""" Examples demonstrating that Python has Lisp-like closures
    (counter()) and that there's a bit of trickyness (counter2())

  >>> from counters import *
  >>> c1 = counter()
  >>> c2 = counter()
  >>> c1()
  1
  >>> c1()
  2
  >>> c2()
  1
  >>> cc1 = counter2()
  >>> cc1()
  Traceback (most recent call last):
    File "<stdin>", line 1, in ?
      File "counters.py", line 12, in _inc
          x += step
          UnboundLocalError: local variable 'x' referenced before assignment
  >>>
"""

def counter(start=0, step=1):

    """Defines and returns a new function that will act as a counter
    with initial value start and increment step.  Each time that
    functions is called it will increment the counter by step and
    return the new value.
    """

    x = [start]
    def _inc():
        x[0] += step
        return x[0]
    return _inc 

def counter2(start=0, step=1):
    """This one does not work"""
    x = start
    def _inc():
        x += step
        return x
    return _inc