How can I convert canvas content to an image?

from Tkinter import *
root = Tk()
cv = Canvas(root)
cv.create_rectangle(10,10,50,50)
cv.pack()
root.mainloop()

I want to convert canvas content to a bitmap or other image, and then do other operations, such as rotating or scaling the image, or changing its coordinates.

Bitmaps can improve efficiency to show if I am no longer drawing.

What should I do?