Tic Tac Toe game in Python

 from tkinter import *

from tkinter import messagebox

import random as r

def button(frame):          #Function to define a button

    b=Button(frame,padx=1,bg="papaya whip",width=3,text="   ",font=('arial',60,'bold'),relief="sunken",bd=10)

    return b

def change_a():             #Function to change the operand for the next player

    global a

    for i in ['O','X']:

        if not(i==a):

            a=i

            break

def reset():                #Resets the game

    global a

    for i in range(3):

        for j in range(3):

                b[i][j]["text"]=" "

                b[i][j]["state"]=NORMAL

    a=r.choice(['O','X'])

def check():                #Checks for victory or Draw

    for i in range(3):

            if(b[i][0]["text"]==b[i][1]["text"]==b[i][2]["text"]==a or b[0][i]["text"]==b[1][i]["text"]==b[2][i]["text"]==a):

                    messagebox.showinfo("Congrats!!","'"+a+"' has won")

                    reset()

    if(b[0][0]["text"]==b[1][1]["text"]==b[2][2]["text"]==a or b[0][2]["text"]==b[1][1]["text"]==b[2][0]["text"]==a):

        messagebox.showinfo("Congrats!!","'"+a+"' has won")

        reset()   

    elif(b[0][0]["state"]==b[0][1]["state"]==b[0][2]["state"]==b[1][0]["state"]==b[1][1]["state"]==b[1][2]["state"]==b[2][0]["state"]==b[2][1]["state"]==b[2][2]["state"]==DISABLED):

        messagebox.showinfo("Tied!!","The match ended in a draw")

        reset()

def click(row,col):

        b[row][col].config(text=a,state=DISABLED,disabledforeground=colour[a])

        check()

        change_a()

        label.config(text=a+"'s Chance")

###############   Main Program #################

root=Tk()                   #Window defined

root.title("Tic-Tac-Toe")   #Title given

a=r.choice(['O','X'])       #Two operators defined

colour={'O':"deep sky blue",'X':"lawn green"}

b=[[],[],[]]

for i in range(3):

        for j in range(3):

                b[i].append(button(root))

                b[i][j].config(command= lambda row=i,col=j:click(row,col))

                b[i][j].grid(row=i,column=j)

label=Label(text=a+"'s Chance",font=('arial',20,'bold'))

label.grid(row=3,column=0,columnspan=3)

root.mainloop()