Question about code writing '% i, callback'

[fl]

On Monday, November 30, 2015 at 12:37:52 PM UTC-5, Terry Reedy wrote:
> On 11/30/2015 11:44 AM, fl wrote:
> 
> > I come across the following code snippet.
> 
> > for i in range(10):
> >      def callback():
> >          print "clicked button", i
> >      UI.Button("button %s" % i, callback)
> 
> > http://effbot.org/zone/default-values.htm
> 
> Note that the above is an intentional example of common buggy code.  It 
> is followed by a version that works, with 'i=i' added to the callback 
> header.
> 
> -- 
> Terry Jan Reedy

With the following code, there is no bug as the original author said.


class buibutton():
    print 'sd'
    def __nonzero__(self):
       return False
       
    def Button(self, ii, callbackk):
        callbackk()
        return

for i in range(10):
    def callback():
        print "clicked button", i
    UI.Button("button %s" % i, callback)

only to find that all callbacks print the same value (most likely 9, in this case). 

Why does it have no bug?