How does slicing with start:stop:step work? | Interview Question

Q: How does slicing with start:stop:step work?

**Target Audience:** Beginners preparing for Python interviews — you only need to know what a list and a string are. **Difficulty:** Easy **Python version:** Checked on CPython 3.11 and the page runner (Pyodide 0.25.1 = CPython 3.11.3); the bytecode comparison was also run on CPython 3.12. > **How to read this doc:** Each idea is explained in plain language first, then tagged with `📌 Interview term:` — the precise word an interviewer expects you to use. Every output shown was produced by actually running the code on CPython 3.11 and in the page runner (Pyodide 0.25.1, which is CPython 3.11.3); the **Verified** section quotes that run exactly. ## 1. Why This Even Matters — A Story First Think of a loaf of bread on a cutting board with marks between the slices: mark 0 before the first slice, mark 1 after it, and so on. To take "slices 2 to 4" you put the knife at mark 1 and at mark 4 and lift what is between. You never ask "is the knife at mark 4 included?" — knives cut between slice

The problem

Question presented to candidate: "Explain how seq[start:stop:step] works in Python. What do s[::-1], s[-3:] and s[4:1:-1] give you, and what happens when you assign to a slice?"

What a strong answer should cover:

The range is half-open: start is included, stop is excluded, so s[:k] + s[k:] == s and len(s[a:b]) == b - a for in-range bounds.
Omitted parts have defaults, negative indices count from the end, and a negative step walks backwards with the defaults flipped (s[::-1] reverses).
Slicing clamps out-of-range bounds instead of raising IndexError; only a step of 0 is an error (ValueError).
On lists a slice is a new, shallow copy (a[:]), and slice assignment / del a[i:j] can replace, insert or remove regions; extended slices (step != 1) require an equal-size replacement.
a[i:j:k] is sugar for a[slice(i, j, k)]; your own __getitem__ receives the slice object, and slice.indices(n) shows the clamped values.

Code / implementation expected: Optional — predicting the output of a few slices on a short string or list is the usual check.

slicing
sequences

Try it yourself

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A short nudge toward the approach. It will not give the answer away.

Solution

Target Audience: Beginners preparing for Python interviews — you only need to know what a list and a string are. Difficulty: Easy Python version: Checked on CPython 3.11 and the page runner (Pyodide 0.25.1 = CPython 3.11.3); the bytecode comparison was also run on CPython 3.12.

How to read this doc: Each idea is explained in plain language first, then tagged with 📌 Interview term: — the precise word an interviewer expects you to use. Every output shown was produced by actually running the code on CPython 3.11 and in the page runner (Pyodide 0.25.1, which is CPython 3.11.3); the Verified section quotes that run exactly.

1. Why This Even Matters — A Story First

Think of a loaf of bread on a cutting board with marks between the slices: mark 0 before the first slice, mark 1 after it, and so on. To take "slices 2 to 4" you put the knife at mark 1 and at mark 4 a

The solution is hidden so you can try first

12 min read · 11 sections

Run the code

Edit the code and run it here. Each console line with an expected value becomes a test.

PythonHalf-open ranges, negative indices, steps, clamping and slice objects

Practice

PythonShallow copies, slice assignment, del and a custom __getitem__

Practice

Discussion

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