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Wed Apr 11 11:22:27 EDT 2001 · http://mail.python.org/pipermail/python-list/2001-April/086010.html

> From:	Roland Oviol [SMTP:roviol at venmex.com]
> Sent:	Wednesday, April 11, 2001 3:49 PM
> To:	python-list at python.org
> Subject:	Newbie, problem array
> 
> hello,
> 
> It's the problem
> 
> >>>d1=[[0,0]]
> >>>d2=d1   #reference no copy
> >>>d2[0][0]=3
> >>>d2
> [[3,0]]
> >>>d1
> [[3,0]]
> 
> ok
> 
> but,
> >>>d1=[[0,0]]
> >>>d2=d1[:]    #copy, no reference

> >>>d2[0][0]=3
> >>>d2
> [[3,0]]
> >>>d1
> [[3,0]]
> 
> Why, is copy, no reference
> 
> How copy a object?

d2 contains a *copy* of the list contained in d1, yes, rather than
containing another reference to the same list. But the list referred to by
d1 contains a *reference* to another list. The list referred to by d2
contains another reference to the same inner list.

Basically, you performed a *shallow* copy, i.e. copied the object, and
copied the references contained within it. If you want to copy the object,
and to copy the *objects* within it, you want to take a *deep* copy:

import copy
d2 = copy.deepcopy(d1)

Cheers,
Simon Brunning
TriSystems Ltd.
sbrunning at trisystems.co.uk

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Newbie, problem array