Clearing Python lists

Is there any reason to do anything more complicated than one of these two lines when you want to clear a list in Python:

old_list = []
old_list = list()

The reason I ask is that I just saw this in some running code:

del old_list[ 0:len(old_list) ]

From stackoverflow

• To clear the list in place, so references "see" the same list.

  For example, this method doesn't clear the referenced list:

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

  But this one does:

  >>> a = [1,2,3]
  >>> b = a
  >>> del a[0:len(a)]
  >>> print a
  []
  >>> print b
  []
  >>> a is b
  True
  

  You could also do:

  >>> a[:] = []
  

  : Good point, thanks!

  Renato Besen : You have a good point

  musicfreak : Hmm, you learn something new every day.

• It appears to me that del will give you the memory back, while assigning a new list will make the old one be deleted only when the gc runs.matter.

  This may be useful for large lists, but for small list it should be negligible.

  Edit: As Algorias, it doesn't matter.

  Note that

  del old_list[ 0:len(old_list) ]
  

  is equivalent to

  del old_list[:]
  

  Algorias : gc is not run in cycles. The list is freed as soon as its last reference is dropped.

• There are two cases in which you might want to clear a list:

  1. You want to use the name old_list further in your code;
  2. You want the old list to be garbage collected as soon as possible to free some memory;

  In case 1 you just go on with the assigment:

      old_list = []    # or whatever you want it to be equal to
  

  In case 2 the del statement would reduce the reference count to the list object the name old list points at. If the list object is only pointed by the name old_list at, the reference count would be 0, and the object would be freed for garbage collection.

      del old_list
  

• You could also do:

  while l:
      l.pop()
  

  FogleBird : LOL. I'm sure that's efficient.

  RoadieRich : Nobody said anything asbout it being efficient...

  Oleksandr Bolotov : +1 for evillness