Different ways of clearing lists

Question

Clearing a list in place will affect all other references of the same list.

For example, this method doesn't affect other references:

>>> 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[:]      # equivalent to   del a[0:len(a)]
>>> print(a)
[]
>>> print(b)
[]
>>> a is b
True

You could also do:

>>> a[:] = []

Doing alist = [] does not clear the list, just creates an empty list and binds it to the variable alist. The old list will still exist if it had other variable bindings.

To actually clear a list in-place, you can use any of these ways:

1. alist.clear() # Python 3.3+, most obvious
2. del alist[:]
3. alist[:] = []
4. alist *= 0 # fastest

See the Mutable Sequence Types documentation page for more details.

There is a very simple way to clear a python list. Use del list_name[:].

For example:

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

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[:]

del list[:] 

Will delete the values of that list variable

del list

Will delete the variable itself from memory

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