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To: python-list@python.org
From: Terry Reedy <tjreedy@udel.edu>
Subject: Re: L[:]
Date: Fri, 10 Jan 2014 17:38:42 -0500
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Xref: csiph.com comp.lang.python:63670

On 1/10/2014 12:38 PM, Albert-Jan Roskam wrote:
> In Python Cookbook, one of the authors (I forgot who) consistently used=
 the "L[:]" idiom like below. If the second line simply starts with "L =3D=
" (so no "[:]") only the name "L" would be rebound, not the underlying ob=
ject. That was the author=C5=9B explanation as far as I can remember. I d=
o not get that. Why is the "L[:]" idiom more memory-efficient here? How c=
ould the increased efficiency be demonstrated?
>
> #Python 2.7.3 (default, Sep 26 2013, 16:38:10) [GCC 4.7.2] on linux2
>>>> L =3D [x ** 2 for x in range(10)]
>>>> L[:] =3D ["foo_" + str(x) for x in L]

Unless L is aliased, this is silly code. The list comp makes a new list=20
object, so if L does not have aliases, it would be best to rebind 'L' to =

the existing list object instead of copying it. To do the replacement=20
'in place':
L =3D [x ** 2 for x in range(10)]
for i, n in enumerate(L):
   L[i] =3D "foo_" + str(n)
print(L)
 >>>
['foo_0', 'foo_1', 'foo_4', 'foo_9', 'foo_16', 'foo_25', 'foo_36',=20
'foo_49', 'foo_64', 'foo_81']
--=20
Terry Jan Reedy