问Python -为什么这个实例方法在另一个实例方法中调用时不需要括号？

EN

# Click Counter
# Demonstrates binding an event with an event handler

from Tkinter import *

class Skeleton(Frame):
   """ GUI application which counts button clicks. """
   def __init__(self, master):
       """ Initialize the frame. """
       Frame.__init__(self, master)
       self.grid()
       self.bttn_clicks = 0 # the number of button clicks
       self.create_widget()

   def create_widget(self):
       """ Create button which displays number of clicks. """
       self.bttn = Button(self)
       self.bttn["text"] = "Total Clicks: 0"
       # the command option invokes the method update_count() on click
       self.bttn["command"] = self.update_count
       self.bttn.grid()

   def update_count(self):
       """ Increase click count and display new total. """
       self.bttn_clicks += 1
       self.bttn["text"] = "Total Clicks: "+ str(self.bttn_clicks)

# main root = Tk() root.title("Click Counter") root.geometry("200x50")

app = Skeleton(root)

root.mainloop()