yield keyword usage

Re: yield keyword usage

On Jul 30, 2007, at 4:13 PM, Ehsan wrote:
As in how it works? Sure, when you use the yield statement in a
function or method you turn it into a generator method that can then
be used for iteration. What happens is that when fibonacci(1000) is
called in the for loop statement it executes up to the yield
statement where it "yields" the then current value of a to the
calling context at which point n in the for loop is bound to that
value and the for loop executes one iteration. Upon the beginning of
the for loop's next iteration the fibonacci function continues
execution from the yield statment until it either reaches another
yield statement or ends. In this case, since the yield occured in a
loop, it will be the same yield statement at which point it will
"yield" the new value of a. It should be obvious now that this whole
process will repeat until the condition a < max is not longer true in
the fibonacci function at which point the function will return
without yielding a value and the main loop (for n in ...) will
terminate.



Erik Jones

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erik@myemma.com
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Re: yield keyword usage

On Jul 30, 4:40 pm, Erik Jones <e...@myemma.co mwrote:Also note that the function could terminate without ever executing
a yield statement (if max<0). In which case nothing would get printed
just like in

for n in []: print n,

You could also force the generator to abort rather than relying
on the while loop not executing by using a return instead of a yield
statement. This can be useful if the while executes even when given
bad arguments.

def fibonacci(max):
if max < 0: # test for invalid argument
print 'max must be >0' # diagnostic message
return # kill generator
a, b = 0, 1
while a < max:
yield a
a, b = b, a+b

for n in fibonacci(1000) :
print n,

print
print

for n in fibonacci(-1000):
print n,


## 0 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987
##
## max must be >0

Re: yield keyword usage

Ehsan wrote:A function body with at least one yield expression (formerly a yield
statement) in its body returns a generator when called:
.... for k in range(i):
.... yield k*k
....<generator object at 0x7ff2822c>The same function can be called multiple times to create independent
generators - the particular example I have given created a generator for
a given number of squares.

The generator can be used in an iterative context (such as a for loop),
but in actual fact it observes the Python iterator protocol, so the
generator has a .next() method, which can be used to extract the next
value from it. Calling the .next() method runs the generator until a
yield expression is encountered, and the value of the yield expression
is the return value of the next method:
014

When the generator function terminates (either with a return statement
or by dropping off the end) it raises a StopIteration exception. This
will terminate an iteration, but in other contexts it can be handled
just like any other exception:
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
StopIterationSo the fibonacci example you quote is a generator function that will
yield a sequence of Fibonacci numbers up to but not including the value
of its argument.

regards
Steve
--
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