关于python：在tkinter中声明全局并导入模块

root = Tk()
root.title("Feet to Metres")

'''
def calculate(*args):
    try:

        value = float(feet.get())
        metres.set((0.3048 * value * 10000.0 + 0.5)/10000.0)
    except ValueError:
        pass
'''

mainframe = ttk.Frame(root, padding="3 3 12 12")
mainframe.grid(column=0, row=0, sticky=(N,W,E,S))
mainframe.columnconfigure(0, weight=1)
mainframe.rowconfigure(0,weight=1)
feet =StringVar()
metres = StringVar()

feet_entry = ttk.Entry(mainframe, width = 7, textvariable = feet)
feet_entry.grid(column=2, row=1, sticky=(W,E))

ttk.Label(mainframe, textvariable = metres).grid(column=2, row=2, sticky=(W,E))
ttk.Button(mainframe, text="Calculate", command = calculate).grid(column=3, row=3, sticky=W)

ttk.Label(mainframe, text ='feet').grid(column=3, row=1, sticky=W)
ttk.Label(mainframe, text ='is equivalent to').grid(column=1, row=2, sticky=E)
ttk.Label(mainframe, text ='metres').grid(column=3, row=2, sticky=W)

for child in mainframe.winfo_children():
   child.grid_configure(padx=5, pady=5)

feet_entry.focus()
root.bind('<Return>', calculate)

root.mainloop()