Slicing

Python · Reference cheat sheet

📋 Overview

Slicing extracts subsequences with seq[start:stop:step]. Works on lists, tuples, strings, bytes, and custom sequences implementing __getitem__. Slices return a new object for built-in sequences (shallow copy of references).

🔧 Core concepts

Omitted bounds default to ends. Out-of-range slice bounds are clipped; single-index OOB raises IndexError.

💡 Examples

Basics:

xs = ["a", "b", "c", "d", "e"]
print(xs[1:4])     # ['b', 'c', 'd']
print(xs[:2])      # ['a', 'b']
print(xs[::2])     # ['a', 'c', 'e']
print(xs[::-1])    # ['e', 'd', 'c', 'b', 'a']
print(xs[-3:-1])   # ['c', 'd']

Strings and bytes:

text = "Python"
print(text[1:4])       # "yth"
print(text[::-1])      # "nohtyP"
data = b"\x00\x01\x02\x03"
print(data[1:3])       # b'\x01\x02'

List slice assign / delete:

nums = [0, 1, 2, 3, 4]
nums[1:3] = [8, 9, 10]   # [0, 8, 9, 10, 3, 4]
del nums[2:4]            # [0, 8, 3, 4]
nums[::2] = [7, 7]       # length must match for extended slice

slice object:

window = slice(1, 4)
print("abcdef"[window])  # "bcd"

⚠️ Pitfalls

• xs[:] is shallow — nested mutables are shared.
• Slice assignment length rules differ for plain vs extended (step != 1) slices.
• s[i:j] never raises for bad bounds; s[i] does.
• Strings/tuples are immutable — cannot slice-assign.
• Copying huge sequences with [:] costs memory; prefer views/iterators when possible.

🔗 Related

• Lists
• Strings
• Tuples
• Bytes / bytearray
• Copy / deepcopy
• Comprehensions