Python List Slicing with None as argument

Question

Via trial and error I found out that

my_list = range(10) my_list[:None] == my_list[:] 

I use this for django query sets so I can define a size or take all:

some_queryset[:length if length else None]  # @IanAuld some_queryset[:length or None]   # @Bakuriu # length works for all numbers and None if you want all elements # does not work with False of any other False values some_queryset[:length] 

• Is this good practice to use None while slicing?
• Can problems occur with this method in any case?

Answer 1

Yes, it is fine to use None, as its behavior is specified by the documentation:

The slice of s from i to j is defined as the sequence of items with index k such that i <= k < j. If i or j is greater than len(s), use len(s). If i is omitted or None, use 0. If j is omitted or None, use len(s). If i is greater than or equal to j, the slice is empty.

Using None for one of the slice parameters is the same as omitting it.

Answer 2

Your way is fine, but I would prefer :

some_queryset[:length] if length else some_queryset 

or

some_queryset[:length] if length else some_queryset[:] 

which are readable with less knowledge of how slicing treats these special cases.