Tkinter Alarm Clock - python-3.x

Question

I am trying to build a countdown timer with tkinter. I want to pass on the values from the entries onto the countdown(count) function. Here is what I tried:
def countdown(count):
label['text'] = count
if count > 0:
top.after(1000, countdown,count-1)
top = tkinter.Tk()
top.geometry("700x100")
hoursT=tkinter.Label(top, text="Hours:")
hoursE=tkinter.Entry(top)
minuteT=tkinter.Label(top, text="Minutes:")
minuteE=tkinter.Entry(top)
secondT=tkinter.Label(top, text="Seconds:")
secondE=tkinter.Entry(top)
hoursT.grid(row=1,column=1)
hoursE.grid(row=1,column=2)
minuteT.grid(row=1,column=3)
minuteE.grid(row=1,column=4)
secondT.grid(row=1,column=5)
secondE.grid(row=1,column=6)
label = tkinter.Label(top)
label.grid(row=3)
t=(int(hoursE.get())*360+int(minuteT.get())*60+int(secondE.get())
button=tkinter.Button(top,text="Start Timer",command=lambda count=t:countdown(count))
button.grid(row=2)
However, I get this error:
Traceback (most recent call last):
File "C:\Users\charley.ACER-PC\AppData\Local\Programs\Python\Python35- 32\tkinterTutorial.py", line 30, in <module>
t=(int(hoursE.get())*360+int(minuteT.get())*60+int(secondE.get()))
ValueError: invalid literal for int() with base 10: ''
How can I run this code:
t=(int(hoursE.get())*360+int(minuteT.get())*60+int(secondE.get())
only when the entries are filled with integers?
Thanks :)

Answer 1

One solution is to have the button there in the first place and change the command at run time with event listeners:
button=tkinter.Button(top,text="Start Timer",command=lambda:None)
button.grid(row=2)
def updateButton():
hour,min,sec=hoursE.get(),minuteT.get(),secondE.get()
if hour.isdigit() and min.isdigit() and sec.isdigit():
time=int(hour)*360+int(min)*60+int(sec)
button.configure(command=lambda count=time:countdown(count))
for widget in (hoursE,minuteT,secondE):
widget.bind("<FocusOut>", updateButton)