On 1/10/14 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 =" (so no "[:]") only the name "L" would be rebound, not the underlying
object. That was the authorś explanation as far as I can remember. I do not get that. Why is the "L[:]" idiom more
memory-efficient here? How could the increased efficiency be demonstrated?
#Python 2.7.3 (default, Sep 26 2013, 16:38:10) [GCC 4.7.2] on linux2
L = [x ** 2 for x in range(10)]
L[:] = ["foo_" + str(x) for x in L]
I'm not sure there is a memory efficiency argument to make here. The
big difference is that the first line make L refer to a completely new
list, while the second line replaces the contents of an existing list.
This makes a big difference if there are other names referring to the list:
>>> L = [1, 2, 3, 4]
>>> L2 = L
>>> L[:] = []
>>> print L2
[]
>>> L = [1, 2, 3, 4]
>>> L2 = L
>>> L = []
>>> print L2
[1, 2, 3, 4]
Names and values in Python can be confusing. Here's an explanation of
the mechanics: http://nedbatchelder.com/text/names.html
HTH,
--Ned.
Thanks!
Regards,
Albert-Jan
--
Ned Batchelder, http://nedbatchelder.com
--
https://mail.python.org/mailman/listinfo/python-list
