[Tutor] Python3 Tutorial question about a[:] = []

[Brett Ritter]

On Wed, Feb 16, 2011 at 12:39 PM, Richard D. Moores <rdmoores at gmail.com> wrote:
> from <http://docs.python.org/py3k/tutorial/introduction.html#lists> :
>
>>>> # Clear the list: replace all items with an empty list
>>>> a[:] = []
>>>> a
> []
>
> I've been using
>>>> a = []
>>>> a
> []
>
> What's the difference?

a = []

This sets the variable "a" to refer to a new list

a[:] = []

This replaces the existing content of a with empty contents.

Check out this example in interactive mode

>>> a = [1,2,3]
>>> b = a
>>> a = []
>>> print a
[]
>>> print b
[1, 2, 3]


Versus:

>>> a = [1,2,3]
>>> b = a
>>> a[:] = []
>>> print a
[]
>>> print b
[]


-- 
Brett Ritter / SwiftOne
swiftone at swiftone.org