Question about classes

Steven Bethard steven.bethard at gmail.com
Mon Nov 22 07:39:53 CET 2004


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



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