Pygame Sprite & Group Shooting Example

import pygame
import random
from pygame.sprite import Group, Sprite, spritecollide, groupcollide

# Replace images with zombie, bullet, and player image.
# You have a start of a zombie madness game.


class Entity(Sprite):
    def __init__(self, image, position, velocity, vector, anchor="topleft"):
        Sprite.__init__(self)
        self.image = image
        self.rect = image.get_rect(**{anchor: position})
        self.position = pygame.Vector2(self.rect.center)
        self.velocity = velocity
        self.vector = vector

    def update(self, delta):
        if self.vector != pygame.Vector2():
            self.position += self.vector * self.velocity * delta
            self.rect.center = self.position

class Bullet(Entity):
    def __init__(self, image, position, velocity, vector, anchor="topleft"):
        Entity.__init__(self, image, position, velocity, vector, anchor)
        angle = vector.as_polar()[1]
        self.image = pygame.transform.rotate(self.image, -angle)

    def update(self, delta, bounds):
        Entity.update(self, delta)
        if not bounds.colliderect(self.rect):
            self.kill()

class Enemy(Entity):
    def __init__(self, image, position, velocity, vector, anchor="topleft"):
        Entity.__init__(self, image, position, velocity, vector, anchor)

    def update(self, delta, player_position):
        vector = (player_position - self.position).normalize()
        self.position += delta * self.velocity * vector
        self.rect.center = self.position

class Game:
    def __init__(self):
        # Basic Pygame Setup
        pygame.display.set_caption("Example")
        self.surface = pygame.display.set_mode((800, 600))
        self.rect = self.surface.get_rect()
        self.clock = pygame.time.Clock()
        self.running = True
        self.delta = 0
        self.fps = 60

        # Game Variables
        self.enemy_velocity = 50
        self.enemy_distance = 50
        self.player_velocity = 70
        self.bullet_velocity = 150
        self.images = {
            "player" : self.create_image(pygame.Color("dodgerblue"), (30, 30)),
            "enemy" : self.create_image(pygame.Color("firebrick"), (15, 15)),
            "bullet" : self.create_image(pygame.Color("lawngreen"), (6, 2))
        }

        self.player = Entity(self.images["player"], self.rect.center, self.player_velocity, pygame.Vector2(0,0))
        self.sprites = Group(self.player)
        self.bullets = Group()
        self.enemies = Group()

    def add_enemy(self):
        rnd = random.randint
        width = self.rect.width - self.images["enemy"].get_width()
        height = self.rect.height - self.images["enemy"].get_height()
        while True:
            position = rnd(0, width), rnd(0, height)
            if pygame.Vector2(position).distance_to(self.player.position) > self.enemy_distance:
                break

        vector = (pygame.Vector2(position) - self.player.position).normalize()
        enemy = Enemy(self.images["enemy"], position, self.enemy_velocity, vector)
        enemy.add(self.sprites, self.enemies)

    def add_bullet(self, mposition):
        vector = (mposition - self.player.position).normalize()
        width = self.rect.width - self.images["bullet"].get_width()
        height = self.rect.height - self.images["bullet"].get_height()
        position = self.player.position + vector * 12
        bullet = Bullet(self.images["bullet"], position, self.bullet_velocity, vector, "center")
        bullet.add(self.sprites, self.bullets)

    def create_image(self, color, size):
        surface = pygame.Surface(size, pygame.SRCALPHA)
        surface.fill(color)
        return surface

    def is_pressed(self, pressed, *keys):
        for key in keys:
            if pressed[key]:
                return True

        return False

    def load_image(self, filename, name, alpha=False):
        if alpha:
            self.images[name] = pygame.image.load(filename).convert_alpha()
        else:
            self.images[name] = pygame.image.load(filename).convert()

    def main_loop(self):
        while self.running:
            for event in pygame.event.get():
                if event.type == pygame.QUIT:
                    self.running = False
                elif event.type == pygame.MOUSEBUTTONDOWN:
                    if event.button == 1:
                        self.add_bullet(event.pos)

            # Update
            direction = pygame.Vector2()
            pressed = pygame.key.get_pressed()

            if self.is_pressed(pressed, pygame.K_UP, pygame.K_w, pygame.K_z):
                direction.y -= 1

            if self.is_pressed(pressed, pygame.K_DOWN, pygame.K_s):
                direction.y += 1

            if self.is_pressed(pressed, pygame.K_LEFT, pygame.K_a):
                direction.x -= 1

            if self.is_pressed(pressed, pygame.K_RIGHT, pygame.K_d):
                direction.x += 1

            # Move in circle instead of
            if direction != pygame.Vector2():
                direction.normalize_ip()

            self.player.vector = direction
            self.player.update(self.delta)
            self.enemies.update(self.delta, self.player.position)
            self.bullets.update(self.delta, self.rect)

            if len(self.enemies) < 2:
                self.add_enemy()

            killed = groupcollide(self.bullets, self.enemies, True, True)

            # Draw
            self.surface.fill(pygame.Color("black"))
            self.sprites.draw(self.surface)

            # Render to screen
            pygame.display.flip()

            # Idle & Sleep
            self.delta = self.clock.tick(self.fps) * 0.001

        # Clean Exit
        pygame.quit()

def main():
    pygame.init()
    game = Game()
    game.main_loop()

main()