Example Pygame Tank Turret Pivoting With Mouse.

import pygame

TRANSPARENT = 0, 0, 0, 0

class ImageHandler:
    def __init__(self):
        color = pygame.Color('darkgreen').lerp('white', 0.1)
        self.tank = self.create_tank(color)
        self.turret = self.create_turret('darkgreen')
        self.shell = self.create_shell('grey10')

    def create_tank(self, color):
        w, h = size = 50, 40
        surface = pygame.Surface(size, pygame.SRCALPHA)
        surface.fill(TRANSPARENT)
        rect = -5, 0, w + 10, 10
        pygame.draw.ellipse(surface, 'black', rect)
        rect = -5, 30, w + 10, 10
        pygame.draw.ellipse(surface, 'black', rect)
        rect = 0, 8, w, h - 16
        surface.fill(color, rect)
        return surface

    def create_turret(self, color):
        size = 35, 12
        surface = pygame.Surface(size, pygame.SRCALPHA)
        surface.fill(color)
        return surface

    def create_shell(self, color):
        size = 20, 10
        surface = pygame.Surface(size, pygame.SRCALPHA)
        surface.fill(color)
        return surface

class Sprite:
    def __init__(self, image, position, anchor):
        self.oimage = image
        self.image = image
        self.rect = image.get_rect(**{anchor:position})

    def draw(self, surface):
        surface.blit(self.image, self.rect)

class Tank:
    def __init__(self, tank, turret, shell, center):
        self.tank = Sprite(tank, center, 'center')
        self.turret = Sprite(turret, center, 'midleft')
        # turret pivot center and pivot point
        self.pcenter = pygame.Vector2(center)
        self.pivot = self.turret.rect.center - self.pcenter
        # turret distance from center
        self.distance = 1.4
        # set turret from center
        self.turret.rect.center += self.pivot * (self.distance - 1)
        self.shell = shell

    def draw(self, surface):
        self.tank.draw(surface)
        self.turret.draw(surface)

    def rotate(self, mpos):
        angle = (mpos - self.pcenter).as_polar()[1]
        pivot = self.pivot.rotate(angle) * self.distance
        self.turret.image = pygame.transform.rotate(self.turret.oimage, -angle)
        self.turret.rect = self.turret.image.get_rect(center=pivot + self.pcenter)

    def shoot(self):
        pass

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

    # Variables
    background = 'lawngreen'
    images = ImageHandler()
    tank = Tank(images.tank, images.turret, images.shell, rect.center)

    # Main Loop
    while running:
        for event in pygame.event.get():
            if event.type == pygame.MOUSEMOTION:
                tank.rotate(event.pos)
            elif event.type == pygame.MOUSEBUTTONDOWN:
                if event.button == 1:
                    tank.shoot()
            elif event.type == pygame.QUIT:
                running = False

        surface.fill(background)
        tank.draw(surface)

        pygame.display.flip()
        delta = clock.tick(fps) * 0.001

main()