python - "Resetting" the arguments of a variable -

i trying reset arguments in variable def sequence when user inputs invalid sequence. otherwise runtimeerror: maximum recursion depth exceeded error because argument stored in def sequence remains invalid. suggestions? thinking of putting "self.sequence(none)" in else: adds argument def sequence seems.

from tkinter import *  class at_content_calculator:      def __init__(self, master):         #initialising various widgets         frame_1 = frame(master)         frame_1.pack()          self.varoutput_1 = stringvar()          self.label_1 = label(frame_1, text="please enter dna sequence:")         self.label_1.pack()         self.entry_1 = entry(frame_1, textvariable=self.sequence)         self.entry_1.pack()         self.output_1 = label(frame_1, textvariable=self.varoutput_1)         self.output_1.pack()         self.button_1 = button(frame_1, text="calculate", command=self.validation_check)         self.button_1.pack()      def sequence(self):      self.dna_sequence = self.entry_1.get()      return self.dna_sequence      def validation_check(self):         #used validate self.dna_sequence contains letters g, c, a, t         valid = 'gcat'         condition = all(i in valid in self.sequence())         if condition:             self.at_calculate()         else:             self.varoutput_1.set("invalid dna sequence. please enter again.")             self.validation_check()      def at_calculate(self):         #used calculate @ content of string stored in self.dna_sequence         self.dna_sequence = self.entry_1.get()         self.total_bases = len(self.dna_sequence)         self.a_bases = self.dna_sequence.count("a")         self.b_bases = self.dna_sequence.count("t")         self.at_content = "%.2f" % ((self.a_bases + self.b_bases) / self.total_bases)         self.varoutput_1.set("at content percentage: " + self.at_content)  root = tk() root.title("at content calculator") root.geometry("320x320") b = at_content_calculator(root) root.mainloop() 

the problem isn't "resetting" value of variable. problem repeatedly call validation_check within without changing value being validated, naturally end recursion error. shouldn't call method again @ all: show message telling user validation has failed, nothing , wait them change value , press button again.

(and looping better here recursion, anyway; don't need either, let method end.)