puzzlement about classmethod

[Faheem Mitha]

Hi,

Consider the following small script.

My understanding of how this works is that, conceptually, class B
holds a separate copy of variable x from class A.

Nearly everything behaves the way I would expect, except that setting
x to 12 in A using class_setx at the beginning also sets the value for
x in B. However, the converse (setting x in B using class_setx), does
not change the value in A, which I would consider to be the expected
behavior.

However, after that the two x variables appear to behave
independently. Can anyone explain to me why this is so?

Regards, Faheem Mitha.

*************** *************** *************** *************** ******
#!/usr/bin/python

class A(object):
x = 0
def class_getx(cls) :
return cls.x
class_getx = classmethod(cla ss_getx)
def class_setx(cls, _x):
cls.x = _x
class_setx = classmethod(cla ss_setx)
def getx(self):
return type(self).x
def setx(self, _x):
type(self).x = _x

class B(A):
pass

def printx():
print "*** begin printing values of x... ***"
print "Fetching from A using class_getx gives %s"%(A.class_ge tx())
print "Fetching from B using class_getx gives %s"%(B.class_ge tx())
print "Fetching from A using instance a and getx gives %s"%(a.getx() )
print "Fetching from B using instance b and getx gives %s"%(b.getx() )
print "*** end printing values of x... ***"

a = A()
b = B()
printx()
print "setting x to 12 in A using class_setx"; A.class_setx(12 )
printx()
print "setting x to 15 in B using class_setx"; B.class_setx(15 )
printx()
print "setting x to 10 in A using class_setx"
A.class_setx(10 )
printx()
print "setting x to 44 in A using instance a and setx"; a.setx(55)
printx()
print "setting x to 22 in B using instance b and setx"; b.setx(22)
printx()

[Duncan Booth]

Re: puzzlement about classmethod

Faheem Mitha wrote:
[color=blue]
> Nearly everything behaves the way I would expect, except that setting
> x to 12 in A using class_setx at the beginning also sets the value for
> x in B. However, the converse (setting x in B using class_setx), does
> not change the value in A, which I would consider to be the expected
> behavior.
>
> However, after that the two x variables appear to behave
> independently. Can anyone explain to me why this is so?[/color]

When you access a class variable B.x before it has been set in B then you
see the value from the base class (A.x). Once you have set a separate value
in the subclass that takes precedence.

Similarly accessing an instance variable b.x first looks in the instance
(b), then in the class (B), then in the base class or classes (here A). The
lookup stops as soon as it finds a value in any of these locations.