generator.py

def foo(size, default=0, existing=None):
  """
  Yield values to a certain size, using an existing value at the same
  index or the default value, in that order.
  
  >>> f = foo(1)
  >>> f.next()
  0
  >>> f.next()
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
  StopIteration
  >>> f = foo(2, 'bar', (5,))
  >>> f.next()
  5
  >>> f.next()
  'bar'

  """
  existing = existing or []
  for x in range(size):
    try:
      yield existing[x]
    except IndexError:
      yield default