yield expression

Oscar Benjamin oscar.j.benjamin at gmail.com
Mon Feb 25 01:51:28 CET 2013

On 25 February 2013 00:39, Ziliang Chen <zlchen.ken at gmail.com> wrote:
> Hi folks,
> When I am trying to understand "yield" expression in Python2.6, I did the following coding. I have difficulty understanding why "val" will be "None" ? What's happening under the hood? It seems to me very time the counter resumes to execute, it will assign "count" to "val", so "val" should NOT be "None" all the time.
> Thanks !
> code snippet:
> ----
>  def counter(start_at=0):
>      count = start_at
>      while True:
>          val = (yield count)
>          if val is not None:
>              count = val
>          else:
>              print 'val is None'
>              count += 1

The value of the yield expression is usually None. yield only returns
a value if the caller of a generator function sends one with the send
method (this is not commonly used). The send method supplies a value
to return from the yield expression and then returns the value yielded
by the next yield expression. For example:

>>> g = counter()
>>> next(g)  # Need to call next() once to suspend at the first yield call
>>> g.send('value for count')  # Now we can send a value for yield to return
'value for count'


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