list.clear() missing?!?

[Felipe Almeida Lessa]

Em Ter, 2006-04-11 às 17:56 +0000, John Salerno escreveu:
> Steven Bethard wrote:
> 
> 
> >     lst[:] = []
> >     lst = []
> 
> What's the difference here?

lst[:] = [] makes the specified slice become []. As we specified ":", it
transforms the entire list into [].

lst = [] assigns the value [] to the variable lst, deleting any previous
one.

This might help:

>>> lst = range(10)
>>> id(lst), lst
(-1210826356, [0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
>>> lst[:] = []
>>> id(lst), lst
(-1210826356, [])

>>> lst = range(10)
>>> id(lst), lst
(-1210844052, [0, 1, 2, 3, 4, 5, 6, 7, 8, 9])
>>> lst = []
>>> id(lst), lst
(-1210826420, [])


You see? lst[:] removes all elements from the list that lst refers to,
while lst = [] just creates a new list and discard the only one. The
difference is, for example:

>>> lst = range(3)
>>> x = [lst, lst, lst]
>>> x
[[0, 1, 2], [0, 1, 2], [0, 1, 2]]
>>> lst[:] = []
>>> x
[[], [], []]

>>> lst = range(3)
>>> x = [lst, lst, lst]
>>> x
[[0, 1, 2], [0, 1, 2], [0, 1, 2]]
>>> lst = []
>>> x
[[0, 1, 2], [0, 1, 2], [0, 1, 2]]

HTH,

-- 
Felipe.