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The Fibonacci sequence is a classic example for understanding recursion and visualizing its tree structure. Each number is the sum of the two preceding ones, creating a binary tree of recursive calls.

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Example code

This example demonstrates how the Fibonacci function creates a branching recursion tree: 
# Author: Bishal Sarang

# Import Visualiser class from module visualiser
from visualiser.visualiser import Visualiser as vs

# Add decorator
# Decorator accepts optional arguments: ignore_args , show_argument_name, show_return_value and node_properties_kwargs
@vs(node_properties_kwargs={"shape":"record", "color":"#f57542", "style":"filled", "fillcolor":"grey"})
def fib(n):
    if n <= 1:
        return n
    return fib(n=n - 1) + fib(n=n - 2)


def main():
    # Call function
    print(fib(n=6))
    # Save recursion tree to a file
    vs.make_animation("fibonacci.gif", delay=2)


if __name__ == "__main__":
    main()

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How it works

The recursive Fibonacci function:
  1. Base case: Returns n when n <= 1
  2. Recursive case: Returns fib(n-1) + fib(n-2), creating two branches at each level
  3. Each call spawns two child calls until reaching the base case
This example demonstrates overlapping subproblems - notice how fib(4), fib(3), fib(2), etc. are calculated multiple times in the recursion tree.

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Decorator configuration

The @vs decorator is configured with custom node styling:
  • shape="record": Uses a record-based node shape
  • color="#f57542": Sets the border color to orange
  • style="filled": Enables filled nodes
  • fillcolor="grey": Sets the fill color to grey

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Output

Running this code with fib(6) generates:
  • Console output: 8 (the 6th Fibonacci number)
  • Animation file: fibonacci.gif showing the complete recursion tree with a 2-second delay between frames
Try experimenting with different values of n to see how the tree grows exponentially. Even fib(10) creates a significantly larger tree than fib(6).

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