python - How to empty a list?

364
votes

It seems so "dirty" emptying a list in this way:

while len(alist) > 0 : alist.pop()

Does a clear way exist to do that?

7
pythonlist
So why do python dicts and sets have a clear() method, but not lists? - job
But if there are multiple references to the object, it might be useful. - Ned Deily
It might be useful if I need to clear a shared list over processes at run time and don't need to wait for garbaging (or am I wrong? did I misunderstand garbage collection?). Also if I want to check each popped element I can debug it while I can't using slicing (or am I wrong). I don't need to stop process execution while clearing my list. - DrFalk3n
@S.Lott Just because you don't understand why this is important doesn't mean the rest of us don't. If multiple objects depend on a common list it will matter. In several design patterns this is important. Garbage collector means you don't have to clean up after yourself; it's not a license to make more of a mess. - UpAndAdam
Notwithstanding any other, better answers, your initial code could have been written: while alist: alist.pop() - MrWonderful

7 Answers

580
votes

This actually removes the contents from the list, but doesn't replace the old label with a new empty list:

del lst[:]

Here's an example:

lst1 = [1, 2, 3]
lst2 = lst1
del lst1[:]
print(lst2)

For the sake of completeness, the slice assignment has the same effect:

lst[:] = []

It can also be used to shrink a part of the list while replacing a part at the same time (but that is out of the scope of the question).

Note that doing lst = [] does not empty the list, just creates a new object and binds it to the variable lst, but the old list will still have the same elements, and effect will be apparent if it had other variable bindings.

191
votes

If you're running Python 3.3 or better, you can use the clear() method of list, which is parallel to clear() of dict, set, deque and other mutable container types:

alist.clear()  # removes all items from alist (equivalent to del alist[:])

As per the linked documentation page, the same can also be achieved with alist *= 0.

To sum up, there are four equivalent ways to clear a list in-place (quite contrary to the Zen of Python!):

  1. alist.clear() # Python 3.3+
  2. del alist[:]
  3. alist[:] = []
  4. alist *= 0
62
votes

You could try:

alist[:] = []

Which means: Splice in the list [] (0 elements) at the location [:] (all indexes from start to finish)

The [:] is the slice operator. See this question for more information.

32
votes

it turns out that with python 2.5.2, del l[:] is slightly slower than l[:] = [] by 1.1 usec.

$ python -mtimeit "l=list(range(1000))" "b=l[:];del b[:]"
10000 loops, best of 3: 29.8 usec per loop
$ python -mtimeit "l=list(range(1000))" "b=l[:];b[:] = []"
10000 loops, best of 3: 28.7 usec per loop
$ python -V
Python 2.5.2
5
votes
lst *= 0

has the same effect as

lst[:] = []

It's a little simpler and maybe easier to remember. Other than that there's not much to say

The efficiency seems to be about the same

-1
votes
list = []

will reset list to an empty list.

Note that you generally should not shadow reserved function names, such as list, which is the constructor for a list object -- you could use lst or list_ instead, for instance.

-3
votes

Another simple code you could use (depending on your situation) is:

index=len(list)-1

while index>=0:
    del list[index]
    index-=1

You have to start index at the length of the list and go backwards versus index at 0, forwards because that would end you up with index equal to the length of the list with it only being cut in half.

Also, be sure that the while line has a "greater than or equal to" sign. Omitting it will leave you with list[0] remaining.