Python yield a list with generator

Question

I was getting confused by the purpose of "return" and "yield"

def countMoreThanOne():
    return (yy for yy in xrange(1,10,2))

def countMoreThanOne():
    yield (yy for yy in xrange(1,10,2))

What is the difference on the above function? Is it impossible to access the content inside the function using yield?

possible duplicate of [Python: generator expression vs. yield](http://stackoverflow.com/questions/1995418/python-generator-expression-vs-yield) — fredtantini, Nov 19 '14 at 11:50
in short - you are returning a generator of generator (with 1 element) in second function and a generator in first function. — Rusty, Nov 19 '14 at 11:52
what do you mean "Is it impossible to access the content inside the function using yield?" ? — Anentropic, Nov 19 '14 at 12:37
when you use the `yield` keyword in a function it becomes a generator function - when you call it you get a generator back. So by yielding a generator expression in the second example you've made a generator inside a generator https://wiki.python.org/moin/Generators — Anentropic, Nov 19 '14 at 12:43
@Anentropic Thanks for clearing the idea behind the second example, I was try to create a function which aim to use only generator then list for returning the result. Since I saw most of the example of using generator are come up with `yield`. — gostriderful, Nov 19 '14 at 13:57
it would be better to just write the function as `def countMoreThanOne(): return xrange(1,10,2)` ...an [xrange object](https://docs.python.org/2/library/stdtypes.html#typesseq-xrange) is iterable and has much the same benefit of a generator (constant memory usage no matter how big the sequence) — Anentropic, Nov 19 '14 at 14:03
Does this answer your question? [What does the "yield" keyword do?](https://stackoverflow.com/questions/231767/what-does-the-yield-keyword-do) — Willy satrio nugroho, Nov 19 '21 at 02:48

Vishnu Upadhyay · Accepted Answer · 2014-11-19T12:05:42.640

4

In first you return a generator

from itertools import chain
def countMoreThanOne():
    return (yy for yy in xrange(1,10,2))

print list(countMoreThanOne())

>>> 
[1, 3, 5, 7, 9]

while in this you are yielding a generator so that a generator within the generator

def countMoreThanOne():
    yield (yy for yy in xrange(1,10,2))

print list(countMoreThanOne())
print list(chain.from_iterable(countMoreThanOne()))

 [<generator object <genexpr> at 0x7f0fd85c8f00>]
[1, 3, 5, 7, 9]

if you use list comprehension then difference can be clearly seen:-

in first:-

def countMoreThanOne():
    return [yy for yy in xrange(1,10,2)]
print countMoreThanOne()

>>> 
[1, 3, 5, 7, 9]

def countMoreThanOne1():
    yield [yy for yy in xrange(1,10,2)]
print countMoreThanOne1()

<generator object countMoreThanOne1 at 0x7fca33f70eb0>
>>>

edited Nov 19 '14 at 12:05

answered Nov 19 '14 at 12:00

Vishnu Upadhyay

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I would add that the _intention_ of the second function (i.e. a worthwhile use of `yield`) would be to change it to: `def countMoreThanOne(): for yy in xrange(1,10,2): yield yy` – Anentropic Nov 19 '14 at 12:19
@Anentropic in this case just a `generator` will return only. – Vishnu Upadhyay Nov 19 '14 at 12:24
yes, I was trying to demonstrate how to use the `yield` keyword in a useful way – Anentropic Nov 19 '14 at 12:34
@Anentropic thanks for your suggestion. i was clearing the problem as it was asked. – Vishnu Upadhyay Nov 19 '14 at 12:39
So in my second example, it is not a proper way of using "yield" to achieve the purpose of my function? – gostriderful Nov 19 '14 at 13:52
@gostriderful No this is not the way to achieve generators. see Anentropic comments, he described it. – Vishnu Upadhyay Nov 19 '14 at 13:53
So, in my example, either I use `yield` or just `return` the `(yy for yy in xrange(1,10,2))` is the right way? – gostriderful Nov 19 '14 at 14:02
@gostriderful you will get data from both but why to use double operations when you can do it in just once. – Vishnu Upadhyay Nov 19 '14 at 14:03
@gostriderful if you find answer helping and correct please mark correct and pramote me on Stack Overflow. :-) – Vishnu Upadhyay Nov 19 '14 at 14:09
Because in my implementation, the function accepts parameter to set the limits steps. At my first thought, I only know that I have to return a sequence of generator. But I have got confused by the `yield` and `return` keyword. @VishnuUpadhyay @Anentropic – gostriderful Nov 19 '14 at 14:18

score 0 · Answer 2 · answered Nov 19 '14 at 14:09

After reading your other comments I think you should write the function like this:

def countMoreThanOne():
    return xrange(1, 10, 2)


>>> print countMoreThanOne()
xrange(1, 11, 2)
>>> print list(countMoreThanOne())
[1, 3, 5, 7, 9]

or even better, to have some point in making it a function:

def oddNumbersLessThan(stop):
    return xrange(1, stop, 2)


>>> print list(oddNumbersLessThan(15))
[1, 3, 5, 7, 9, 11, 13]

Python yield a list with generator

2 Answers2