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 Ziliang Chen 02-25-2013 12:36 AM

yield expression

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

 Colin J. Williams 02-26-2013 04:34 PM

Re: yield expression

On 24/02/2013 7:36 PM, 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

Perhaps it's becaoue (teild count) is a statement. Statements do not
return a value.

Colin W.
>

 Ian Kelly 02-26-2013 04:50 PM

Re: yield expression

On Tue, Feb 26, 2013 at 9:34 AM, Colin J. Williams <cjw@ncf.ca> wrote:
> Perhaps it's becaoue (teild count) is a statement. Statements do not return
> a value.

yield is a bit of an odd duck in that it's both a statement and an
expression. Compare:

http://docs.python.org/3/reference/s...ield-statement

http://docs.python.org/3/reference/e...html#yieldexpr

 Dave Angel 02-26-2013 05:05 PM

Re: yield expression

On 02/26/2013 11:34 AM, Colin J. Williams wrote:
> On 24/02/2013 7:36 PM, 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

>
> Perhaps it's becaoue (teild count) is a statement. Statements do not
> return a value.
>
> Colin W.
>>

>

'yield count' is a yield_expression, not always a statement. If it were
the first thing in a statement, it'd be a yield_stmt

See the docs: http://docs.python.org/2/reference/simple_stmts.html

assignment_stmt ::= (target_list "=")+ (expression_list | yield_expression)

and http://docs.python.org/2/reference/expressions.html

yield_atom ::= "(" yield_expression ")"
yield_expression ::= "yield" [expression_list]

The value produced by the yield expression is produced by a.send()
method. This allows an approximation to coroutines.

I believe this dual usage of yield started in Python 2.5

--
DaveA

 Vytas D. 02-26-2013 05:07 PM

Re: yield expression

Hi,

You are using "yield" incorrectly. "yield" works like return, but it can
return more than once from the same function. Functions that "yield"
produce a so called "generator" object. This generator object gives you
values every time you call it.

The generator works very interesting way. It starts like normal function
and goes until it finds "yield" and returns the value. The state of
generator is saved - it is like it is put to sleep until you call it again.
So the next time you call generator() it runs from the point it returned
last time and will return you another value.

Simple sample of making and using generator (prints forever, so just kill
with CTRL+C).

def counter(start_at=0):
"""Returns integer each time called"""

count = start_at
while True:
yield count
count += 1

def main():
generator = counter()

while True:
print(next(generator))

if __name__ == '__main__':
main()

Hope helps.

Vytas D.

On Tue, Feb 26, 2013 at 4:34 PM, Colin J. Williams <cjw@ncf.ca> wrote:

> On 24/02/2013 7:36 PM, 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
>>

>
> Perhaps it's becaoue (teild count) is a statement. Statements do not
> return a value.
>
> Colin W.
>
>>
>>

> --
> http://mail.python.org/**mailman/listinfo/python-list<http://mail.python.org/mailman/listinfo/python-list>
>

 Colin J. Williams 02-26-2013 06:44 PM

Re: yield expression

On 26/02/2013 12:07 PM, Vytas D. wrote:
> Hi,
>
> You are using "yield" incorrectly. "yield" works like return, but it can
> return more than once from the same function. Functions that "yield"
> produce a so called "generator" object. This generator object gives you
> values every time you call it.
>
> The generator works very interesting way. It starts like normal function
> and goes until it finds "yield" and returns the value. The state of
> generator is saved - it is like it is put to sleep until you call it
> again. So the next time you call generator() it runs from the point it
> returned last time and will return you another value.
>
> Simple sample of making and using generator (prints forever, so just
> kill with CTRL+C).
>
> def counter(start_at=0):
> """Returns integer each time called"""
>
> count = start_at
> while True:
> yield count
> count += 1
>
> def main():
> generator = counter()
>
> while True:
> print(next(generator))
>
>
> if __name__ == '__main__':
> main()
>
>
> Hope helps.
>
> Vytas D.
>
>
>
> On Tue, Feb 26, 2013 at 4:34 PM, Colin J. Williams <cjw@ncf.ca
> <mailto:cjw@ncf.ca>> wrote:
>
> On 24/02/2013 7:36 PM, 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
>
>
> Perhaps it's becaoue (teild count) is a statement. Statements do
> not return a value.
>
> Colin W.
>
>
>
> --
> http://mail.python.org/__mailman/listinfo/python-list
> <http://mail.python.org/mailman/listinfo/python-list>
>
>

Yes, it's very helpful. Thanks also to the other two responders.

