why UnboundLocalError?

[Alex Gittens]

I'm trying to define a function that prints fields of given widths
with specified alignments; to do so, I wrote some helper functions
nested inside of the print function itself. I'm getting an
UnboundLocalError, and after reading the Naming and binding section in
the Python docs, I don't see why.

Here's the error:
>>> fieldprint([5, 4], 'rl', ['Ae', 'Lau'])
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "fieldprint.py", line 35, in fieldprint
    str += cutbits()
  File "fieldprint.py", line 11, in cutbits
    for i in range(0, len(fields)):
UnboundLocalError: local variable 'fields' referenced before assignment

This is the code:
def fieldprint(widths,align,fields):

        def measure():
                totallen = 0
                for i in range(0, len(fields)):
                        totallen += len(fields[i])
                return totallen

        def cutbits():
                cutbit = []
                for i in range(0, len(fields)):
                        if len(fields[i]) >= widths[i]:
                                cutbit.append(fields[i][:widths[i]])
                                fields = fields[widths[i]:]
                        elif len(fields[i]) > 0:
                                leftover = widths[i] - len(fields[i])
                                if align[i] == 'r':
                                        cutbit.append(' '*leftover + fields[i])
                                elif align[i] == 'l':
                                        cutbit.append(fields[i] + ' '*leftover)
                                else:
                                        raise 'Unsupported alignment option'
                                fields[i] = ''
                        else:
                                cutbit.append(' '*widths[i])
                return cutbit.join('')

        if len(widths) != len(fields) or len(widths)!=len(align) or
len(align)!=len(fields):
                raise 'Argument mismatch'

        str = ''


        while measure()!=0:
                str += cutbits()

What's causing the error?

Thanks,
Alex
-- 
ChapterZero: http://tangentspace.net/cz/