You don't understand the point because you don't understand the feature
request or PEP. That is probably my fault for not communicating the intent
better in the past. The feature request and PEP were written to offer
something like the below (or at least enough that the below could be built
with minimal effort):
def do_login(...):
proc = subprocess.Popen(...)
current = proc.recv(timeout=5)
last_line = current.rstrip().rpartition('\n')[-1]
if last_line.endswith('login:'):
proc.send(username)
if proc.readline(timeout=5).rstrip().endswith('password:'):
proc.send(password)
if 'welcome' in proc.recv(timeout=5).lower():
return proc
proc.kill()
The API above can be very awkward (as shown :P ), but that's okay. From
those building blocks a (minimally) enterprising user would add
functionality to suit their needs. The existing subprocess module only
offers two methods for *any* amount of communication over pipes with the
subprocess: check_output() and communicate(), only the latter of which
supports sending data (once, limited by system-level pipe buffer lengths).
Neither allow for nontrivial interactions from a single subprocess.Popen()
invocation. The purpose was to be able to communicate in a bidirectional
manner with a subprocess without blocking, or practically speaking,
blocking with a timeout. That's where the "async" term comes from. Again,
there was never any intent to have the functionality be part of asyncore or
any other asynchronous sockets framework, which is why there are no
handle_*() methods, readable(), writable(), etc.
Your next questions will be: But why bother at all? Why not just build the
piece you need *inside* asyncio? Why does this need anything more? The
answer to those questions are wants and needs. If I'm a user that needs
interactive subprocess handling, I want to be able to do something like the
code snippet above. The last thing I need is to have to rewrite the way my
application/script/whatever handles *everything* just because a new
asynchronous IO library has been included in the Python standard library -
it's a bit like selling you a $300 bicycle when you need a $20 wheel for
your scooter.
That there *now* exists the ability to have async subprocesses as part of
asyncio is a fortunate happenstance, as the necessary underlying tools for
building the above now exist in the standard library. It's a matter of
properly embedding the asyncio-related bits inside a handful of functions
to provide something like the above, which is what I was offering to write.
But why not keep working on the subprocess module? Yep. Tried that. Coming
up on 9 years since I created the feature request and original Activestate
recipe. To go that route is going to be 2-3 times as much work as has
already been dedicated to get somewhere remotely acceptable for inclusion
in Python 3.5, but more likely, subsequent rejection for similar reasons
why it has been in limbo.
But here's the thing: I can build enough using asyncio in 30-40 lines of
Python to offer something like the above API. The problem is that it really
has no natural home. It uses asyncio, so makes no sense to put in
subprocess. It doesn't fit the typical asyncio behavior, so doesn't make
sense to put in asyncio. The required functionality isn't big enough to
warrant a submodule anywhere. Heck, it's even way too small to toss into an
external PyPI module. But in the docs? It would show an atypical, but not
wholly unreasonable use of asyncio (the existing example already shows what
I would consider to be an atypical use of asyncio). It would provide a good
starting point for someone who just wants/needs something like the snippet
above. It is *yet another* use-case for asyncio. And it could spawn a
larger library for offering a more fleshed-out subprocess-related API,
though that is probably more wishful thinking on my part than anything.
- Josiah
On Thu, Mar 27, 2014 at 4:24 PM, Victor Stinner <victor.stin...@gmail.com>wrote:
> 2014-03-27 22:52 GMT+01:00 Josiah Carlson <josiah.carl...@gmail.com>:
> > * Because it is example docs, maybe a multi-week bikeshedding discussion
> > about API doesn't need to happen (as long as "read line", "read X bytes",
> > "read what is available", and "write this data" - all with timeouts - are
> > shown, people can build everything else they want/need)
>
> I don't understand this point. Using asyncio, you can read and write a
> single byte or a whole line. Using functions like asyncio.wait_for(),
> it's easy to add a timeout on such operation.
>
> Victor
>
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