NameError: name 'clear_message' is not defined - python-3.x

Question

from Tkinter import *
import tkinter as tk # python3
TITLE_FONT = ("Helvetica", 18, "bold")
class SampleApp(tk.Tk):
def __init__(self, *args, **kwargs):
tk.Tk.__init__(self, *args, **kwargs)
# the container is where I stack a bunch of frames
# on top of each other, then the one we want visible
# will be raised above the others
container = tk.Frame(self)
container.pack(side="top", fill="both", expand=True)
container.grid_rowconfigure(0, weight=1)
container.grid_columnconfigure(0, weight=1)
self.frames = {}
for F in (MainPage,StorageOrMotor,Storage,Motor):
page_name = F.__name__
frame = F(container, self)
self.frames[page_name] = frame
# put all of the pages in the same location;
# the one on the top of the stacking order
# will be the one that is visible.
frame.grid(row=0, column=0, sticky="nsew")
self.show_frame("MainPage")
def show_frame(self, page_name):
'''Show a frame for the given page name'''
frame = self.frames[page_name]
frame.tkraise()
class MainPage(tk.Frame):
global user_key
global psw_key
user_key=""
psw_key=""
#Here is the defined for clear_message `def`
def clear_message():
user_key.delete(0, 'END')
psw_key.delete(0, 'END')
def __init__(self, parent, controller):
tk.Frame.__init__(self, parent)
self.controller = controller
Username = tk.Label(self, text="Username:",font=("Helvetica", "20","bold"))
Username.grid(row=2, column=3,columnspan=2)
Password = tk.Label(self, text="Password:",font=("Helvetica", "20","bold"))
Password.grid(row=3, column=3,columnspan=2)
#............
Username_key = tk.Entry(self, textvariable = user_key, width=19, font=("Helvetica", "15"))
Username_key.grid(row=2, column=5,columnspan=5)
Password_key = tk.Entry(self, textvariable = psw_key, width=19, font=("Helvetica", "15"))
Password_key.grid(row=3, column=5,columnspan=5)
log_in = tk.Button(self, width=7, text="Log In", command=lambda: controller.show_frame("StorageOrMotor"))
log_in.grid(row=5,column=8,columnspan=2)
#............`I try to create a clear button`
Clear = tk.Button(self, width=7, text=" Clear " ,command=lambda:clear_message())
Clear.grid(row=5,column=5,columnspan=2)

Answer 1

clear_message() belongs to MainPage class so when calling it inside that class, you need to use self. prefix. Also, you need to use self as a parameter when defining the method since it is a member of a class.
def clear_message(self): #self here
user_key.delete(0, 'END')
psw_key.delete(0, 'END')
Additionally, since clear_message doesn't take any arguments, you don't need a lambda expression there.
#self here and removed lambda
Clear = tk.Button(self, width=7, text=" Clear " ,command=self.clear_message)