Is the Slicing lesson free?

Yes. The first 5 lessons on PyRun are free forever — no signup, no card. Slicing runs real Python in your browser via Pyodide.

How long does the Slicing lesson take?

About 5 minutes end-to-end. It's a hands-on lesson — you read a paragraph, edit the code, and run it in the same tab.

Do I need to install Python to try the Slicing lesson?

No install required. PyRun runs real CPython in your browser via Pyodide (Python compiled to WebAssembly). Works on Chrome, Firefox, Safari, and Edge — desktop or mobile.

What level is the Slicing lesson?

Beginner level. No prior Python required — good starting point.

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Beginner·5 min·basics · slicing

Slicing

You’ll be able to

Extract sub-sequences with [start:stop:step] syntax
Reverse sequences with a negative step
Use slices to insert, delete, and replace multiple items
Understand the difference between shallow copies and references

Why this matters

Slicing shows up in every pandas DataFrame, NumPy array, and Django queryset. Negative indices and stride tricks like s[::-1] for reversal are staples of LeetCode problems and one-line interview answers.

Common pitfalls

Off-by-one: s[0:3] returns 3 chars, not 4. The end index is exclusive; internalize this before touching pandas.
Assuming slicing raises on out-of-range: s[100:200] on a short string returns '', not an error. Silent bugs.
Confusing s[-1] (last item) with s[:-1] (everything but last). Both are common but do opposite things.

seq[start:stop:step] — any of the three can be omitted.

Rules

start defaults to 0, stop to len(seq), step to 1
Negative indices count from the end (-1 is the last)
The slice never raises on out-of-range — it just clamps

Slicing returns a copy, not a view

b = a[:] is the idiomatic shallow copy of a list.

Try it

Take every third character of "abcdefghij".
Get the middle third of a 30-element list.

Practice

3 graded exercises

Write the code, click Check. We’ll tell you exactly what to fix.

Progress0 / 3

Exercise 1
From nums = [10, 20, 30, 40, 50, 60, 70] print the last three elements — expected: [50, 60, 70].
Exercise 2
Take word = "playground" and print it reversed — expected: dnuorgyalp.
Exercise 3
From s = "abcdefghij" print every third character, starting at the first — expected: adgj.

← PreviousLists & Looping Next →Tuples & Sets

Slicing

You’ll be able to

Extract sub-sequences with [start:stop:step] syntax
Reverse sequences with a negative step
Use slices to insert, delete, and replace multiple items
Understand the difference between shallow copies and references

Why this matters

Common pitfalls

Off-by-one: s[0:3] returns 3 chars, not 4. The end index is exclusive; internalize this before touching pandas.
Assuming slicing raises on out-of-range: s[100:200] on a short string returns '', not an error. Silent bugs.
Confusing s[-1] (last item) with s[:-1] (everything but last). Both are common but do opposite things.

seq[start:stop:step] — any of the three can be omitted.

Rules

start defaults to 0, stop to len(seq), step to 1
Negative indices count from the end (-1 is the last)
The slice never raises on out-of-range — it just clamps

Slicing returns a copy, not a view

b = a[:] is the idiomatic shallow copy of a list.

Try it

Take every third character of "abcdefghij".
Get the middle third of a 30-element list.

Practice

3 graded exercises

Write the code, click Check. We’ll tell you exactly what to fix.

Progress0 / 3

Exercise 1
From nums = [10, 20, 30, 40, 50, 60, 70] print the last three elements — expected: [50, 60, 70].
Exercise 2
Take word = "playground" and print it reversed — expected: dnuorgyalp.
Exercise 3
From s = "abcdefghij" print every third character, starting at the first — expected: adgj.