from tkinter import *
import datetime
import pytz         # Definitions of time zones

def main():
    root = Tk()
    root.title("Digital Clock")
    root.geometry("400x120")
    panel = Frame(root)
    panel.pack(side=TOP, fill=X)

    tz = None               # Timezone definition, None means local timezone
    timeZoneIdx = IntVar()  # Control variable for the group of radio buttons

    def setTimeZone():
        nonlocal tz
        n = timeZoneIdx.get()
        # print("setTimeZone: n =", n)
        if n == 1:  # Moscow
            tz = pytz.timezone("Europe/Moscow")
        elif n == 2: # Shanghai    
            tz = pytz.timezone("Asia/Shanghai")
        else:
            tz = None
        redraw()

    localTime = Radiobutton(
        panel,    
        text="Local time", variable=timeZoneIdx, value=0, 
        command=setTimeZone
    )
    moscowTime = Radiobutton(
        panel,    
        text="Moscow", variable=timeZoneIdx, value=1, 
        command=setTimeZone
    )
    shanghaiTime = Radiobutton(
        panel,    
        text="Shanghai", variable=timeZoneIdx, value=2, 
        command=setTimeZone
    )
    localTime.pack(side=LEFT, padx=4, pady=4)
    moscowTime.pack(side=LEFT, padx=4, pady=4)
    shanghaiTime.pack(side=LEFT, padx=4, pady=4)

    drawArea = Canvas(root, bg="silver")
    drawArea.pack(side=TOP, expand=True, fill=BOTH)
    
    def drawTime():
        dt = datetime.datetime.now(tz)
        txt = "{0:02d}:{1:02d}:{2:02d}".format(
            dt.hour, dt.minute, dt.second
        )
        w = drawArea.winfo_width()
        h = drawArea.winfo_height()   
        drawArea.create_text(
            w/2, h/2, anchor=CENTER, text=txt, font=("Times", 32)
        ) 

    def redraw():
        drawArea.delete("all")
        drawTime()
        
    def animate():
        redraw()
        drawArea.after(1000, animate)

    drawArea.bind("<Configure>", lambda e: redraw())
    
    root.update()

    timeZoneIdx.set(0)    
    drawTime()
    drawArea.after(100, animate)

    root.mainloop() 

if __name__ == "__main__":
    main()