An arc that follows my mouse but stays on the circle

Question

I am trying to make the arc follow my mouse while staying on the circle path.

I do not know why it is not working.

When I run it, it just create a pygame white screen with no error.

Here is my code:

from __future__ import division 
from math import atan2, degrees, pi
import math
import pygame

pygame.init()
screen = pygame.display.set_mode((800, 800))
pygame.display.set_caption("globe")
CENTER = (400, 400)
RADIUS = 350

running = True

def drawCircleArc(screen,color,center,radius,startDeg,endDeg,thickness):

    (x,y) = center
    rect = (x-radius,y-radius,radius*2,radius*2)
    startRad = math.radians(startDeg)
    endRad = math.radians(endDeg)
    pygame.draw.arc(screen,color,rect,startRad,endRad,thickness)


while running:

  for event in pygame.event.get():
    if event.type == pygame.QUIT: 
        running = False

  mouse = pygame.mouse.get_pos()

  relx = mouse[0] - CENTER[0]
  rely = mouse[1] - CENTER[1]
  rad = atan2(-rely,relx)
  rad %= 2*pi
  degs = degrees(rad)


  screen.fill((152,206,231))

  drawCircleArc(screen,(243,79,79),CENTER,RADIUS, degs + 90,degs + 100 ,10)


  pygame.draw.circle(screen, (71,153,192), CENTER, RADIUS)
  pygame.display.update()

pygame.quit()

Picture Picture2

What I really want is the following picture Thank you

Answer 1

I think the follow will do what you want. I fixed two problems:

1. You were drawing the graphics in the wrong order and covering up the short reddish arc (they need to be drawn from back to front), and

2. The two literal values you were adding to the calcuated degs angle were too large.

I also made a several other changes that weren't strictly needed, including reformatting the code follow the PEP 8 - Style Guide for Python Code guidelines and adding a pygame.time.Clock to slow down the refresh rate to something I though was more reasonable.

from __future__ import division
from math import atan2, degrees, pi
import math
import pygame

pygame.init()
screen = pygame.display.set_mode((800, 800))
pygame.display.set_caption("globe")
clock = pygame.time.Clock()
FPS = 60  # Frames per sec
CENTER = (400, 400)
RADIUS = 350

running = True

def draw_circle_arc(screen, color, center, radius, start_deg, end_deg, thickness):
    x, y = center
    rect = (x-radius, y-radius, radius*2, radius*2)
    start_rad = math.radians(start_deg)
    end_rad = math.radians(end_deg)
    pygame.draw.arc(screen, color, rect, start_rad, end_rad, thickness)

while running:
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            running = False

    mouse = pygame.mouse.get_pos()
    relx = mouse[0] - CENTER[0]
    rely = mouse[1] - CENTER[1]
    rad = atan2(-rely, relx)
    degs = degrees(rad)

    screen.fill((152,206,231))
    pygame.draw.circle(screen, (71,153,192), CENTER, RADIUS)
    draw_circle_arc(screen, (243,79,79), CENTER, RADIUS, degs-10, degs+10, 10)
    pygame.display.update()
    clock.tick(FPS)

pygame.quit()

Here's what it looks like running