import slither, pygame, time


snakey = slither.Sprite()
snakey.setCostumeByName("costume0")

SoExcited = slither.Sprite()
SoExcited.addCostume("SoExcited.png", "avatar")
SoExcited.setCostumeByNumber(0)

SoExcited.goTo(300, 300)
SoExcited.setScaleTo(0.33)

slither.slitherStage.setColor(40, 222, 40)

screen = slither.setup() # Begin slither

continueLoop = True
while continueLoop:
    slither.blit(screen) # Display
    snakey.changeXBy(1)
    SoExcited.changeDirectionBy(1)
    # Handle quitting
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            continueLoop = False
    time.sleep(0.01)    

import slither, pygame, time


snakey = slither.Sprite()
snakey.setCostumeByName("costume0")

SoExcited = slither.Sprite()
SoExcited.addCostume("SoExcited.png", "avatar")
SoExcited.setCostumeByNumber(0)

SoExcited.goTo(300, 300)
SoExcited.setScaleTo(0.33)

slither.slitherStage.setColor(40, 222, 40)

screen = slither.setup() # Begin slither

def run_a_frame():
    snakey.changeXBy(1)
    SoExcited.changeDirectionBy(1)

slither.runMainLoop(run_a_frame)

Update basic test Now uses the new format by @BookOwl.
Update basic test
Now uses the new format by @BookOwl.
