re.match in AT content calculator not working

Question

Working out a few kinks in this code and for some reason my method validation check is not quite working. All I want it to do is to validate that the input from the user ONLY contains letters G, C, A, T before moving onto the method at_calculate which performs the maths on the input sequence. Any help/tips would be appreciated.

import re

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 a DNA sequence:")
        self.label_1.pack()
        self.entry_1 = Entry(frame_1, textvariable=self.dna_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 dna_sequence(self):
        self.dna_sequence = ()

    def validation_check(self):
        #used to validate that self.dna_sequence only contains letters G, C, A, T
        if re.match(r"GCAT", self.dna_sequence):
            self.at_calculate()
        else:
            self.varoutput_1.append = "Invalid DNA sequence. Please enter again."
            self.validation_check()

    def at_calculate(self):
        #used to calculate AT 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()

Answer 1

If you want to validate the input from the user ONLY contains letters G, C, A, T you need to put the characters within a character class that will match any combinations of this characters :

Note :self.dna_sequence is a function and you can't pass it to match function although its incorrect.you need to return the input value within that function :

def dna_sequence(self):
     dna_sequence = self.entry_1.get()
     return dna_sequence

and then do :

if re.match(r"[GCAT]+", self.dna_sequence()):

[GCAT]+ will match any combinations of that characters with length 1 or more. if you want that be in length 4 you can use [GCAT]+{4}.

But this also will match duplicated characters. like GGCC.If you don't want such thing you can use set.intersection :

if len(self.dna_sequence())==4 and len(set(GCAT).intersection(self.dna_sequence()))==4:
      #do stuff

Or as a better way :

if sorted(self.dna_sequence)==ACGT:
      #do stuff