newbie OO question

[Larry Bates]

Jeremy Winters wrote:
> class SegmentValue:
> 	def
> __init__(self,seg=[0,0,0,0,0,0],value=0,description=""):
> 		self.segment=seg
> 		self.value=value
> 		self.description=description
> 
> #that's my class! note the default of a 6 item list
> for the seg parameter... which is then bound(?) to the
> segment attribute of the instance...
> 
> now here's the session I just did
> 
>>>> from SegmentValue import *
>>>> a=SegmentValue([1,2,3,4,5,6],22,"segment a")
>>>> b=SegmentValue()
>>>> c=SegmentValue() 
>>>> b.segment[0]=1
>>>> b.segment
> [1, 0, 0, 0, 0, 0]
>>>> c.segment
> [1, 0, 0, 0, 0, 0]
>>>> a.segment
> [1, 2, 3, 4, 5, 6]
> 
> so... what I'm seeing here is that when I explicitly
> set seg during instantiation... it creates a unique
> sequence that can be manipulated without affecting
> other instances... but if I instantiate using the
> default... it seems to refer back to the default when
> maniuplating the segment attribute.
> 
> in a continuation of the session...
> 
>>>> d=SegmentValue()
>>>> d.segment
> [1, 0, 0, 0, 0, 0]
> 
> ...you'll note that any new instances now refer to the
> *modified default sequence*.
> 
> what is going on here?  how do I instantiate without
> explicitly defining a new sequence each time? or is
> this even possible?
> 
> thanks in advance,
> jeremy
> 
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It trips up almost everyone at first.

You should not use a list (mutable) as a default keyword argument.
>From Python Language Reference Manual by Guido van Rossum and
Fred L. Drake, Jr.:

6.3.4 Calls

<snip>
Default values are calculated, once, when the function is defined; thus, a
mutable object such as a list or dictionary used as default value will be shared
by all calls that don't specify an argument value for the corresponding slot;
this should usually be avoided.
<end snip>

Change to something like:

__init__(self,seg=None,value=0,description=""):
         self.segment=seg or [0,0,0,0,0,0]
         self.value=value
         self.description=description

-Larry Bates