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Newsgroups: comp.lang.python
Date: Sun, 24 Feb 2013 16:59:06 -0800 (PST)
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Subject: Re: yield expression
From: Ziliang Chen <zlchen.ken@gmail.com>
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Xref: csiph.com comp.lang.python:39842

On Monday, February 25, 2013 8:51:28 AM UTC+8, Oscar Benjamin wrote:
> On 25 February 2013 00:39, Ziliang Chen  wrote:
>=20
> > Hi folks,
>=20
> > When I am trying to understand "yield" expression in Python2.6, I did t=
he following coding. I have difficulty understanding why "val" will be "Non=
e" ? 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.
>=20
> >
>=20
> > Thanks !
>=20
> >
>=20
> > code snippet:
>=20
> > ----
>=20
> >  def counter(start_at=3D0):
>=20
> >      count =3D start_at
>=20
> >      while True:
>=20
> >          val =3D (yield count)
>=20
> >          if val is not None:
>=20
> >              count =3D val
>=20
> >          else:
>=20
> >              print 'val is None'
>=20
> >              count +=3D 1
>=20
>=20
>=20
> The value of the yield expression is usually None. yield only returns
>=20
> a value if the caller of a generator function sends one with the send
>=20
> method (this is not commonly used). The send method supplies a value
>=20
> to return from the yield expression and then returns the value yielded
>=20
> by the next yield expression. For example:
>=20
>=20
>=20
> >>> g =3D counter()
>=20
> >>> next(g)  # Need to call next() once to suspend at the first yield cal=
l
>=20
> 0
>=20
> >>> g.send('value for count')  # Now we can send a value for yield to ret=
urn
>=20
> 'value for count'
>=20
>=20
>=20
>=20
>=20
> Oscar

Thanks Oscar !
I am cleared. Only when "send" is used to feed "yield" a new value, the "yi=
eld" expression has none "None", otherwise, "yield" expression has "None" v=
alue.