Button 类将其所有关键字参数 (**kw) 转发到其基类 (Widget)。你可以在这里看到它是如何做到的
class Button(Widget):
"""Button widget."""
def __init__(self, master=None, cnf={}, **kw):
"""Construct a button widget with the parent MASTER.
STANDARD OPTIONS
activebackground, activeforeground, anchor,
background, bitmap, borderwidth, cursor,
disabledforeground, font, foreground
highlightbackground, highlightcolor,
highlightthickness, image, justify,
padx, pady, relief, repeatdelay,
repeatinterval, takefocus, text,
textvariable, underline, wraplength
WIDGET-SPECIFIC OPTIONS
command, compound, default, height,
overrelief, state, width
"""
Widget.__init__(self, master, 'button', cnf, kw)