[Distutils] PEP 439 and pip bootstrap updated

[Brett Cannon]

On Wed, Jul 10, 2013 at 9:09 PM, Richard Jones <richard at python.org> wrote:

> On 11 July 2013 06:50, Paul Moore <p.f.moore at gmail.com> wrote:
> > I think "python -m pip" should be the canonical form (used in
> documentation,
> > examples, etc). The unittest module has taken this route, as has timeit.
> > Traditionally, python-dev have been lukewarm about the -m interface, but
> its
> > key advantage is that it bypasses all the issues around versioned
> > executables, cross-platform issues, the general dreadfulness of script
> > wrappers on Windows, etc, in one fell swoop.
>
> "python -m pip" does make the bootstrapping a more complex proposition
> - the stdlib would have to have something called "pip" that could be
> overridden (while it is actually *running*) by something installed in
> site-packages. Not easy.
>

It's also fraught with historical baggage; remember xmlplus? That was
extremely painful and something I believe everyone was glad to see go away.

Having said that, there are two solutions to this.

The compatible solution with older Python versions is to have the bootstrap
download pip and have it installed as piplib or some other alternative name
that is not masked by a pip stub in the stdlib.

The dead-simple, extremely elegant solution (starting in Python 3.4) is to
make pip a namespace package in the stdlib with nothing more than a
__main__.py file that installs pip; no checking if it's installed and then
running it, etc, just blindly install pip. Then, if you install pip as a
regular package, it takes precedence and what's in the stdlib is completely
ignored (this helps with any possible staleness with the stdlib's bootstrap
script vs. what's in pip, etc.). You don't even need to change the
__main__.py in pip as it stands today since namespace packages only work if
no regular package is found.

In case that didn't make sense, here is the file structure:

python3.4/
    pip/
        __main__.py  # Install pip, nothing more
~/.local/
    bin/
        pip  # Literally a shebang and two lines of Python; see below
    lib/python3.4/site-packages
        pip/  # As it stands today
            __init__.py
            __main__.py
            ...

This also means pip3 literally becomes ``import runpy;
runpy.run_module('pip')``, so that is even easier to maintain (assuming
pip's bin/ stub isn't already doing that because of backwards-compatibility
concerns or something with __main__.py or runpy not existing far enough
back, otherwise it should =).

-Brett
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