Re: how to cause a request for a missing class attribute cause its calculation

[John Posner]

On 5/18/2010 4:54 PM, Chris Rebert wrote:

snip



Suggested reading: http://docs.python.org/library/functions.html#property


I've placed a revision to this official *property* documentation at:

  http://wiki.python.org/moin/AlternativeDescriptionOfProperty

There's also a gentle (I hope) intro to the *property* feature on the Wiki:

  http://wiki.python.org/moin/ComputedAttributesUsingPropertyObjects

-John
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how to cause a request for a missing class attribute cause its calculation

[Vincent Davis]

Lets say I have
class foo(object):
def __init__(self, x, y):
self.x=x
self.y=y
def xplusy(self):
self.xy = x+y

inst = foo(1,2)
inst.xy   # no value, but I what this to cause the calculation of inst.xy

I don't what to have self.xy calculated before it is called.


Thanks

*Vincent Davis
720-301-3003 *
vinc...@vincentdavis.net
 my blog http://vincentdavis.net |
LinkedInhttp://www.linkedin.com/in/vincentdavis
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Re: how to cause a request for a missing class attribute cause its calculation

[Chris Rebert]

On Tue, May 18, 2010 at 1:39 PM, Vincent Davis vinc...@vincentdavis.net wrote:

 Lets say I have
 class foo(object):
     def __init__(self, x, y):
         self.x=x
         self.y=y
     def xplusy(self):
         self.xy = x+y
 inst = foo(1,2)
 inst.xy   # no value, but I what this to cause the calculation of inst.xy
 I don't what to have self.xy calculated before it is called.

class Foo(object):
def __init__(self, x, y):
self.x = x
self.y = y

@property
def xy(self):
'''Will calculate x+y every time'''
return self.x + self.y

Or if Foo is immutable and you wish to cache x+y:

class Foo(object):
def __init__(self, x, y):
self.x = x
self.y = y
self._xy = None

@property
def xy(self):
if self._xy is None:
self._xy = self.x + self.y
return self._xy

Suggested reading: http://docs.python.org/library/functions.html#property

Cheers,
Chris
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Re: how to cause a request for a missing class attribute cause its calculation

[Ethan Furman]

Vincent Davis wrote:
Lets say I have 
class foo(object):

def __init__(self, x, y):
self.x=x
self.y=y
def xplusy(self):
self.xy = x+y

 ^ this needs to be self.x + self.y


inst = foo(1,2)
inst.xy   # no value, but I what this to cause the calculation of inst.xy

I don't what to have self.xy calculated before it is called.


My current favorite method:

def __getattr__(self, name):
if name != 'xy':
raise AttributeError(%s not found % name)
self.xy = self.x + self.y
return self.xy


~Ethan~
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Re: how to cause a request for a missing class attribute cause its calculation

[Ethan Furman]

Ethan Furman wrote:

Vincent Davis wrote:

Lets say I have class foo(object):
def __init__(self, x, y):
self.x=x
self.y=y
def xplusy(self):
self.xy = x+y

 ^ this needs to be self.x + self.y


inst = foo(1,2)
inst.xy   # no value, but I what this to cause the calculation of inst.xy

I don't what to have self.xy calculated before it is called.


My current favorite method:

def __getattr__(self, name):
if name != 'xy':
raise AttributeError(%s not found % name)
self.xy = self.x + self.y
return self.xy


~Ethan~


Chris' reply is more on-point, I think -- I was thinking of attributes 
that are only calculated once.  *sigh*


~Ethan~
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