Python tkinter, do something before subprocess starts -

i'm having problem python tkinter program. want press button start subprocess , indicate subprocess running changing label's value. subprocess takes time , problem label waits subprocess finished change, don't understand, because used variable first change label , go on subprocess. here code:

def program_final():     start = false     while true:         if start == false:              v.set("scanning...")             label.pack()             start = true         else:             # p = subprocess.popen('sudo nfc-poll', shell=true, stdout=subprocess.pipe, stderr=subprocess.stdout)     # opens subporocess starts nfc-polling in background             p = subprocess.popen('ping 8.8.8.8', shell=true, stdout=subprocess.pipe, stderr=subprocess.stdout)             counter = 0             output = ""             lines = []             while true:                 line = p.stdout.readline() # while loop iterates through output lines of                  lines.insert(counter, line) # subproccess , saves whole result list                 counter = counter +1 # result needed set output                 if counter == 9:                     break              if lines[6][7:10] == 'uid': # check if uid line present                 output = output + "tag found!\n" + lines[6][7:] # if yes output string gets added uid of tag             elif lines[6][7:10] != 'uid': # if uid line not present means no tag found                 output = output + "no tag found!\n" # output set no tag found              text.delete(1.0, end) # old tag infos getting deleted out of texfield             text.insert(insert, output) # tag infos or 'no tag found' message added textfield             break 

thanks in advance.

tkinter update graphic when there nothing else do. can force gui refresh if use update method on widgets want update.

tk.updates()