title: Python, Problem get value form combobox with button
body: <p>I try to get the value of the current combobox with a button and display the message.
  But i do something wrong to take correctly the value selected.</p>
  
  <pre><code>AttributeError: 'SelectDB' object has no attribute 'cmb'
  </code></pre>
  
  <p>How i do to get the combobox value ?</p>
  
  <pre><code>import sqlite3
  from tkinter import *
  from tkinter import ttk
  from tkinter import messagebox
  
  class SelectDB:
      def __init__(self, wind) :
          self.wind = wind
          self.wind.title ('MyApp')
          ttk.Label(self.wind, text="Select you're Database:").grid (row = 0, column = 0)
          ttk.Combobox(self.wind, width="10", values=("Local (sqllite)","MYSQL")).grid (row = 0, column = 1)
          ttk.Button(text="Start", command=self.checkcmbo).grid (row = 1, column = 0)
  
      def checkcmbo(self):
          if self.cmb.get() == "Local (sqllite)":
              messagebox.showinfo("What user choose", "you choose Local (sqllite)")
          elif self.cmb.get(self) == "MYSQL":
              messagebox.showinfo("What user choose", "you choose MYSQL")
          else:
              messagebox.showinfo("What user choose", "NOTHING")
  
  if __name__ == '__main__':
      wind = Tk()
      application = SelectDB(wind)
      wind.mainloop()
  </code></pre>