Python Slicing Cheatsheet

The syntax is seq[start:stop:step]

Rules

• start — inclusive (default: 0, or last item if step is negative)
• stop — exclusive (default: end of sequence)
• step — how many to skip (default: 1). Negative = reverse

Negative indices

-1 is the last element, -2 second-to-last, and so on.

Common patterns

• a[:] — full copy
• a[::-1] — reverse
• a[::2] — every other element
• a[1:-1] — everything except first & last
• a[-3:] — last three elements

Edge cases

• Out-of-bounds indices are clamped — no IndexError
• If start ≥ stop with positive step → empty result
• Slice assignment can change length: a[1:3] = [9]