Is the Recursion lesson free?

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

How long does the Recursion lesson take?

About 7 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 Recursion 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 Recursion lesson?

Intermediate level. Best after finishing the Beginner track (syntax, control flow, functions, data types).

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Intermediate·7 min·intermediate · algorithms

Recursion

You’ll be able to

Write recursive functions with clear base cases
Recognize when recursion is clearer than iteration
Avoid stack overflow with tail-call awareness
Convert simple recursion to iteration when needed

Why this matters

Python caps recursion at ~1000 frames (sys.getrecursionlimit), so tree traversals in AST tooling, JSON walkers, and directory scanners often need iterative rewrites. functools.lru_cache on recursive functions is the standard interview trick for memoization.

Common pitfalls

Missing or wrong base case — you'll hit RecursionError fast; write the base case first, then the recursive step.
Deep recursion on user input — an attacker can crash your parser; convert to a stack-based loop for JSON/XML.
Recomputing overlapping subproblems — wrap the function with @lru_cache(maxsize=None) for instant speedup.

A function that calls itself. Every recursion has two parts:

Base case — when do we stop?
Recursive case — call ourselves with a smaller subproblem.

When it shines

Tree / graph traversals
Anything naturally defined "in terms of itself" (factorial, fib, parsers)
Quick & dirty divide-and-conquer

Pitfalls

Forgetting the base case → RecursionError
Python's default recursion limit is ~1000; deep recursion needs sys.setrecursionlimit or an iterative rewrite.

Try it

Write a recursive flatten([1, [2, [3, 4]]]) → [1, 2, 3, 4].
Use recursion to reverse a list.

Practice

3 graded exercises

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

Progress0 / 3

Exercise 1
Write recursive factorial(n) — base case n <= 1 returns 1, else n * factorial(n - 1). Print factorial(6). Expected: 720.
Exercise 2
Write recursive flatten(nested) that takes a nested list (any depth) and returns a flat list. Test with flatten([1, [2, [3, [4, 5]]], 6]) — expected: [1, 2, 3, 4, 5, 6].
Exercise 3
Write recursive power(base, exp) for non-negative exp — base case exp == 0 returns 1, else base * power(base, exp - 1). Print power(2, 10). Expected: 1024.

← PreviousCounter, defaultdict, namedtuple Next →In-memory I/O

Recursion

You’ll be able to

Write recursive functions with clear base cases
Recognize when recursion is clearer than iteration
Avoid stack overflow with tail-call awareness
Convert simple recursion to iteration when needed

Why this matters

Common pitfalls

Missing or wrong base case — you'll hit RecursionError fast; write the base case first, then the recursive step.
Deep recursion on user input — an attacker can crash your parser; convert to a stack-based loop for JSON/XML.
Recomputing overlapping subproblems — wrap the function with @lru_cache(maxsize=None) for instant speedup.

A function that calls itself. Every recursion has two parts:

Base case — when do we stop?
Recursive case — call ourselves with a smaller subproblem.

When it shines

Tree / graph traversals
Anything naturally defined "in terms of itself" (factorial, fib, parsers)
Quick & dirty divide-and-conquer

Pitfalls

Forgetting the base case → RecursionError
Python's default recursion limit is ~1000; deep recursion needs sys.setrecursionlimit or an iterative rewrite.

Try it

Write a recursive flatten([1, [2, [3, 4]]]) → [1, 2, 3, 4].
Use recursion to reverse a list.

Practice

3 graded exercises

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

Progress0 / 3

Exercise 1
Write recursive factorial(n) — base case n <= 1 returns 1, else n * factorial(n - 1). Print factorial(6). Expected: 720.
Exercise 2
Write recursive flatten(nested) that takes a nested list (any depth) and returns a flat list. Test with flatten([1, [2, [3, [4, 5]]], 6]) — expected: [1, 2, 3, 4, 5, 6].
Exercise 3
Write recursive power(base, exp) for non-negative exp — base case exp == 0 returns 1, else base * power(base, exp - 1). Print power(2, 10). Expected: 1024.