Python！—— GUI程序设计

from tkinter import *

def processOK():
    print("OK button is clicked")

def processCancel():
    print("Cancel button is clicked")

window = Tk()
btOK = Button(window, text = "OK", fg = "red", command = processOK) # fg指定按钮的前景色（字体颜色）
btCancel = Button(window, text = "Cancel", bg = "yellow", command = processCancel) # bg = background

btOK.pack()
btCancel.pack()

window.mainloop()

上面是正常代码，但我发现一个有趣的现象。如果我把上面的部分代码改成：

btOK = Button(window, text = "OK", fg = "red", command = processOK()) # fg指定按钮的前景色（字体颜色）
btCancel = Button(window, text = "Cancel", bg = "yellow", command = processCancel()) # bg = background

就会发现控制台会如下显示，而且不论我怎么按界面的两个按钮，都不会有新的打印内容出现。这是为什么？