ensure the setup.py directory is in sys.path because of PEP 517

[ngoldbaum]

Rationale

Currently cartopy is failing to build on e.g. Travis with the following traceback:

 Traceback (most recent call last):
    File "/home/travis/build/ngoldbaum/yt/venv/lib/python2.7/site-packages/pip/_vendor/pep517/_in_process.py", line 207, in <module>
      main()
    File "/home/travis/build/ngoldbaum/yt/venv/lib/python2.7/site-packages/pip/_vendor/pep517/_in_process.py", line 197, in main
      json_out['return_val'] = hook(**hook_input['kwargs'])
    File "/home/travis/build/ngoldbaum/yt/venv/lib/python2.7/site-packages/pip/_vendor/pep517/_in_process.py", line 54, in get_requires_for_build_wheel
      return hook(config_settings)
    File "/tmp/pip-build-env-dD1vz4/overlay/lib/python2.7/site-packages/setuptools/build_meta.py", line 130, in get_requires_for_build_wheel
      return self._get_build_requires(config_settings, requirements=['wheel'])
    File "/tmp/pip-build-env-dD1vz4/overlay/lib/python2.7/site-packages/setuptools/build_meta.py", line 112, in _get_build_requires
      self.run_setup()
    File "/tmp/pip-build-env-dD1vz4/overlay/lib/python2.7/site-packages/setuptools/build_meta.py", line 126, in run_setup
      exec(compile(code, __file__, 'exec'), locals())
    File "setup.py", line 36, in <module>
      import versioneer
  ImportError: No module named versioneer

The issue is that in the latest version of pip, they enabled PEP 517 and PEP 518 support, which means that the directory of setup.py is no longer included in sys.path. Since cartopy uses versioneer and we want that behavior, the fix is to explicitly add the path to the setup.py directory to sys.path.

This was suggested to me by @pganssle on IRC.

Implications

I think this is harmless for older setuptools and pip verisons, because . is already in sys.path.

[ngoldbaum]

I signed the CLA.

[pganssle]

One thing to note is that even though, after the next pip release, this won't be necessary, you will need to deal with this when you eventually explicitly specify setuptools.build_meta as your PEP 517 backend. As long as your build system works with PEP 517, you can also explicitly specify setuptools.build_meta as your backend in your pyproject.toml, since you won't be relying on the legacy behavior.

[build-system]
requires = [
    "setuptools >= 0.40.8",
    "wheel",
    "Cython >= 0.28",
    "numpy >= 1.10",
]
build-backend="setuptools.build_meta"

I recommend doing this in a separate PR, though. Your build is complicated enough that you should probably test the transition better than you need to test this.

Note: I have not tested this, but I think that you can also add "versioneer" to your requires and remove HERE from your setup.py's sys.path once you switch to a PEP 517 build.

[ngoldbaum]

And I guess this isn't strictly necessary to build cartopy because pyproject.toml was removed in #1264. But it would be necessary if you ever re-added pyproject.toml.

[ocefpaf]

I was looking into this recently and maybe the best course of action is to drop versioneer instead of append the current path to sys.path. At @ngoldbaum and @pganssle do you know any nice versioneer alternatives out there?

[ngoldbaum]

I think setuptools_scm plays more nicely with pep517.

[ocefpaf]

I think setuptools_scm plays more nicely with pep517.

Thanks! I'll play with it in other projects I have more control and, once I feel comfortable with it, I'll send a PR here. (Unless someone beats me to it first.)

[QuLogic]

I do like setuptools_scm, especially since we have some patched copy of versioneer, which also seems dead upstream. If it fixes things, I would be happy to start using it.

[dopplershift]

I've switched MetPy over to setuptools_scm (if for no other reason than it's alive and under the PyPA umbrella). So far, no complaints. The only gotcha is that it's actual main purpose in life is to include everything under source control in release tarballs; for MetPy this took some adjusting of things, but in the end I think the product was better.

IIRC, the versioneer here is tweaked, so we'd have to look at either changing the versioning behavior or adjusting to something new.

[QuLogic]

I looked at setuptools_scm, and I think it requires a new-ish version of setuptools. I don't know if we can rely on that for Python 2, but as soon as 0.18 is out and we drop support for Py2, then I'll look at it again.

[QuLogic]

I've opened #1545 to switch to setuptools-scm.

[QuLogic]

So I think we don't need this any more with the above PR merged.