Hughes, Chad O wrote: Is there any way to create a class method? I can create a class variable like this:
Hmm, seeing this post, I have decided to implement a 'classproperty'
descriptor.
But I could not. This is what I imagined:
class A(object):
_x = 0
@classmethod
def get_x(cls):
print "Getting x..."
return cls._x
@classmethod
def set_x(cls,value):
print "Setting x..."
cls._x = value
x = classproperty(get_x,set_x)
Usage example: print A.x
Getting x
0A.x = 8
Setting xprint A.x
Getting x
8
I was trying for a while, but I could not implement a 'classproperty'
function. Is it possible at all?
Thanks,
Les 5 2049
Laszlo Zsolt Nagy wrote: I was trying for a while, but I could not implement a 'classproperty' function. Is it possible at all?
You could define a "normal" property in the metaclass: > class A:
.... class __metaclass__(type):
.... @property
.... def clsprp(cls): return 42
.... A.clsprp
42
Peter
Peter Otten wrote: Laszlo Zsolt Nagy wrote: I was trying for a while, but I could not implement a 'classproperty' function. Is it possible at all?
You could define a "normal" property in the metaclass:
The only way I could do this is:
class MyXMetaClass(type):
_x = 0
def get_x(cls):
print "Getting x"
return cls._x
def set_x(cls,value):
cls._x = value
print "Set %s.x to %s" % (cls.__name__,value)
x = property(get_x,set_x)
class A(object):
__metaclass__ = MyXMetaClass
print A.x
A.x = 8
Results in:
Getting x
0
Set A.x to 8
But of course this is bad because the class attribute is not stored in
the class. I feel it should be.
Suppose we want to create a class property, and a class attribute; and
we would like the property get/set methods to use the values of the
class attributes.
A real example would be a class that keeps track of its direct and
subclassed instances:
class A(object):
cnt = 0
a_cnt = 0
def __init__(self):
A.cnt += 1
if self.__class__ is A:
A.a_cnt += 1
class B(A):
pass
print A.cnt,A.a_cnt # 0,0
b = B()
print A.cnt,A.a_cnt # 1,0
a = A()
print A.cnt,A.a_cnt # 2,1
But then, I may want to create read-only class property that returns the
cnt/a_cnt ratio.
This now cannot be implemented with a metaclass, because the metaclass
cannot operate on the class attributes:
class A(object):
cnt = 0
a_cnt = 0
ratio = a_class_property_that_returns_the_cnt_per_a_cnt_ra tio() # ????
def __init__(self):
A.cnt += 1
if self.__class__ is A:
A.a_cnt += 1
Any ideas?
Les
Laszlo Zsolt Nagy wrote: class A(object): cnt = 0 a_cnt = 0 def __init__(self): A.cnt += 1 if self.__class__ is A: A.a_cnt += 1 class B(A): pass print A.cnt,A.a_cnt # 0,0 b = B() print A.cnt,A.a_cnt # 1,0 a = A() print A.cnt,A.a_cnt # 2,1
But then, I may want to create read-only class property that returns the cnt/a_cnt ratio. This now cannot be implemented with a metaclass, because the metaclass cannot operate on the class attributes:
Huh? Every function in the metaclass takes the class object as the
first parameter. So they can all operate on the class attributes:
py> class A(object):
.... cnt = 0
.... a_cnt = 0
.... def __init__(self):
.... A.cnt += 1
.... if self.__class__ is A:
.... A.a_cnt += 1
.... class __metaclass__(type):
.... @property
.... def ratio(cls):
.... return cls.a_cnt/float(cls.cnt)
....
py> class B(A):
.... pass
....
py> A.cnt, A.a_cnt
(0, 0)
py> A.ratio
Traceback (most recent call last):
File "<interactive input>", line 1, in ?
