How do I make the enemy float around randomly?

Question

I have pretty much everything done for this little game except I can't seem to get the enemy to just aimlessly float around. They spawn at the top of the window but it's rather bland having them stand in line Civil War style. I'm pretty sure it's something to do with class Enemy, but not sure. Any tips on how to get the player and aliens moving around would be appreciated!

import sys, logging, os, random, math, open_color, arcade

#check to make sure we are running the right version of Python
version = (3,7)
assert sys.version_info >= version, "This script requires at least Python {0}.{1}".format(version[0],version[1])

#turn on logging, in case we have to leave ourselves debugging messages
logging.basicConfig(format='[%(filename)s:%(lineno)d] %(message)s', level=logging.DEBUG)
logger = logging.getLogger(__name__)

SCREEN_WIDTH = 800
SCREEN_HEIGHT = 600
MARGIN = 30
SCREEN_TITLE = "Intergalactic slam"
NUM_ENEMIES = 5
STARTING_LOCATION = (400,100)
BULLET_DAMAGE = 10
ENEMY_HP = 10
HIT_SCORE = 10
KILL_SCORE = 100
PLAYER_HP = 100

class Bullet(arcade.Sprite):
    def __init__(self, position, velocity, damage):
        ''' 
        initializes the bullet
        Parameters: position: (x,y) tuple
            velocity: (dx, dy) tuple
            damage: int (or float)
        '''
        super().__init__("assets/Player/PNG/Sprites/Missiles/spaceMissiles_012.png", 0.5)
        (self.center_x, self.center_y) = position
        (self.dx, self.dy) = velocity
        self.damage = damage

    def update(self):
        '''
        Moves the bullet
        '''
        self.center_x += self.dx
        self.center_y += self.dy

class Enemy_Bullet(arcade.Sprite):
    def __init__(self, position, velocity, damage):
        super().__init__("PNG/laserGreen1.png", 0.5)
        (self.center_x, self.center_y) = position
        (self.dx, self.dy) = velocity
        self.damage = damage
    def update(self):
        self.center_x += self.dx
        self.center_y += self.dy

class Player(arcade.Sprite):
    def __init__(self):
        super().__init__("assets/Player/PNG/Sprites/Ships/spaceShips_005.png", 0.5)
        (self.center_x, self.center_y) = STARTING_LOCATION
        self.hp = PLAYER_HP

class Enemy(arcade.Sprite):
    def __init__(self, position):
        '''
        initializes an alien enemy
        Parameter: position: (x,y) tuple
        '''
        super().__init__("PNG/shipGreen_manned.png", 0.5)
        self.hp = ENEMY_HP
        (self.center_x, self.center_y) = position





class Window(arcade.Window):

    def __init__(self, width, height, title):
        super().__init__(width, height, title)
        file_path = os.path.dirname(os.path.abspath(__file__))
        os.chdir(file_path)

        self.set_mouse_visible(True)
        arcade.set_background_color(open_color.black)
        self.bullet_list = arcade.SpriteList()
        self.enemy_list = arcade.SpriteList()
        self.enemy_bullet_list = arcade.SpriteList()
        self.player = Player()
        self.score = 0
        self.win = False
        self.lose = False


    def setup(self):
        '''
        Set up enemies
        '''
        for i in range(NUM_ENEMIES):
            x = 120 * (i+1) + 40
            y = 500
            enemy = Enemy((x,y))
            self.enemy_list.append(enemy)            

    def update(self, delta_time):
        self.bullet_list.update()
        self.enemy_bullet_list.update()
        if (not (self.win or self.lose)): 
            for e in self.enemy_list:
                for b in self.bullet_list:
                    if (abs(b.center_x - e.center_x) <= e.width / 2 and abs(b.center_y - e.center_y) <= e.height / 2):
                        self.score += HIT_SCORE
                        e.hp -= b.damage
                        b.kill()
                        if (e.hp <= 0):
                            e.kill()
                            self.score += KILL_SCORE
                            if (len(self.enemy_list) == 0):
                                self.win = True
                if (random.randint(1, 75) == 1):
                    self.enemy_bullet_list.append(Enemy_Bullet((e.center_x, e.center_y - 15), (0, -10), BULLET_DAMAGE))
                for b in self.enemy_bullet_list:
                    if (abs(b.center_x - self.player.center_x) <= self.player.width / 2 and abs(b.center_y - self.player.center_y) <= self.player.height / 2):
                        self.player.hp -= b.damage
                        b.kill()
                        if (self.player.hp <= 0):
                            self.lose = True                




    def on_draw(self):
        arcade.start_render()
        arcade.draw_text(str(self.score), 20, SCREEN_HEIGHT - 40, open_color.white, 16)
        arcade.draw_text("HP: {}".format(self.player.hp), 20, 40, open_color.white, 16)

        if (self.player.hp > 0):
            self.player.draw()

        self.bullet_list.draw()
        self.enemy_bullet_list.draw()
        self.enemy_list.draw()
        if (self.lose):
            self.draw_game_loss()
        elif (self.win):
            self.draw_game_won()

    def draw_game_loss(self):
        arcade.draw_text(str("LOSER!"), SCREEN_WIDTH / 2 - 90, SCREEN_HEIGHT / 2 - 10, open_color.white, 30)

    def draw_game_won(self):
        arcade.draw_text(str("WINNER!"), SCREEN_WIDTH / 2 - 90, SCREEN_HEIGHT / 2 - 10, open_color.white, 30)

    def on_mouse_motion(self, x, y, dx, dy):
        '''
        The player moves left and right with the mouse
        '''
        self.player.center_x = x

    def on_mouse_press(self, x, y, button, modifiers):
        if button == arcade.MOUSE_BUTTON_LEFT:
            x = self.player.center_x
            y = self.player.center_y + 15
            bullet = Bullet((x,y),(0,10),BULLET_DAMAGE)
            self.bullet_list.append(bullet)


def main():
    window = Window(SCREEN_WIDTH, SCREEN_HEIGHT, SCREEN_TITLE)
    window.setup()
    arcade.run()


if __name__ == "__main__":
    main()

Answer 1

You'd want to change the self.center_x and self.center_y of each Enemy on every update, like you're already doing for each Bullet, but make the dx and dy values random in some way. For example:

class Enemy(arcade.Sprite):
    def __init__(self, position):
        ...
        (self.center_x, self.center_y) = position
        (self.dx, self.dy) = (0, 0)

    def update(self):
        self.dx += random.random() - 0.5
        self.dy += random.random() - 0.5
        self.center_x += self.dx
        self.center_y += self.dy

Now, this may look more like "twitching wildly" than "floating": many times a second, the thing potentially changes course completely. That's technically random movement, but it's not something a spaceship would do.

If it's too twitchy, make it so that dx and dy change more slowly, for example by dividing the random.random() - 0.5 by a fixed number. If it's too floaty, make it so that every update changes it more.

If you want the enemy to prefer moving down, or towards the player, get out the trigonometry and adjust dx and dy to match.