Confused about class behavior

Mark Jackson mjackson at wc.eso.mc.xerox.com
Sat Sep 16 20:01:14 CEST 2000


Opinderjit <bhellao at my-deja.com> writes:
> I don't seem to understand the following behavior. I create this class
> having one attribute, color, set to 'Green'
> 
> class C:
>     color = "Green"

Note color is a class variable.  There is no code present that would
create instance attributes.

> I create two instances of class C, instance 'a' and 'b', and call the
> access the color attribute. The output is just as expected.
> >> a = C(); b = C()
> >> a.color; b.color
> 'Green'
> 'Green'

Since neither instance has any attributes of its own (look at the
output from 'dir(a)' at this point) you have examined C.color twice.

> When I do the following, both instances a and b were affected.
> >> C.color = 'Red'
> >> a.color; b.color
> 'Red'
> 'Red'

Neither instance is affected.  Both continue to refer all inquiries on
to the base class.

> Is this correct? Futher more, if I change the color attribute of one of
> the instance classes, b,  followed by changing the color of the base
> class, I get the follwoing:
> >> b.color = 'Yellow'

This creates a new attribute, color, for the instance b.

> >> C.color = 'Blue'
> >> a.color; b.color
> 'Blue'

Since a still has no attributes, references the base class.

> 'Yellow'

The value referenced by the color attribute of the instance b.

Clearer now?  To get the behavior you probably wanted, try this:

class D:
	def __init__(self):
		self.color = "Green"

-- 
Mark Jackson - http://www.alumni.caltech.edu/~mjackson
	Some circumstantial evidence is very strong,
	as when you find a trout in the milk.
				- Henry David Thoreau




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