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To: python-list@python.org
From: Dennis Lee Bieber <wlfraed@ix.netcom.com>
Subject: Re: what does 'a=b=c=[]' do
Date: Wed, 21 Dec 2011 18:20:16 -0500
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On Wed, 21 Dec 2011 14:25:17 -0800 (PST), Eric <einazaki668@yahoo.com>
wrote:


>Except it seems that I didn't create three different arrays, I created
>one array that goes by three different names (i.e. x[], y[] and z[]
>all reference the same pile of numbers, no idea which pile).
>
>This surprises me, can someone tell me why it shouldn't?  I figure if
>I want to create and initialize three scalars the just do "a=b=c=7",
>for example, so why not extend it to arrays.  Also, is there a more
>pythonic way to do "x=[], y=[], z=[]"?
>
>It's a slick language but I still have trouble wrapping my brain
>around some of the concepts.
>
	The key one is that lists ([] defines a list, not an array) are
"mutable". Your "7" is not mutable.

	a = b = c = 7
does the same thing as
	a = b = c = []
which is to define the names "a", "b", and "c", and connects the three
names to the single object (integer 7 or new empty list).

	b = 8
then /disconnects/ the name "b" from the object (empty list or integer
7) and connects the name to an integer 8 object.

	b = [1, 2, 3]
disconnects the name "b" from the object and connects it to a list
object containing three elements.

	b.append("something")
however is going "inside" the object to change what it contains -- the
connection remains the same object however.

>>> a,b,c=[[] for x in range(3)]
>>> a.append(1)
>>> b
[]
>>> c
[]
>>> a
[1]
>>>

	For the amount of typing, it's easier to just do a straight line
tuple unpack

>>> a,b,c = ([],[],[])
>>> a.append(2)
>>> a
[2]
>>> b
[]
>>> c
[]
>>>

-- 
	Wulfraed                 Dennis Lee Bieber         AF6VN
        wlfraed@ix.netcom.com    HTTP://wlfraed.home.netcom.com/