<p>I create a new Button object but did not specify the <code>command</code> option upon creation. Is there a way in Tkinter to change the command (onclick) function after the object has been created?</p>

Sure; just use the
```
bind
```
method to specify the callback after the button has been created.  I've just written and tested the example below.  You can find a nice tutorial on doing this at
http://www.pythonware.com/library/tkinter/introduction/events-and-bindings.htm
```
```
from Tkinter import Tk, Button

root = Tk()
button = Button(root, text="Click Me!")
button.pack()

def callback(event):
    print "Hello World!"

button.bind("<Button-1>", callback)
root.mainloop()
```
```