Entry

How do I empty a list?

May 10th, 2006 14:41
stephen brown, Jonathan Schroder, Michael Chermside, Asher Amoretto,

Although there are other methods, the standard idiom looks like this: 
>>> aBigList = range(20) 
>>> del aBigList[:] 
>>> aBigList 
[] 
That funny-looking [:] thing is not a sideways view of a fellow peeking  
through a window... it's a slice operator. "aBigList[2:5]" means all  
items in aBigList starting with item #2 and ending just before item #5  
(so it's items 2, 3, and 4). Either boundary can be ommitted,  
so "aBigList[:5]" means all items from the start of the list up to just  
before #5, and "aBigList[2:]" means all items from #2 through the end  
of the list. "aBigList[:]" means all items from the start to the end.  
So "del aBigList[:]" clears the entire list. 
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why not yourlist = [] ?
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Because yourlist = [] doesn't empty the existing list, it just makes
yourlist a reference to a different list.  The distinction only matters
if you have multiple references to the same list (which isn't uncommon.)
>>> a = range(5)
>>> b = a
Because python uses references, a and b are now references to the same
list.  This can be proven by changing one of them.
>>> a[2] = 'a'
>>> b
[0, 1, 'a', 3, 4]
>>> 
Re-assigning a makes it refer to a different list, but doesn't
change the list that it used to refer to.
>>> a = []
>>> b
[0, 1, 'a', 3, 4]
>>> 
If you want to change the object, not just the name, you need to use
the del operator.
>>> a = b
>>> a
[0, 1, 'a', 3, 4]
>>> del b[:]
>>> a
[]
>>>
Admittedly, the distinction gets confusing when the slice notation is
used on the left-hand-side of the assignment.
>>> a = range(5)
>>> b = a
>>> a[:] = []
>>> b
[]
>>>