import Tkinter

def set_menu_font(m, f):
    for i in range(m.index("end") + 1):
        t = m.type(i)
        if t in ("tearoff", "separator"): continue
        m.entryconfigure(i, font=f)

t = Tkinter.Tk()
m = Tkinter.Menu(t)
t.configure(menu=m)
m.insert_cascade("end", label="File", underline=0)
m.insert_cascade("end", label="Edit", underline=0)
m.insert_separator("end")
m.insert_cascade("end", label="Help", underline=0)

b = Tkinter.Button(t, command=lambda: set_menu_font(m, ("Helvetica", 24)),
                    text="Font 1")
c = Tkinter.Button(t, command=lambda: set_menu_font(m, ("Courier", 12)),
                    text="Font 2")
b.pack(side="left")
c.pack(side="left")
t.mainloop()