PyGame - Drag Circles & Click Buttons

#!/usr/bin/env python3

#
# pygame (simple) template - by furas
#
# https://github.com/furas/my-python-codes/tree/master/pygame/__template__/
#
# http://pastebin.com/9VdUEPXi
#

# ---------------------------------------------------------------------

__author__  = 'Bartlomiej "furas" Burek'
__webpage__ = 'http://blog.furas.pl'

# ---------------------------------------------------------------------

import pygame

# === CONSTANS === (UPPER_CASE names)

BLACK = (  0,   0,   0)
WHITE = (255, 255, 255)

RED   = (255,   0,   0)
GREEN = (  0, 255,   0)
BLUE  = (  0,   0, 255)

SCREEN_WIDTH  = 600
SCREEN_HEIGHT = 400

BLOCK_SIZE = 50
CIRCLE_RADIUS = int(BLOCK_SIZE/2)

# === CLASSES === (CamelCase names)

class Button():

    def __init__(self, text='OK', pos=(0,0), size=(100,50), command=None):
        font = pygame.font.SysFont(None, 35)

        self.text = text
        self.rect = pygame.Rect((0,0), size)

        self.image_normal = pygame.Surface(size)
        self.image_normal.fill(WHITE)
        txt_image = font.render(self.text, True, RED)
        txt_rect = txt_image.get_rect(center=self.rect.center)
        self.image_normal.blit(txt_image, txt_rect)

        self.image_hover = pygame.Surface(size)
        self.image_hover.fill(RED)
        txt_image = font.render(self.text, True, WHITE)
        txt_rect = txt_image.get_rect(center=self.rect.center)
        self.image_hover.blit(txt_image, txt_rect)

        self.rect.topleft = pos

        self.hover = False

        if command:
            self.command = command

    def draw(self, screen):
        if self.hover:
            screen.blit(self.image_hover, self.rect)
        else:
            screen.blit(self.image_normal, self.rect)

    def handle_event(self, event):
        if event.type == pygame.MOUSEMOTION:
            self.hover = self.rect.collidepoint(event.pos)

        if self.hover and self.command:
            if event.type == pygame.MOUSEBUTTONDOWN:
                if event.button == 1:
                    self.command()

    def command(self):
        print("Click")

# === FUNCTIONS === (lower_case names)

def print_hello():
    print("Click HELLO")

def print_world():
    print("Click WORLD")

# === MAIN === (lower_case names)

# --- (global) variables ---

# --- init ---

pygame.init()

screen = pygame.display.set_mode((SCREEN_WIDTH, SCREEN_HEIGHT))
screen_rect = screen.get_rect()

# --- objects ---

# - buttons -

button1 = Button(text="HELLO") # create button
button1.command = print_hello # assign function to button

button2 = Button(text="WORLD", pos=(110,0), command=print_world) # create button and assign function

# - circles -

circles = []

for x in range(10):
    circles.append( pygame.Rect(x*(BLOCK_SIZE+5), BLOCK_SIZE, BLOCK_SIZE, BLOCK_SIZE) )

# - drag -

selected = None

# --- mainloop ---

clock = pygame.time.Clock()
is_running = True

while is_running:

    # --- events ---

    for event in pygame.event.get():

        # --- global events ---

        if event.type == pygame.QUIT:
            is_running = False

        elif event.type == pygame.KEYDOWN:
            if event.key == pygame.K_ESCAPE:
                is_running = False

        elif event.type == pygame.MOUSEBUTTONDOWN:
            if event.button == 1:
                for i, c in enumerate(circles):
                    # Pythagoras A^2 + B^2 = C^2
                    dx = c.centerx - event.pos[0] # A
                    dy = c.centery - event.pos[1] # B
                    distance_square = dx**2 + dy**2 # C^2

                    if distance_square <= CIRCLE_RADIUS**2: # C^2 <= RADIUS^2
                        selected = i
                        selected_offset_x = c.x - event.pos[0]
                        selected_offset_y = c.y - event.pos[1]

        elif event.type == pygame.MOUSEBUTTONUP:
            if event.button == 1:
                selected = None

        elif event.type == pygame.MOUSEMOTION:
            if selected is not None: # selected can be `0` so `is not None` is required
                # move object
                circles[selected].x = event.pos[0] + selected_offset_x
                circles[selected].y = event.pos[1] + selected_offset_y

        # --- objects events ---

        button1.handle_event(event)
        button2.handle_event(event)

    # --- updates ---

        # empty

    # --- draws ---

    screen.fill(BLACK)

    button1.draw(screen)
    button2.draw(screen)

    # draw rect
    for c in circles:
        pygame.draw.circle(screen, RED, c.center, CIRCLE_RADIUS)

    pygame.display.update()

    # --- FPS ---

    clock.tick(25)

# --- the end ---

pygame.quit()