>>>def myG():
...  yield 1
...  yield 2
...  yield 3
...
>>>g=myG()
>>>next(g)
1
>>>next(g)
2
>>>next(g)
3
>>>next(g)
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
StopIteration
>>>g2=myG()
>>>for i in g2:
...  print(i)
1
2
3

>>>a=[7,8,9]
>>>b=[i**2 for i in a]
>>>b
[49, 64, 81]
>>>ib=(i**2 for i in a)
>>>ib
<generator object <genexpr> at 0x7f72291217e0>
>>>next(ib)
49
>>>next(ib)
64
>>>next(ib)
81
>>>next(ib)
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
StopIteration

def squares(start, stop):
  for i in xrange(start, stop):
    yield i*i

（i*i for i in xrange(start, stop))

[i*i for i in xrange(start, stop)]

class Squares(object):
  def __init__(self, start, stop):
    self.start = start
    self.stop = stop
  def __iter__(self):
    return self
  def next(self):
    if self.start >= self.stop:
      raise StopIteration
    current = self.start * self.start
    self.start += 1
    return current

def current(self):
  return self.start