File "<interactive input>", line 11, in ratio
ZeroDivisionError: float division
py> b = B()
py> A.cnt, A.a_cnt, A.ratio
(1, 0, 0.0)
py> a = A()
py> A.cnt, A.a_cnt, A.ratio
(2, 1, 0.5)
STeVe
On Thu, 06 Oct 2005 11:05:10 +0200, Laszlo Zsolt Nagy <ga*****@designaproduct.biz> wrote: Hughes, Chad O wrote:
Is there any way to create a class method? I can create a class variable like this: Hmm, seeing this post, I have decided to implement a 'classproperty' descriptor. But I could not. This is what I imagined:
class A(object): _x = 0 @classmethod def get_x(cls): print "Getting x..." return cls._x @classmethod def set_x(cls,value): print "Setting x..." cls._x = value x = classproperty(get_x,set_x)
Usage example:
print A.xGetting x 0A.x = 8Setting xprint A.xGetting x 8
I was trying for a while, but I could not implement a 'classproperty' function. Is it possible at all? Thanks,
Les
Using Peter's advice (not tested beyond what you see): class A(object):
... _x = 0
... class __metaclass__(type):
... def get_x(cls):
... print "Getting x..."
... return cls._x
... def set_x(cls,value):
... print "Setting x..."
... cls._x = value
... x = property(get_x, set_x)
... A.x
Getting x...
0 A.x = 8
Setting x... A.x
Getting x...
8 vars(A).items()
[('__module__', '__main__'), ('__metaclass__', <class '__main__.__metaclass__'>), ('_x', 8), ('_
_dict__', <attribute '__dict__' of 'A' objects>), ('__weakref__', <attribute '__weakref__' of 'A
' objects>), ('__doc__', None)] A._x
8 vars(A).keys()
['__module__', '__metaclass__', '_x', '__dict__', '__weakref__', '__doc__']
Regards,
Bengt Richter
On Thu, 06 Oct 2005 16:09:22 +0200, Laszlo Zsolt Nagy <ga*****@designaproduct.biz> wrote: Peter Otten wrote:
Laszlo Zsolt Nagy wrote: I was trying for a while, but I could not implement a 'classproperty' function. Is it possible at all?
You could define a "normal" property in the metaclass:
The only way I could do this is:
class MyXMetaClass(type): _x = 0 def get_x(cls): print "Getting x" return cls._x def set_x(cls,value): cls._x = value print "Set %s.x to %s" % (cls.__name__,value) x = property(get_x,set_x)
class A(object): __metaclass__ = MyXMetaClass
print A.x A.x = 8
Results in:
Getting x 0 Set A.x to 8
But of course this is bad because the class attribute is not stored in the class. I feel it should be. Suppose we want to create a class property, and a class attribute; and we would like the property get/set methods to use the values of the class attributes. A real example would be a class that keeps track of its direct and subclassed instances:
class A(object): cnt = 0 a_cnt = 0 def __init__(self): A.cnt += 1 if self.__class__ is A: A.a_cnt += 1
class B(A): pass
print A.cnt,A.a_cnt # 0,0 b = B() print A.cnt,A.a_cnt # 1,0 a = A() print A.cnt,A.a_cnt # 2,1
But then, I may want to create read-only class property that returns the cnt/a_cnt ratio. This now cannot be implemented with a metaclass, because the metaclass cannot operate on the class attributes:
But it can install a property that can. class A(object): cnt = 0 a_cnt = 0 ratio = a_class_property_that_returns_the_cnt_per_a_cnt_ra tio() # ???? def __init__(self): A.cnt += 1 if self.__class__ is A: A.a_cnt += 1
Any ideas? class A(object):
... cnt = 0
... a_cnt = 0
... def __init__(self):
... A.cnt += 1
... if self.__class__ is A:
... A.a_cnt += 1
... class __metaclass__(type):
... def ratio(cls):
... print "Getting ratio..."
... return float(cls.a_cnt)/cls.cnt #
... ratio = property(ratio)
...
I inverted your ratio to lessen the probability if zero division...
class B(A): pass
... A.ratio
Getting ratio...
Traceback (most recent call last):
File "<stdin>", line 1, in ?
File "<stdin>", line 11, in ratio
ZeroDivisionError: float division
Oops ;-)
A.cnt, A.a_cnt
(0, 0) b=B() A.cnt, A.a_cnt
(1, 0) A.ratio
Getting ratio...
0.0 a=A() A.ratio
Getting ratio...
0.5
a=A() A.ratio
Getting ratio...
0.66666666666666663
The old instance is no longer bound, so should it still be counted as it is?
You might want to check how to use weak references if not...
b2=B() B.ratio
Getting ratio...
0.5 b3=B() B.ratio
Getting ratio...
0.40000000000000002
Regards,
Bengt Richter This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics
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