Slicing

Indexing pulls out one element; slicing pulls out a whole run of them. A slicing operation creates a new list by taking out elements from an existing list — you name the range of indexes you want, and Python hands back those elements as a fresh list, leaving the original untouched.

All the examples below work on one list:

>>> list1 = ['Red','Green','Blue','Cyan','Magenta','Yellow','Black']

The basic form: list1[start:stop]

The slice list1[start:stop] takes the elements from index start up to but not including index stop:

>>> list1[2:6]        # elements at indexes 2 to 5
['Blue', 'Cyan', 'Magenta', 'Yellow']

The element at index 6 ('Black') is not included — the second index marks where the slice stops, not the last element taken. This half-open behaviour is the heart of every slicing rule that follows.

When the indexes are unusual

Slicing is forgiving where plain indexing is strict:

>>> list1[2:20]       # second index out of range
['Blue', 'Cyan', 'Magenta', 'Yellow', 'Black']
>>> list1[7:2]        # first index greater than second
[]

• If the second index runs beyond the end of the list, there is no IndexError — the slice is simply truncated at the end of the list.
• If the first index is greater than the second, the range is empty, and the result is an empty list [].

Leaving an index out

Either boundary may be omitted, and Python fills in the obvious default:

>>> list1[:5]         # first index missing: start from index 0
['Red', 'Green', 'Blue', 'Cyan', 'Magenta']

A missing first index means "from the beginning"; the slice above returns the sublist from index 0 to 4.

Slicing with a step size

A third value inside the brackets sets a step size — how far to jump between the elements taken:

>>> list1[0:6:2]      # every second element from index 0 to 5
['Red', 'Blue', 'Magenta']

With step 2, the slice picks the elements at indexes 0, 2 and 4 within the given range.

Negative indexes in slices

Negative index values work in slices just as in indexing, counting from the right:

>>> list1[-6:-2]      # elements at indexes -6, -5, -4, -3
['Green', 'Blue', 'Cyan', 'Magenta']

The stop position -2 is again excluded — only the elements at -6, -5, -4 and -3 are sliced out.

Omitting both boundaries

With both the first and the last index missing, the slice covers the entire list, and the step still applies:

>>> list1[::2]        # step size 2 over the whole list
['Red', 'Blue', 'Magenta', 'Black']

Reversing a list with a negative step

A negative step size walks the list backwards, so the whole list in reverse order is one slice away:

>>> list1[::-1]
['Black', 'Yellow', 'Magenta', 'Cyan', 'Blue', 'Green', 'Red']