[Distutils] Entry points: specifying and caching

Wes Turner wes.turner at gmail.com
Fri Oct 20 10:41:02 EDT 2017

On Friday, October 20, 2017, Donald Stufft <donald at stufft.io> wrote:

> On Oct 20, 2017, at 9:35 AM, Nick Coghlan <ncoghlan at gmail.com
> <javascript:_e(%7B%7D,'cvml','ncoghlan at gmail.com');>> wrote:
> On 20 October 2017 at 23:19, Donald Stufft <donald at stufft.io
> <javascript:_e(%7B%7D,'cvml','donald at stufft.io');>> wrote:
>> One that I was helping someone debug just the other day is that they’re
>> super non-debuggable and the behavior when you have two things providing
>> the same entry point is basically ???? (If I remember correctly, the
>> behavior is that the first thing found is the one that “wins”, which means
>> the ordering of sys.path and the names of the projects supply it is what
>> determines it). This got exposed to the end user that they installed
>> something that they thought was going to add support for something, but
>> which silently did nothing because two different project happened to pick
>> the same name for their entry point (not the group, it was two things
>> providing plugins for the same system).
> While I agree with this, I think that's a combination of pkg_resources
> itself being hard to debug in general, and the fact that pkg_resources
> doesn't clearly define the semantics of how it resolves name conflicts
> within an entry point group - as far as I know, it's largely an accident of
> implementation.
> The interoperability spec is going to state that conflict resolution when
> the same name within a group is declared by multiple packages is the
> responsibility of the group consumer, so documenting the format should
> actually improve this situation, since it allows for the development of
> competing conflict resolution strategies in different runtime libraries.
> I think it makes it *worse*, because now the behavior isn’t just a
> entrypoints weirdness, but now it changes based on which runtime library
> you use (which isn’t something that end users are likely to have much
> insight into) and it represents a footgun that package authors are unlikely
> to be aware of. If mycoolentrypointslib comes out that is faster, but
> changes some subtle behavior like this it’ll break people, but that is
> unlikely going to be an effect that people expect to happen just because
> they switched between two things both implementing the same standard.
> So effectively this means that not only is the fact you’re using
> entrypoints part of your API, but now which entry point library you’re
> using at runtime is now also part of your API.

When should the check for duplicate entry points occur?

- At on_install() time (+1)
- At runtime

Is a sys.path-like OrderedDict preemptive strategy preferable or just as
dangerous as importlib?
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