Is it possible that a python def contains both “yield” and “return”?

Question

Currently I am learning the python tornado, and I find an interesting def here , the sample code goes as

@gen.coroutine
def fetch_coroutine(url):
    http_client = AsyncHTTPClient()
    response = yield http_client.fetch(url)
    return response.bodyere

As you see the def function contains both yield and return... So, is it follow the python rules ? How can we use this kind of def ? anyone give me some samples will be very appreciate...

Answer 1

>>> def f():
...     yield 1
...     return 2
... 
>>> g = f()
>>> next(g)
1
>>> next(g)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
StopIteration: 2

return in a generator stops its execution and ends the iteration by raising a StopIteration . Apparently, giving a value in return just passes it as an argument to the StopIteration exception.

It was pointed out in the comments that passing a value like this is only allowed since Python 3.3.

This value can't be seen in normal iteration (ie for x in f() ).

It seems like Tornado does something special with it, by iterating using next and catching that exception. Coroutines are a complex topic. This could be the "result" of the coroutine, and the yield s in it would be just to suspend execution and exchange data.

Answer 2

Not in Python 2. In Python 2, a function that contains "yield" can have a bare "return" with no value, but it is not allowed to return a value. Tornado has a solution to this: you can yield and then raise gen.Return(value):

@gen.coroutine
def fetch_coroutine(url):
    http_client = AsyncHTTPClient()
    response = yield http_client.fetch(url)
    raise gen.Return(response.body)

In Python 3.3 and later, a function that contains "yield" can also return a value:

@gen.coroutine
def fetch_coroutine(url):
    http_client = AsyncHTTPClient()
    response = yield http_client.fetch(url)
    return response.body

Python 3.3 gained the ability to return a value from a generator in PEP 380 , along with the new statement "yield from".