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I haven't been able to find any examples of return values from the yield from expression. I have tried this simple code, without success:
def return4():
return 4
def yield_from():
res = yield from range(4)
res = yield from return4()
def test_yield_from():
for x in yield_from():
print(x)
test_yield_from()
Which produces:
» python test.py
0
1
2
3
Traceback (most recent call last):
File "test.py", line 52, in <module>
test_yield_from()
File "test.py", line 48, in test_yield_from
for x in yield_from():
File "test.py", line 44, in yield_from
res = yield from return4()
TypeError: 'int' object is not iterable
But I was expecting:
» python test.py
0
1
2
3
4
Because, as stated in the PEP:
Furthermore, when the iterator is another generator, the subgenerator is allowed to execute a return statement with a value, and that value becomes the value of the yield from expression.
Obviously, I am not getting this explanation. How does a return in a "subgenerator" work with regards to yield from?
592
The Yield keyword in Python is similar to a return statement used for returning values or objects in Python. However, there is a slight difference. The yield statement returns a generator object to the one who calls the function which contains yield, instead of simply returning a value.
It's allowed in Python 3. x, but is primarily meant to be used with coroutines - you make asynchronous calls to other coroutines using yield coroutine() (or yield from coroutine() , depending on the asynchronous framework you're using), and return whatever you want to return from the coroutine using return value .
The yield keyword pauses generator function execution and the value of the expression following the yield keyword is returned to the generator's caller. It can be thought of as a generator-based version of the return keyword. yield can only be called directly from the generator function that contains it.
yield in Python can be used like the return statement in a function. When done so, the function instead of returning the output, it returns a generator that can be iterated upon. You can then iterate through the generator to extract items. Iterating is done using a for loop or simply using the next() function.
Generators can return a value when they are exhausted:
def my_gen():
yield 0
return "done"
g = my_gen()
next(g)
next(g) # raises StopIteration: "done"
The returned value in a yield from statement will be this value. eg.
def yield_from():
res = yield from my_gen()
assert res == "done"
By default this value is None. That is res = yield from range(4) will set res as None.
184
yield from generator is short for
for i in generator:
yield i
well it's a bit more commplicated than that: https://www.python.org/dev/peps/pep-0380/#formal-semantics .
this will not work well if generator = 4. (your return4() is not a generator. it's a function.)
in order to get what you wand you would just do this:
def yield_from():
yield from range(4)
yield 4
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