Generate labels for a multi-level outline

[castironpi at gmail.com]

'''
On May 6, 4:43 pm, pyt... at bdurham.com honoring:

>>> a.next( )
( "I", )
>>> a.throw( up )
( "I", "A" )
>>> a.next( )
( "I", "B" )
>>> a.next( )
( "I", "C" )
>>> a.throw( down )
( "II", )
'''

#funny declaration
class up( Exception ): pass
class down( Exception ): pass

def outline( ):
    stack= [ 1 ]
    while 1:
        try:
            yield stack
            stack[ -1 ]+= 1
        except up:
            stack.append( 1 )
        except down:
            stack.pop( -1 )
            stack[ -1 ]+= 1

a= outline( )
print a.next( )
print a.throw( up )
print a.next( )
print a.next( )
print a.throw( down )
print a.throw( up )
print a.throw( up )
print a.next( )
print a.next( )
print a.throw( down )

##output:

[1]
[1, 1]
[1, 2]
[1, 3]
[2]
[2, 1]
[2, 1, 1]
[2, 1, 2]
[2, 1, 3]
[2, 2]

##

cf.

formatter.NullFormatter.format_counter
formatter.NullFormatter.format_letter
formatter.NullFormatter.format_roman