[Distutils] PEP 438, pip and --allow-external (was: "pip: cdecimal an externally hosted file and may be unreliable" from python-dev)

[Chris Jerdonek]

On Fri, May 9, 2014 at 3:16 AM, Paul Moore <p.f.moore at gmail.com> wrote:
> So there's an ongoing debate over pip's behaviour around disallowing
> external hosting by default (see thread "pip: cdecimal an externally
> hosted file and may be unreliable" over on python-dev for the latest
> round).
>
> It appears that the reason for disallowing external hosting (as
> opposed to unverifiable downloads) is purely about reliability - we
> can't be sure that an external host provides the same level of uptime
> as PyPI[1]. Given that, it seems to me that the situation is, for an
> externally hosted package foo:
>
>     `pip install foo` - fails immediately, 100% of the time
>     `pip install --allow-external foo foo` - works in all but a few
> cases where foo's host is down[2]
>
> I cannot understand how guaranteed failure is ever better than
> "occasional but rare" failure.

With regard to this general question, I think it's pretty common to
want guaranteed failure over flaky failure.  Think of flaky tests
versus deterministic tests, for example.  With guaranteed failure, you
find out right away that something's wrong with how you're doing
things, whereas with flaky failure, you might not find out until days
later when you're not around or not watching, etc.

--Chris