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RecursionError · Exercises
Core Python

RecursionError Exercises

4 exercises that build on the Python RecursionError lesson, from easy to hard. Each one is checked in your browser when you press Check. Stuck? Use the hint before peeking at the solution.

What this lesson covers: A RecursionError means a function kept calling itself (directly or indirectly) until Python's call-stack depth limit was hit, almost always because the recursion never reaches a base case. By the end you can read a RecursionError traceback, find the missing or unreachable base case, and choose between fixing the recursion, using a loop or an explicit stack. You will practise: Add the missing base case; Predict the outcome; Sum the digits recursively; Replace recursion with a stack.

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1. Add the missing base case

Fix the bug Easy

countdown(n) should return a list like [3, 2, 1] for countdown(3) and an empty list for 0, but it recurses forever. Fix it without using a loop.

Your code 
def countdown(n):
    return [n] + countdown(n - 1)

print(countdown(3))




  
  

  
  
  



    
  
    

  

    
2. Predict the outcome

    Predict the output
    Easy
    
  

  
Read the code and type exactly what it prints, then press Check.

  
    

  

    Read this code
    

      
      
    

  

  
import sys

def grow(n):
    return grow(n + 1)

def shrink(n):
    if n == 0:
        return "done"
    return shrink(n - 1)

sys.setrecursionlimit(150)
try:
    grow(1)
except RecursionError:
    print("grow failed")
print(shrink(10))
sys.setrecursionlimit(1000)

  
  
  

  

    
      
      
      
      
    
    
  

  




  
  

  
  
  



    

  
    

  

    
3. Sum the digits recursively

    Write code
    Medium
    
  

  
Write a recursive function digit_sum(n) for a non-negative integer n that returns the sum of its digits, e.g. digit_sum(1234) is 10. Use n // 10 and n % 10, and give it a base case so it never raises RecursionError.

  
    

  

    Your code
    

      
      
      
      
    

  

  
def digit_sum(n):
    pass


  
  
  
  

    
      
      
      
      
    
    
  

  




  
  

  
  
  



    
  
    

  

    
4. Replace recursion with a stack

    Write code
    Hard
    
  

  
Write count_items(nested) that returns how many non-list items appear in a list that may contain lists inside lists, e.g. [1, [2, [3, 4]], 5] holds 5 items. It must handle nesting 3000 levels deep, so do not recurse: use a while loop with a list as an explicit stack.

  
    

  

    Your code
    

      
      
      
      
    

  

  
def count_items(nested):
    pass


  
  
  
  

    
      
      
      
      
    
    
  

  




  
  

  
  
  



    
  
  

    Next lessonPython AssertionError
    
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