Why can't I 'yield from' inside an async function?
Question:
In Python 3.6, I am able to use yield
inside a coroutine. However I am not able to use yield from
.
Below is my code. On line 3 I await another coroutine. On line 4 I try to yield from
a file. Why won’t Python 3.6 allow me to do that?
async def read_file(self, filename):
with tempfile.NamedTemporaryFile(mode='r', delete=True, dir='/tmp', prefix='sftp') as tmp_file:
await self.copy_file(filename, tmp_file)
yield from open(tmp_file)
Here’s the exception Python 3.6 raises for the above code:
File "example.py", line 4
yield from open(tmp_file)
^
SyntaxError: 'yield from' inside async function
Answers:
According to PEP 525, which introduces asyncronous generators in Python 3.6:
Asynchronous yield from
While it is theoretically possible to implement yield from
support for
asynchronous generators, it would require a serious redesign of the
generators implementation.
yield from
is also less critical for asynchronous generators, since
there is no need provide a mechanism of implementing another
coroutines protocol on top of coroutines. And to compose asynchronous
generators a simple async for
loop can be used:
async def g1():
yield 1
yield 2
async def g2():
async for v in g1():
yield v
As you can see, the answer boils down to “it would be too hard to implement, and you don’t need it anyway”.
In Python 3.6, I am able to use yield
inside a coroutine. However I am not able to use yield from
.
Below is my code. On line 3 I await another coroutine. On line 4 I try to yield from
a file. Why won’t Python 3.6 allow me to do that?
async def read_file(self, filename):
with tempfile.NamedTemporaryFile(mode='r', delete=True, dir='/tmp', prefix='sftp') as tmp_file:
await self.copy_file(filename, tmp_file)
yield from open(tmp_file)
Here’s the exception Python 3.6 raises for the above code:
File "example.py", line 4
yield from open(tmp_file)
^
SyntaxError: 'yield from' inside async function
According to PEP 525, which introduces asyncronous generators in Python 3.6:
Asynchronous
yield from
While it is theoretically possible to implement
yield from
support for
asynchronous generators, it would require a serious redesign of the
generators implementation.
yield from
is also less critical for asynchronous generators, since
there is no need provide a mechanism of implementing another
coroutines protocol on top of coroutines. And to compose asynchronous
generators a simpleasync for
loop can be used:async def g1(): yield 1 yield 2 async def g2(): async for v in g1(): yield v
As you can see, the answer boils down to “it would be too hard to implement, and you don’t need it anyway”.