[Python-Dev] PEP 492: async/await in Python; version 5

Guido van Rossum guido at python.org
Tue May 5 22:55:33 CEST 2015

One small clarification:

On Tue, May 5, 2015 at 12:40 PM, Jim J. Jewett <jimjjewett at gmail.com> wrote:

> [...] but I don't understand how this limitation works with things like a
> per-line file iterator that might need to wait for the file to
> be initially opened.

 Note that PEP 492 makes it syntactically impossible to use a coroutine
function to implement an iterator using yield; this is because the
generator machinery is needed to implement the coroutine machinery.
However, the PEP allows the creation of asynchronous iterators using
classes that implement __aiter__ and __anext__. Any blocking you need to do
can happen in either of those. You just use `async for` to iterate over
such an "asynchronous stream".

(There's an issue with actually implementing an asynchronous stream mapped
to a disk file, because I/O multiplexing primitives like select() don't
actually support waiting for disk files -- but this is an unrelated
problem, and asynchronous streams are useful to handle I/O to/from network
connections, subprocesses (pipes) or local RPC connections. Checkout the
streams <https://docs.python.org/3/library/asyncio-stream.html> and
subprocess <https://docs.python.org/3/library/asyncio-subprocess.html>
submodules of the asyncio package. These streams would be great candidates
for adding __aiter__/__anext__ to support async for-loops, so the idiom for
iterating over them can once again closely resemble the idiom for iterating
over regular (synchronous) streams using for-loops.)

--Guido van Rossum (python.org/~guido)
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