Python Slices

1. What is a Slice?

A slice extracts a subset of a collection (list, string, tuple) and returns it as a new object — the original is never modified.

my_list = [2, 3, 4, 2, 1, 3, 2, 4, 3, 1]
new_list = my_list[0:2]   # → [2, 3]

2. Slice Syntax

collection[start : stop : step]

Works exactly like range(start, stop, step) — the stop value is never included.

Only the colon is required. Start, stop, and step are all optional.

3. Examples

Stop only

my_list[:4]      # → first 4 elements (indices 0–3)

Start and stop

my_list[1:4]     # → elements at indices 1, 2, 3

Start, stop, and step

my_list[1:6:2]   # → indices 1, 3, 5

Step only (every other element)

my_list[::2]     # → every other element from start to end

4. Negative Values

Just like negative indexing, slices support negative start, stop, and step.

Negative start

my_list[-4:]     # → last 4 elements

Negative start and stop

my_list[-4:-1]   # → 4th-last up to (not including) last element

Negative step (go backwards)

my_list[8:0:-1]  # → from index 8 down to (not including) index 0

⚠️ When using a negative step, make sure start > stop, otherwise you get an empty result.

5. Special Shortcuts

Copy a list / string / tuple

my_list[:]       # → a brand new copy of the whole collection

Reverse a collection ⭐

my_list[::-1]    # → reversed version
"hello"[::-1]    # → "olleh"
(1, 2, 3)[::-1]  # → (3, 2, 1)

This is one of the most useful slice tricks — memorize it!

6. Slices on Strings and Tuples

Slices work identically on strings and tuples.

String

s = "hello world"
s[::2]     # → every other character: "hlowrd"
s[1:6:2]   # → "el "

Tuple

tup = 1, 2, 3, 4, 5
tup[1:6:2]    # → (2, 4)
tup[::-1]     # → (5, 4, 3, 2, 1)

7. Slices Always Return a New Object

The original collection is never changed by a slice.

my_list = [1, 2, 3, 4, 5]
new_list = my_list[1:3]   # → [2, 3]
# my_list is still [1, 2, 3, 4, 5]

Cheat Sheet

lst = [2, 3, 4, 2, 1, 3, 2, 4, 3, 1]

lst[:]        # copy entire list
lst[:3]       # first 3 elements
lst[2:]       # from index 2 to end
lst[1:4]      # indices 1, 2, 3
lst[1:6:2]    # indices 1, 3, 5
lst[::2]      # every other element
lst[::-1]     # reverse the list
lst[-4:-1]    # 4th-last up to (not including) last