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From: Terry Reedy <tjreedy@udel.edu>
Subject: Re: root[:]=[root,root]
Date: Sat, 17 Dec 2011 22:08:24 -0500
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Xref: x330-a1.tempe.blueboxinc.net comp.lang.python:17426

On 12/16/2011 9:40 PM, YAN HUA wrote:
> Hi,all. Could anybody tell how this code works?
>  >>> root = [None, None]
>  >>> root[:] = [root, root]
>  >>> root
> [[...], [...]]
>  >>> root[0]
> [[...], [...]]
>  >>> root[0][0][1][1][0][0][0][1][1]
> [[...], [...]]

A simpler example:
 >>> l = []
 >>> l.append(l)
 >>> l
[[...]]

Python is (now) smart enough to recognize a recursive list and print 
'...' instead of going into an infinite loop printing '['s (as it once 
did, I believe).

-- 
Terry Jan Reedy