A Button widget displays a textual string, bitmap or image and invokes a command when the user presses it (by clicking mouse button 1 over the button or, when the button has focus, by pressing the space key). Inherits from Widget. In addition to the standard widget options, a button accepts the options documented in the Tk button manual page, such as command (the callback invoked when the button is pressed), textvariable, state and default.