Slicing Lists and Tuples in Python

Internal

• Lists
• Tuples

Overview

Slicing means extracting of a section of a list of tuple with the slice notation:

[<start-index>:<end-index>:<step>]

All of the indices mentioned above may be omitted, or may have negative values.

Step

If the step is omitted, it defaults to 1. [1:3], [1:3:] and [1:3:1] are equivalent and result in:

l = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h']
l[1:3]   # ['b', 'c']
l[1:3:]  # ['b', 'c']
l[1:3:1] # ['b', 'c']

If the step has a value larger than 1, the elements are selected starting from the start index (inclusive) and then successively adding the step value to obtain the value of the selected indexes, until the index value is equal or larger than the end index.

---

A sub-sequence of a list can be extracted with a slice. The slice [<start-index>:<end-index>:<step>] specified the index of the last element, and the index of the first element not included in slice, and the step. The step is optional, the default value is 1.

l = ['a', 'b', 'c', 'd', 'e', 'f']
assert ['a', 'b', 'c', 'd', 'e', 'f'] == l[:] # starts from the beginning and ends after the end of the list
assert ['c', 'd'] == l[2:4]
assert ['c'] == l[2:4:2]

The step can be negative, which means the slice starts at the end and goes from right to left.

l = ['a', 'b', 'c', 'd', 'e', 'f']
assert ['f', 'e', 'd', 'c', 'b', 'a'] == l[::-1]

[:] creates a copy of the list.

l = ['a', 'b', 'c']
l2 = l[:]
l2[0] = 'x'
assert ['a', 'b', 'c'] == l
assert ['x', 'b', 'c'] == l2

Remove the last element of the list:

l = ['a', 'b', 'c']
l2 = l[:-1]
assert ['a', 'b'] == l2

Extract Elements from the Tail of the List Starting with a Certain Index

l = [1, 2, 3]
assert l[0:] == [1, 2, 3]
assert l[1:] == [2, 3]
assert l[2:]== [3]
assert l[3:] == [] # empty list
assert l[4:] == [] # empty list

Extract Elements from the Head of the List Counting from the Tail

TODO