[Distutils] PEP 517 again

Nathaniel Smith njs at pobox.com
Sat Aug 26 16:42:02 EDT 2017

On Sat, Aug 26, 2017 at 12:54 PM, Paul Moore <p.f.moore at gmail.com> wrote:
> On 26 August 2017 at 20:17, Nathaniel Smith <njs at pobox.com> wrote:
>> Eh... I would really prefer something that's (a) more explicit about what
>> specifically went wrong, and (b) harder to return by accident. It's not at
>> all obvious that if the list of requirements is 'None' that means 'this
>> build supports making sdists in general but cannot make them from this
>> source tree but might still be able to make a wheel'. And if you forget to
>> put in a return statement, then python returns None for you, which seems
>> like it could lead to some super confusing error modes.
> Well, we've had an extensive discussion about how frontends need to
> trust backends to get things right. I don't really see it as
> reasonable to now argue that backends might "forget" to return the
> right value - they might just as well "forget" to properly isolate
> builds...

It's not about division of responsibilities, it's about handling
errors gracefully when they happen. There are three bins:

- creating an sdist succeeded
- creating an sdist failed for expected reasons, and a clever frontend
might be able to handle the problem automatically if it understands
what the problem is (sdist creation isn't supported in this case) and
understands its goals (just trying to build a wheel really, so the
sdist isn't crucial)
- creating an sdist failed for unexpected reasons, that need a human
to sort out (due to a broken system, or bugs – hey, they happen – or

The whole discussion has been about how we can most reliably
distinguish between the second and third categories, and give good
error messages for the third category. The argument for NotImplemented
is that it avoids cases where some internal call raises
NotImplementedError and it "leaks out" accidentally, causing a
unexpected error to be incorrectly treated as expected error -- we
don't want pip to be hiding real bugs in backend code. The argument
for NotImplementedError is that it produces better error messages on
buggy frontends. 'return None' is kind of the worst of both worlds, in
that it's an easy thing to return accidentally, and it gives confusing
error messages if the frontend fails to handle it properly. (Even more
confusing, actually, because 'NoneType object has no attribute ...' is
even harder to track down than 'NotImplementedType object has no
attribute ...'.)

> As regards an explicit description of what went wrong, why can't we
> just use the same reporting methods that we will for any other build
> issue (backends simply report the problem on stdout/stderr)? I don't
> see why the backend has to package up its error information and send
> it to the frontend to report, when we already have a perfectly
> effective way for backends to report errors and/or warnings to the
> user. If you're worried that the frontend might suppress the
> information (maybe because it's planning on falling back to a direct
> wheel build) then isn't that just the converse - backends need to
> trust frontends to do the right thing?

What I mean is more, if you're some random user and you see this in a
build backend, what do you guess it means?

  def get_requires_for_build_sdist(config_settings=None):
      return None

Now how about these?

  def get_requires_for_build_sdist(config_settings=None):
      return NotImplemented

  def get_requires_for_build_sdist(config_settings=None):
      raise NotImplementedError

  def get_requires_for_build_sdist(config_settings=None):
      raise SdistBuildNotSupported

I mean, obviously return None will work. Basically anything that's
different from "return a list or string" will work :-). That's what
makes this a bikeshed topic, and I still think we're mostly just
spinning our wheels here until Nick and Donald have a chance to hash
something out that they both can agree on. But I really don't see any
advantages to 'return None' compared to the other options that have
been discussed


Nathaniel J. Smith -- https://vorpus.org

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