def action(event):
        
        flag=0
        Text=event.widget.cget("text")
        full_text=display.get()
       
        if(full_text and full_text[-1] not in "+-*/" and Text not in'=1234567890'):
            display.insert(END,Text)
        elif(Text not in "+-*/="):
            if(flag==1):
                print(flag)
                display.delete(0,END)
                display.insert(END,Text)
                flag=0
                
            else:
                
                display.insert(END,Text)
                
        elif(Text == '='):
            try:
                display.delete(0, END)  
                result = str(eval(full_text)) 
                display.insert(END, result)
                flag=1
       
            except ZeroDivisionError:
                display.insert(END,"UNDEFINED")

In this code when equal is clicked flag will be 1. Then after equals result. Any number clicked it should delete and and add the text. But it is not working.