Different ways of clearing lists

Solution 1:

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[:] = []

Solution 2:

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:

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

See the Mutable Sequence Types documentation page for more details.

Solution 3:

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
[] []

Solution 4:

~~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[:]

Solution 5:

del list[:]

Will delete the values of that list variable

del list

Will delete the variable itself from memory