def calculate_area(shape, side1, side2=None):
  """Calculates the area of a square or rectangle."""
  if shape == "square":
    return side1 * side1
  elif shape == "rectangle":
    return side1 * side2
  else:
    print("Invalid shape. Please enter 'square' or 'rectangle'.")
    return None

# Example usage for square and rectangle
area_square = calculate_area("square", 5)
area_rectangle = calculate_area("rectangle", 3, 4)

print("Area of square:", area_square)
print("Area of rectangle:", area_rectangle)

# Now trying to add circle functionality
def calculate_circle_area(radius):
  """Calculates the area of a circle."""
  # Here's where I'm stuck!
  # I know the formula for circle area is pi * radius^2,
  # but I'm getting an error "NameError: name 'pi' is not defined".
  return radius * radius  # This line causes the error

# Trying to call the circle function
# circle_area = calculate_circle_area(7)  # This line would also cause an error
# print("Area of circle:", circle_area)

def calc(shape, arg1, arg2=None):
    pi = 3.14
    if shape == 'square':
        return f'Square: {arg1*arg1}'

    elif shape == 'rectangle':
        return f'Rectangle: {(2*arg1) + (2*arg2)}'

    elif shape == 'circle':
        return f'Circle: {pi * arg1**2}'

    else:
        return 'Error! Wrong shape'


print(calc('square', 8))
print(calc('rectangle', 5,3))
print(calc('circle', 4))

Output:
Square: 64
Rectangle: 16
Circle: 50.24

import math

print( math.pi )

from numpy import pi

print(pi)