Class property

[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__(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