Question about code writing '% i, callback'

Mon Nov 30 12:50:42 EST 2015

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?