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References: <1k19ubi.dwyvio12tkc4kN%see@sig.for.address>
Date: Sat, 14 May 2011 17:47:00 -0700
Subject: Re: I don't understand generator.send()
From: Chris Rebert <crebert@ucsd.edu>
Cc: python-list@python.org
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On Sat, May 14, 2011 at 5:08 PM, Victor Eijkhout <see@sig.for.address> wrot=
e:
> #! /usr/bin/env python
>
> def ints():
> =C2=A0 =C2=A0i=3D0
> =C2=A0 =C2=A0while True:
> =C2=A0 =C2=A0 =C2=A0 =C2=A0yield i
> =C2=A0 =C2=A0 =C2=A0 =C2=A0i +=3D 1
>
> gen =3D ints()
> while True:
> =C2=A0 =C2=A0i =3D gen.next()
> =C2=A0 =C2=A0print i
> =C2=A0 =C2=A0if i=3D=3D5:
> =C2=A0 =C2=A0 =C2=A0 =C2=A0r =3D gen.send(2)
> =C2=A0 =C2=A0 =C2=A0 =C2=A0print "return:",r
> =C2=A0 =C2=A0if i>10:
> =C2=A0 =C2=A0 =C2=A0 =C2=A0break
>
> I thought the send call would push the value "2" at the front of the
> queue. Instead it coughs up the 2, which seems senseless to me.
>
> 1/ How should I view the send call? I'm reading the manual and dont' get
> it

`yield` is an expression. Within the generator, the result of that
expression is [, ignoring the complications of .throw() etc.,] the
argument to .send(). You're currently using `yield` only as a
statement, so it's no wonder you're not quite understanding .send(). I
think this example should clarify things somewhat:

>>> def example(start):
...     i =3D ord(start)
...     while True:
...         sent =3D (yield chr(i)) # Note use of yield as expression
...         print('was sent', sent)
...         i +=3D 1
...
>>> g =3D example('a')
>>> g.send(3)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: can't send non-None value to a just-started generator
>>> # Ok, so we can't send something back to `yield`
>>> # until we hit the first `yield`.
>>> g.send(None) # Follow the error message's advice
'a'
>>> g.send(3) # Let's try again now.
was sent 3
'b'
>>> g.send(5)
was sent 5
'c'
>>> g.send(9)
was sent 9
'd'
>>>

Cheers,
Chris
--
http://rebertia.com