Class property (was: Class methods)

[Laszlo Zsolt Nagy]

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(g et_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

[Peter Otten]

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__(t ype):
.... @property
.... def clsprp(cls): return 42
.... A.clsprp

42

Peter

[Laszlo Zsolt Nagy]

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(ty pe):
_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__,v alue)
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_propert y_that_returns_ the_cnt_per_a_c nt_ratio() # ????
def __init__(self):
A.cnt += 1
if self.__class__ is A:
A.a_cnt += 1

Any ideas?

Les

[Steven Bethard]

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__(t ype):
.... @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 "<interacti ve input>", line 1, in ?
File "<interacti ve input>", line 11, in ratio
ZeroDivisionErr or: 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

[Bengt Richter]

On Thu, 06 Oct 2005 11:05:10 +0200, Laszlo Zsolt Nagy <ga*****@design aproduct.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(g et_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__(t ype):
... 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__.__met aclass__'>), ('_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

[Bengt Richter]

On Thu, 06 Oct 2005 16:09:22 +0200, Laszlo Zsolt Nagy <ga*****@design aproduct.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(ty pe):
_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__,v alue)
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_propert y_that_returns_ the_cnt_per_a_c nt_ratio() # ????
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__(t ype):
... 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
ZeroDivisionErr or: 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...
0.6666666666666 6663

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...
0.4000000000000 0002

Regards,
Bengt Richter