yield expression
Ziliang Chen
zlchen.ken at gmail.com
Sun Feb 24 19:59:06 EST 2013
On Monday, February 25, 2013 8:51:28 AM UTC+8, Oscar Benjamin wrote:
> On 25 February 2013 00:39, Ziliang Chen 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
>
> 0
>
> >>> g.send('value for count') # Now we can send a value for yield to return
>
> 'value for count'
>
>
>
>
>
> Oscar
Thanks Oscar !
I am cleared. Only when "send" is used to feed "yield" a new value, the "yield" expression has none "None", otherwise, "yield" expression has "None" value.
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