Scope and Recursion

Control where names exist and solve repeatable problems with safe recursive functions.

12 min read 10 quiz questions

Where variables live

A variable made inside a function is local to that function. A variable made outside functions is global and can be read from inside a function.

A local name can shadow a global name with the same spelling.

Example 1: Local scope and shadowing

message = "outside"

def show_message():
    message = "inside"
    print(message)

show_message()
print(message)

The local message shadows the global one only while show_message runs. After the call, the global value is still "outside".

Changing global variables

Example 2: Change a global name

score = 0

def add_point():
    global score
    score = score + 1

add_point()
print(score)

Use global score only when a function must reassign a global variable. Passing values in and returning new values is usually easier to test and reuse.

Example 3: A parameter can shadow a global

color = "blue"

def paint(color):
    print(color)

paint("red")
print(color)

A function that calls itself

Recursion means a function calls itself to solve a smaller version of the same problem. Every recursive function needs a base case, a condition that stops the calls.

Example 4: Factorial with recursion

def factorial(number):
    if number == 0:
        return 1
    return number * factorial(number - 1)

print(factorial(5))

Example 5: Fibonacci with recursion

def fibonacci(number):
    if number <= 1:
        return number
    return fibonacci(number - 1) + fibonacci(number - 2)

print(fibonacci(7))

Key points

Local variables exist inside their function.
A local name can shadow a global name.
global allows reassignment of a global variable.
Recursion solves a smaller version of the same problem.
A base case stops recursive calls.

Worked example

Count down exam days with recursion — the function calls itself on a smaller problem until it hits the base case.

Example

def countdown(days):
    if days == 0:
        return "Exam day!"
    print(days)
    return countdown(days - 1)

print(countdown(3))

How it works, step by step

Each call prints the current number, then delegates to days-1.
The base case (days == 0) stops the descent.
Without a base case Python raises RecursionError.

References & Further Reading

Related entries from PyLabs's own reference library.

globals() (Return the current global namespace dictionary.)
locals() (Get the local names currently available in this part of the code.)

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Practice exercise

Not passed yet

Write a recursive function named countdown. It should print the number, call itself with one less, and stop after printing 0. Call it with 3.

Expected output: 3 2 1 0

# Write your code below

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Scope and Recursion quiz

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