[Distutils] Announcement: Pip 10 is coming, and will move all internal APIs

[Donald Stufft]

> On Oct 21, 2017, at 10:30 PM, Nick Coghlan <ncoghlan at gmail.com> wrote:
> 
> However, none of that impacts the question of whether `pip.main()` runs code in the current process or implicitly runs it in a subprocess - `pip` doesn't import the modules it installs either way, so it will all look the same as far as the import system is concerned.


That of course also ignores things like “foo.py optionally imports bar.py if it is available”, with something like:

try:
    import bar
except ImportError:
    bar = None


If you then did ``import foo``, noticed the optional features powered by bar weren’t added, you would also have to reload ``foo`` (and track down any references to it!). Reloading modules in Python is hard :/


The additional thing here is that the import system isn’t the only cache at play either— pkg_resources builds a cache of installed packages at import time too and that has to get invalidated somehow as well. This took pip like 3 or 4 versions to get right because we tried to detect what version of pip was installed _after_ we installed all the versions in order to stop doing the outdated warning with ``pip install -U pip`` and I’m still not entirely convinced we’re not erroneously spitting out the warning in some cases.

The end result of all of this is unless you’re really really careful and design things just right and know exactly how not only yourself, but all your dependencies are written (and track how they change in different versions!) and also how things that use _you_ are going to import you and your dependencies… it’s basically playing whack a mole and is almost never worth the effort.
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