> On Oct 21, 2017, at 10:30 PM, Nick Coghlan <ncogh...@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outdated warning with ``pip install -U pip`` and I’m still not entirely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.
