How does slicing work in Python?

Question

Accepted Answer

**Slicing** extracts a portion of a sequence (list, tuple, string) using the syntax `sequence[start:stop:step]`. It's a powerful, concise feature used constantly in Python.

## Basic slicing

```python
nums = [0, 1, 2, 3, 4, 5]

nums[1:4]     # [1, 2, 3]   — start at index 1, up to (NOT including) 4
nums[:3]      # [0, 1, 2]   — from the start (start defaults to 0)
nums[3:]      # [3, 4, 5]   — to the end (stop defaults to len)
nums[:]       # [0, 1, 2, 3, 4, 5] — a full (shallow) COPY
```

The key rule: `start` is **inclusive**, `stop` is **exclusive** (`nums[1:4]` gives indices 1, 2, 3 — not 4).

## Negative indices count from the end

```python
nums[-1]      # 5  — last element
nums[-2]      # 4  — second to last
nums[-3:]     # [3, 4, 5] — last three elements
nums[:-1]     # [0, 1, 2, 3, 4] — everything except the last
```

## The step parameter

```python
nums[::2]     # [0, 2, 4]      — every 2nd element
nums[1::2]    # [1, 3, 5]      — every 2nd, starting at index 1
nums[::-1]    # [5, 4, 3, 2, 1, 0] — REVERSE the sequence (a famous idiom)
```

`nums[::-1]` reversing a sequence is one of the most recognizable Python idioms.

## Slicing strings

```python
s = "Hello"
s[1:4]        # "ell"
s[::-1]       # "olleH" — reverse a string
s[0]          # "H"
```

## Slice assignment (for mutable lists)

```python
nums = [0, 1, 2, 3, 4]
nums[1:3] = [10, 20, 30]   # replace a slice (can change length)
# nums → [0, 10, 20, 30, 3, 4]
```

## Slicing is safe (no IndexError)

```python
nums[10:20]   # [] — out-of-range slices return empty, don't crash (unlike nums[10])
```

Unlike single-index access (which raises `IndexError`), slicing gracefully returns what's available.

## Why it matters

Slicing is a ubiquitous, distinctly-Pythonic feature for extracting, copying, and reversing sequences cleanly.

Understanding the `start:stop:step` syntax (especially inclusive-start/exclusive-stop), negative indices (counting from the end), the reverse idiom (`[::-1]`), and that slices are safe (no crash on out-of-range) lets you manipulate lists and strings concisely and correctly.

It replaces verbose loops for common operations and appears throughout idiomatic Python code.