alien_invasion

my first python game playing rules: you can use up,down,left and right key to control the airship to move, use Key_space to control it to fire bullet，once you can fire most 100 bullets, use Key_Q to quit the game, use Key_P or mouse to click the play button to start game, use Key_home to restart the game,and if you click the Home key,the game will gives you three seconds to prepare the new game. Meanwhile,the game have the background music in the backage music. In addtion,when you start a new game,you have three lives ,if your airship have a collision with any aliens or the aliens arrive at the bottom of the screen,you will lose one life。In the upper left corner of the screen，you can see how many lifes you have now.In the middle of the screen，you can see your highest score，and in the upper right corner of the screen,you can see your current score and grade.Once you win a game, your level will be increased by one level, and your spacecraft and alien spacecraft will be speeded up at the same time. Thanks.