On 08/10/18 12:21, Stefan Behnel wrote:
Petr Viktorin schrieb am 10.08.2018 um 11:51:
On 08/10/18 11:21, Stefan Behnel wrote:
coming back to PEP 489 [1], the multi-phase extension module
initialization. We originally designed it as an "all or nothing" feature,
but as it turns out, the "all" part is so difficult to achieve that most
potential users end up with "nothing". So, my question is: could we split
it up so that projects can get at least the main advantages: module spec
and unicode module naming.

PEP 489 is a great protocol in the sense that it allows extension modules
to set themselves up in the same way that Python modules do: load, create
module, execute module code. Without it, creating the module and executing
its code are a single step that is outside of the control of CPython, which
prevents the module from knowing its metadata and CPython from knowing
up-front what the module will actually be.

Now, the problem with PEP 489 is that it requires support for reloading and
subinterpreters at the same time [2]. For this, extension modules must
essentially be free of static global state, which comprises both the module
code itself and any external native libraries that it uses. That is
somewhere between difficult and impossible to achieve. PEP 573 [3] explains
some of the reasons, and lists solutions for some of the issues, but cannot
solve the general problem that some extension modules simply cannot get rid
of their global state, and are therefore inherently incompatible with
reloading and subinterpreters.

Are there any issues that aren't explained in PEP 573?
I don't think Python modules should be *inherently* incompatible with
subinterpreters. Static global state is perhaps unavoidable in some cases,
but IMO it should be managed when it's exposed to Python.
If there are issues not in the PEPs, I'd like to collect the concrete cases
in some document.

There's always the case where an external native library simply isn't
re-entrant and/or requires configuration to be global. I know, there's
static linking and there are even ways to load an external shared library
multiple times, but that's just adding to the difficulties. Let's just
accept that some things are not easy enough to make for a good requirement.

For that case, I think the right thing to do is for the module to raise an extension when it's being initialized for the second time, or when the underlying library would be initialized for the second time.

"Avoid static global state" is a good rule of thumb for supporting subinterpreters nicely, but other strategies are possible. If an underlying library just expects to be initialized once, and then work from several modules, the Python wrapper should ensure that (using global state, most likely). Other ways of handling things should be possible, depending on the underlying library.

I would like the requirement in [2] to be lifted in PEP 489, to make the
main features of the PEP generally available to all extension modules.

The question is then how to opt out of the subinterpreter support. The PEP
explicitly does not allow backporting new init slot functions/feeatures:

"Unknown slot IDs will cause the import to fail with SystemError."

But at least changing this in Py3.8 should be doable and would be really

I don't think we can just silently skip unknown slots -- that would mean
modules wouldn't be getting features they asked for.
Do you have some more sophisticated model for slots in mind, or is this
something to be designed?

Sorry for not being clear here. I was asking for changing the assumptions
that PEP 489 makes about modules that claim to support the multi-step
initialisation part of the PEP. Adding a new (flag?) slot was just one idea
for opting out of multi-initialisation support.

Would this be better than a flag + raising an error on init?
One big disadvantage of a big opt-out-of-everything button is that it doesn't encourage people to think about what the actual non-reentrant piece of code is.
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