This brings us back to the OP question. Why not " val = (yield count)"?

Colin W.

 Colin J. Williams 02-26-2013 06:44 PM

Re: yield expression

On 26/02/2013 12:07 PM, Vytas D. wrote:
> Hi,
>
> You are using "yield" incorrectly. "yield" works like return, but it can
> return more than once from the same function. Functions that "yield"
> produce a so called "generator" object. This generator object gives you
> values every time you call it.
>
> The generator works very interesting way. It starts like normal function
> and goes until it finds "yield" and returns the value. The state of
> generator is saved - it is like it is put to sleep until you call it
> again. So the next time you call generator() it runs from the point it
> returned last time and will return you another value.
>
> Simple sample of making and using generator (prints forever, so just
> kill with CTRL+C).
>
> def counter(start_at=0):
> """Returns integer each time called"""
>
> count = start_at
> while True:
> yield count
> count += 1
>
> def main():
> generator = counter()
>
> while True:
> print(next(generator))
>
>
> if __name__ == '__main__':
> main()
>
>
> Hope helps.
>
> Vytas D.
>
>
>
> On Tue, Feb 26, 2013 at 4:34 PM, Colin J. Williams <cjw@ncf.ca
> <mailto:cjw@ncf.ca>> wrote:
>
> On 24/02/2013 7:36 PM, 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
>
>
> Perhaps it's becaoue (teild count) is a statement. Statements do
> not return a value.
>
> Colin W.
>
>
>
> --
> http://mail.python.org/__mailman/listinfo/python-list
> <http://mail.python.org/mailman/listinfo/python-list>
>
>

Yes, it's very helpful. Thanks also to the other two responders.

This brings us back to the OP question. Why not " val = (yield count)"?

Colin W.

 Dave Angel 02-27-2013 02:59 AM

Re: yield expression

On 02/26/2013 01:44 PM, Colin J. Williams wrote:
> On 26/02/2013 12:07 PM, Vytas D. wrote:
>> Hi,
>>
>> You are using "yield" incorrectly. "yield" works like return, but it can
>> return more than once from the same function. Functions that "yield"
>> produce a so called "generator" object. This generator object gives you
>> values every time you call it.
>>
>> The generator works very interesting way. It starts like normal function
>> and goes until it finds "yield" and returns the value. The state of
>> generator is saved - it is like it is put to sleep until you call it
>> again. So the next time you call generator() it runs from the point it
>> returned last time and will return you another value.
>>
>> Simple sample of making and using generator (prints forever, so just
>> kill with CTRL+C).
>>
>> def counter(start_at=0):
>> """Returns integer each time called"""
>>
>> count = start_at
>> while True:
>> yield count
>> count += 1
>>
>> def main():
>> generator = counter()
>>
>> while True:
>> print(next(generator))
>>
>>
>> if __name__ == '__main__':
>> main()
>>
>>
>> Hope helps.
>>
>> Vytas D.
>>
>>
>>
>> On Tue, Feb 26, 2013 at 4:34 PM, Colin J. Williams <cjw@ncf.ca
>> <mailto:cjw@ncf.ca>> wrote:
>>
>> On 24/02/2013 7:36 PM, 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
>>
>>
>> Perhaps it's becaoue (teild count) is a statement. Statements do
>> not return a value.
>>
>> Colin W.
>>
>>
>>
>> --
>> http://mail.python.org/__mailman/listinfo/python-list
>> <http://mail.python.org/mailman/listinfo/python-list>
>>
>>

> Yes, it's very helpful. Thanks also to the other two responders.
>
> This brings us back to the OP question. Why not " val = (yield count)"?
>
> Colin W.
>
>

Repeating a misconception doesn't make it any more true. yield is both
an expression and a statement. Nothing wrong with the statement
val = (yield count)

although he apparently wasn't making any good use of the possible return
value, since he observed it was always None. That's just because he
didn't use the send method in the calling function.

Most links I see on a web search explain only the expression form of
yield, which is why i took a lot of time building the response I did
earlier.

--
DaveA

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