def check():
    if check_var.get() == True:
        while True:
            with open('get.txt', 'r') as file:
                data = file.read()
            if data == 'get':
                print('Something')
                with open('get.txt', 'w') as file:
                    file.write('')

 exitflag = False # a small hole, that will be used below
while True:
    with open('get.txt', 'r') as myfile:
        while True:
            myfile.seek(0)
            data = myfile.read()
            if 'get' in data:
                # Do some stuff here... 
                print 'I have found get... etc'
                print 'The file will be cleared now!'
                break
            
            # If data includes `exitnow` all nested loops are terminated 
            # (just a hole to quit all loops)
            if 'exitnow' in data:
                exitflag = True
                break
    try:
        with open('get.txt', 'w') as myfile:
            myfile.write('')
    except IOError:
        # Do exception stuff here
        print "Couldn't open the file for writing/clearing..."
    
    if exitflag:
        print "I've got exit-flag. Exiting immediately!"
        break

 import threading, time
def file_checking():
    exitflag = False # a small hole, that will be used below
    while True:
        with open('get.txt', 'r') as myfile:
            while True:
                time.sleep(1) # This is optional, i.e. the file checkd every 1 s.
                myfile.seek(0)
                data = myfile.read()
                if 'get' in data:
                    # Do some stuff here... 
                    print 'I have found get... etc'
                    print 'The file will be cleared now!'
                    break
                
                # If data includes `exitnow` all nested loops are terminated 
                # (just a hole to quit all loops)
                if 'exitnow' in data:
                    exitflag = True
                    break
        try:
            with open('get.txt', 'w') as myfile:
                myfile.write('')
        except IOError:
            # Do exception stuff here
            print "Couldn't open the file for writing/clearing..."
        
        if exitflag:
            print "I've got exit-flag. Exiting immediately!"
            break
checker = threading.Thread(target=file_checking)
checker.daemon = True
checker.start()
# This emulates main gui loop
ind = 0
for x in range(500):
    time.sleep(1)
    ind += 1
    print "I am executing... but the file is still checked, time={}".format(ind)