Question about classes

[Steve Holden]

Steven Bethard wrote:

> Ben wrote:
> 
>> class foo(xstart):
>>     x = 5
>>     if xstart != 0:
>>         x = xstart
>>
>> a = foo(8)
>>
>> What I am curious about is why not? What am I missing about classes here?
>> Is the functionality delivered in some other fashion, or must I:
>>
>> class foo:
>>     x = 5
>>
>> a = foo()
>> a.x = 8
> 
> 
> The parentheses after a class name do not indicate a parameter list; 
> they indicate the list of base classes.  So generally, they must by 
> classes/types:
> 
>  >>> def make_class(base):
> ....     class C(base):
> ....         pass
> ....     return C
> ....
>  >>> make_class(object)
> <class '__main__.C'>
>  >>> make_class(1)
> Traceback (most recent call last):
>   File "<interactive input>", line 1, in ?
>   File "<interactive input>", line 2, in make_class
> TypeError: Error when calling the metaclass bases
>     int() takes at most 2 arguments (3 given)
> 
> If you want to set a class or instance variable for an object at 
> instantiation time, you probably want to supply the __init__ method:
> 
>  >>> class C(object):
> ....     x = 5
> ....     def __init__(self, x):
> ....         if x != 0:
> ....             C.x = x
> ....            
>  >>> c = C(0)
>  >>> c.x
> 5
>  >>> c = C(8)
>  >>> c.x
> 8
> 
> It seems strange to me that you want to set a *class* variable here, not 
> an instance variable, though perhaps you have your reasons.  Note that 
> by doing so, every time you make a new instance, you'll change the x 
> attribute for all objects:
> 
>  >>> C.x
> 5
>  >>> c1 = C(0)
>  >>> c1.x
> 5
>  >>> C.x
> 5
>  >>> c2 = C(8)
>  >>> c2.x
> 8
>  >>> c1.x
> 8
>  >>> C.x
> 8
> 
> If instead, you intended to set an instance variable, you might write it 
> like:
> 
>  >>> class C(object):
> ....     def __init__(self, x):
> ....         if x != 0:
> ....             self.x = x
> ....         else:
> ....             self.x = 5
> ....            
>  >>> c = C(0)
>  >>> c.x
> 5
>  >>> c = C(8)
>  >>> c.x
> 8
> 
> Steve

Don't forget, though, that due to the attribute resolution order, if an 
instance doesn't have a particular attribute but the class does then a 
reference to the attribute in a method will get the class attribute.

This has been used to implement class defaults in the past.

regards
  another Steve
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