The problem is that circle_locs remains an empty list.

Modify handle_mouse_click: depending on current_shape, append to either square_locs, circle_locs, or path_verts (use if-elif-else):

if current_shape == 's':
 square_locs.append( [x, y, current_size] )
elif current_shape == 'p':
 circle_locs.append( [x, y, current_size] )
else:
 (do this one)

Still no circles! You need to implement draw_circles (very similar to draw_squares). Also, implement draw_circle. It’s similar to draw_square, but instead of the forloop, just call t.circle(radius). The radius is half the current_size.

Now, circles and squares get drawn. Are you having problems? Add some print statements in every function, and run the program. Look in the IDLE console for your debug print output.

I am aware that I have to implement that code but I am struggling with where to put the code. The work that I have currently changes the size of a square but I need to be able to change the pen to be able to do that with circles now. This is for a homework.

import turtle

"""
This program lets the user draw squares, circles, and polygonal paths.
"""

# The locations of the squares.  Each entry is a list, with [x, y, size]
square_locs= []

# The locations of the circles.  Each entry is a list, with [x, y, size]
circle_locs = []

# The vertexes of the path.  Each entry is a list, with [x, y]

path_verts = []

"""
When the user clicks, either a square, circle, or vertex
gets added.  Which one, depends on the current shape:

  's' : add a square
  'c' : add a circle
  'p' : add a vertex to the path
"""
current_shape = 's'

"""
Each added shape has a size.  It's whatever the current size
was at the time of the click.  This gets changed with the
+ and - keys.
"""
current_size = 15

def draw_square(x, y, size):
    """
    Draw a square, at the given x, y position, with the given size
    """
    t = turtle
    t.up()
    t.goto(x, y)
    t.down()
    for side in range(4):
        t.fd(size)
        t.left(90)

def draw_circle(x, y, diameter):
    """
    Draw a circle at the given x y position, with the given diameter
    """
    t = turtle
    t.penup()
    t.goto(x, y)
    t.down()
    t.circle()
    # ******************************
    # You must implement this function.
    # ******************************

    pass

def draw_squares():
    """
    Draw all the existing squares
    """
    for point in square_locs:
        x, y, size = point
        draw_square(x, y, size)

def draw_circles():
    """
    Draw all the existing circles
    """
    for point in circle_locs:
        x, y, size = point
        draw_square(x, y, size)
   
        

    # ******************************
    # You must implement this function
    # ******************************
    pass

def draw_path():
    """
    Draw the current path, from its existing vertices
    """
    # ******************************
    # You must change this function;
    # it doesn't quite work.
    # ******************************

    t = turtle
    t.down()
    for x, y, size in path_verts:
        t.goto(x, y)

def redraw_scene():
    """
    Draw all the objects, squares, circles, and path
    """
    # Prepare to do fast drawing
    turtle.tracer(False)
    turtle.hideturtle()
    # erase the window
    turtle.clear()

    # Draw all the object
    draw_squares()
    draw_circles()
    draw_path()



    # show the window again
    turtle.update()

def handle_mouse_click(x, y):
    """
    This is called when the user clicks somewhere
    """
    # Add an [x, y, size] list to the list of square locations.
    square_locs.append( [x, y, current_size] )
    if current_shape == 's':
        square_locs.append( [x, y, current_size] )
    elif current_shape == 'p':
        circle_locs.append( [x, y, current_size] )
    
    # ******************************
    # You must modify this function:
    # Instead of just adding to square_locs, check current_shape
    # and then either add to square_locs, circle_locs, or path_verts
    # ******************************

    pass

    # Don't remove this call to redraw_scene().
    # We just added something, so we need to re-draw everything.
    redraw_scene()

def handle_key_plus():
    """
    This is called when the user presses the + key
    """
    # ******************************
    # You must change this function
    # ******************************
    global current_size

    current_size += 5

def handle_key_minus():
    """
    This is called when the user presses the - key
    """
    # ******************************
    # You must change this function
    # ******************************
    global current_size
    current_size -= 5
    if current_size < 5:
        current_size==5
    pass

def handle_key_S():
    """
    This is called when the user presses the S key
    """
    # ******************************
    # You must change this function
    # ******************************
    global current_shape
    current_shape = 's'

def handle_key_C():
    """
    This is called when the user presses the C key
    """

    # ******************************
    # You must change this function
    # ******************************
    global current_shape
    current_shape = 'c'
    pass

def handle_key_P():
    """
    This is called when the user presses the P key
    """

    # ******************************
    # You must change this function
    # ******************************
    global current_shape
    current_shape = 'p'
    pass


def main():
    t = turtle
    # Create the turtle drawing window
    t.setup(500, 500)

    # register callbacks for all the user actions
    t.onscreenclick(handle_mouse_click)

    t.onkey(handle_key_S, 's')
    t.onkey(handle_key_C, 'c')
    t.onkey(handle_key_P, 'p')

    t.onkey(handle_key_plus,  '+')
    t.onkey(handle_key_minus, '-')

    # get keyboard focus (so that keyboard events will trigger onkey() calls)
    t.listen()

    # wait for keyboard, mouse, and timer events.
    t.mainloop()

if __name__ == '__main__':
    # Run the main() function when we run this module,
    # but not when we "import" this module.
    main()