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From: Peter Otten <__peter__@web.de>
Subject: Re: So what's happening here?
Date: Fri, 05 Jun 2015 15:09:03 +0200
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Xref: csiph.com comp.lang.python:92141

Paul Appleby wrote:

> I saw somewhere on the net that you can copy a list with slicing. So
> what's happening when I try it with a numpy array?
> 
>>>> a = numpy.array([1,2,3])
>>>> b = a[:]
>>>> a is b
> False
>>>> b[1] = 9
>>>> a
> array([1, 9, 3])

Copy or view -- have a look under the hood:

>>> a = numpy.array([1,2,3])
>>> v = a.view()
>>> c = a.copy()
>>> s = a[:]
>>> a.flags["OWNDATA"]
True
>>> v.flags["OWNDATA"]
False
>>> c.flags["OWNDATA"]
True
>>> s.flags["OWNDATA"]
False

You only get a copy if you ask for one; slicing produces a view.