yield expression

Dave Angel davea at davea.name
Tue Feb 26 18:05:15 CET 2013


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



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