login_example.py

from tkinter import *
from tkinter.ttk import *

class App(Frame):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.count_failed_logins = 0
        self.logged_in_user = None
        # sits at bottom of login screen; invisible until invalid login
        self.error_label = Label(self, foreground='red')
        
        self.show_login_screen()
        
    def clear_widgets(self):
        for w in self.winfo_children():
            w.pack_forget() 
        
    def logout(self, *events):
        self.clear_widgets()
        self.logged_in_user = None
        self.show_login_screen()
        
    def show_main_window(self):
        Label(self, text='Welcome, {}'.format(self.logged_in_user)).pack()
        
        Frame(self).pack(fill=Y, expand=True)  # vertical padding
        l = Label(self, text='logout', font='helvitica 7')
        l.pack(anchor='e')
        l.bind('<Button-1>', self.logout)
        
    def show_login_screen(self):
        Frame(self).pack(fill=Y, expand=True)  # vertical padding
        f = Frame(self)
        f.pack()
        
        Label(f, text='Username:').grid(row=0,column=0, pady=4)
        Label(f, text='Password:').grid(row=1,column=0, pady=4)
        
        self.name_input = Entry(f, width=20)
        self.pass_input = Entry(f, width=20, show='☺')
        
        self.name_input.grid(row=0,column=1, pady=4)
        self.pass_input.grid(row=1,column=1, pady=4)
        
        Button(self, text='login', command=self.verify_creds).pack()
        Frame(self).pack(fill=Y, expand=True)  # vertical padding
        
        self.error_label.pack()
        
        # set cursor on the username
        self.name_input.focus_set()
        
        # allow the user to press Enter instead of clicking login button
        self.name_input.bind('<Return>', self.verify_creds)
        self.pass_input.bind('<Return>', self.verify_creds)
    
    def verify_creds(self, *events):  # events are discarded if it's bound
        u = self.name_input.get()
        p = self.pass_input.get()
        if u == 'admin' and p == 'admin':
            # login correct, do whatever
            self.clear_widgets()
            self.logged_in_user = u
            self.error_label.configure(text='')
            self.show_main_window()
        else:
            if self.count_failed_logins == 0:
                self.error_label.configure(text='invalid credentials')
            self.count_failed_logins += 1


class Root(Tk):
    '''
    main window object serves as style manager and entry point for 
    the main app widget    
    '''
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.title('Login Example')
        self.geometry('500x400')
        self.style = Style(self)    # ttk makes it look nicer
        self.style.configure("TEntry", padding=4)
        self.style.configure("TLabel", font='courier 10 bold', padding=3)
        
        App(self).pack(expand=True, fill=BOTH)


def main():
    Root().mainloop()


if __name__ == '__main__':
    main()