instance to have a different
 property (data descriptor) that can be accessed with a unique attribute
 name, it seems to me that there is no solution with data descriptors.
 There is any workaround to this? Thank in advance for any help.

You can invent a naming convention and then invoke the getter/setter
explicitly:

 class A(object):
... def __getattr__(self, name):
... if not name.startswith(_prop_):
... return getattr(self, _prop_ + name).__get__(self)
... raise AttributeError(name)
...
 a = A()
 a.p
Traceback (most recent call last):
  File stdin, line 1, in module
  File stdin, line 4, in __getattr__
  File stdin, line 5, in __getattr__
AttributeError: _prop_p
 a._prop_p = property(lambda self: self.x * self.x)
 a.x = 42
 a.p
1764

But what are you really trying to do?

Peter
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Re: different instances with different data descriptors with the same name

[grflanagan]

On Feb 18, 11:21 am, Fabrizio Pollastri [EMAIL PROTECTED] wrote:
 Data descriptors are set as attributes of object types. So if one has many
 instances of the same class and wants each instance to have a different 
 property
 (data descriptor) that can be accessed with a unique attribute name, it seems 
 to
 me that there is no solution with data descriptors. There is any workaround to
 this? Thank in advance for any help.

 F. Pollastri

If you explain your intent you might get some good advice. Here's one
idea:

[code]

class Descriptor(object):
val = 0

def __init__(self, initval=0):
self.val = initval

def __get__(self, obj, objtype):
return '%05d' % self.val

def __set__(self, obj, val):
self.val = val

def Factory(attrname, initval=0):

class Obj(object):
pass

setattr(Obj, attrname, Descriptor(initval))

return Obj

X = Factory('x')
Y = Factory('y', 1)

obj1 = X()

print obj1.x

obj2 = Y()

print obj2.y

obj2.y = 5

print obj2.y

print obj2.x

[/code]

Outputs:

0
1
5
Traceback (most recent call last):
  ...
AttributeError: 'Obj' object has no attribute 'x'


Gerard
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Re: different instances with different data descriptors with the same name

[Gabriel Genellina]

En Mon, 18 Feb 2008 08:21:11 -0200, Fabrizio Pollastri  
[EMAIL PROTECTED] escribi�:

 Data descriptors are set as attributes of object types. So if one has  
 many
 instances of the same class and wants each instance to have a different  
 property
 (data descriptor) that can be accessed with a unique attribute name, it  
 seems to
 me that there is no solution with data descriptors. There is any  
 workaround to
 this? Thank in advance for any help.

Use __getattr__ or change dynamically each instance's class. See this  
recent thread:
http://groups.google.com/group/comp.lang.python/browse_frm/thread/bfc093464dd6ba9/

-- 
Gabriel Genellina

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