def currency(value, group_sep=' ', decimal_sep='.'):
   """Format currency by digits.

   >>> currency(100)
   '100.00'
   >>> currency(3.1415)
   '3.14'
   >>> currency(31415)
   '31 415.00'
   >>> currency(3141500.1)
   '3 141 500.10'
   >>> currency(31415000.1)
   '31 415 000.10'
   """
   head,tail = ('%.2f'%value).split('.')
   l = len(head)
   return group_sep.join([head[r-3:l+r] for r in xrange(0,-l,-3)][::-1]) + decimal_sep + tail