Class instance problem?

[David Eppstein]

In article <c0c22f25.0405122043.2f11549d at posting.google.com>,
 zhuang at princeton.edu (Zhao Huang) wrote:

>   I'm new to python and I've come across a problem with my objects
> corrupting each other.
> 
> class aclass:
>     num = 0
>     l = [[],[]]
> 
> a = aclass()
> b = aclass()
> a.l.append(1)
> print "a.l",a.l
> print "b.l",b.l
> 
>   My expectation is that a,b are separate objects, but appending to
> a's l also appends to b's l. Why does this occur? On the other hand,
> the command
> 
> a.num = 1
> 
> doesn't change b.num's. This is inconsistency is driving me crazy.
> What am I doing wrong?

When you set num and l in the class body, you're making them class-wide 
variables (like static in Java).  If you want a different one per 
object, you need to set them in the __init__ method.

-- 
David Eppstein                      http://www.ics.uci.edu/~eppstein/
Univ. of California, Irvine, School of Information & Computer Science