Frame button binding

Hi all,

I want to create a Scrolled Frame, it works nicely with the place command
however when I bind the B2-Motion, and I click the middle mouse button
on the child buttons instead of dragging it does nothing, since they are 
capturing the B2 and not propagate it to the master.

SimplifiedExample:

def dragStart(e):
    print "B2-pressed")

f = Frame(tk)
f.pack(expand=YES, fill=BOTH)
f.bind("<Button-2>", dragStart)

Button(f, text="Button").pack()

I found a not elegant solution of setting the bind to the toplevel like
f.winfo_toplevel().bind("<Button-2>", dragStart)

and in dragStart I am checking
     if not str(e.widget).startswith(str(f)): return

Is there a better way of doing it?

Thanks in advance
Vasilis