Reintroduce get_plugin_manager() for backward-compatibility

[nicoddemus]

PyCharm depends on get_plugin_manager() for its pytest runner.

Using the pytest runner in PyCharm as of 2.8.0.dev4 raises this exception:

Traceback (most recent call last):
  File "D:\Programming\PyCharm\helpers\pycharm\pytestrunner.py", line 60, in <module>
    main()
  File "D:\Programming\PyCharm\helpers\pycharm\pytestrunner.py", line 30, in main
    _pluginmanager = get_plugin_manager()
  File "D:\Programming\PyCharm\helpers\pycharm\pytestrunner.py", line 20, in get_plugin_manager
    from _pytest.core import PluginManager
ImportError: No module named '_pytest.core'

The related code in PyCharm\helpers\pycharm\pytestrunner.py is:

def get_plugin_manager():
  try:
    from _pytest.config import get_plugin_manager
    return get_plugin_manager()
  except ImportError:
    from _pytest.core import PluginManager
    return PluginManager(load=True)

get_plugin_manager was removed during the refactoring that lead to pluggy, I guess.

While it could be argued that PyCharm could fix it on their side, keeping backward compatibility in pytest is simple and would prevent a ton of PyCharm users not being able to run py.test tests when we release 2.8.0, at least until a hotfix for PyCharm is released.

[RonnyPfannschmidt]

nicely done
please add a deprecationwarning with stacklevel 2, so its noticeable more easily

[hpk42]

sure,a dding it back is fine but could you add a test noting pycharms needs it so the next person doesn't accidentally delete this method?

[nicoddemus]

Applied suggested changes. 😄

[hpk42]

sorry i didn't say this earlier but i think we don't need to bother with deprecating it. @RonnyPfannschmidt , do you insist to deprecate it? I think it's ok to offer a method for IDEs and others to get a plugin manager instance.

[nicoddemus]

I agree we should just keep this around and remove the warning... waiting for @RonnyPfannschmidt's take on it.

[RonnyPfannschmidt]

i'm wondering more about having a pytest-controller package,
that would support running py.test slave processes in various configurations

[nicoddemus]

That's an interesting idea, but certainly out of the scope of this PR.

[hpk42]

Let's strike the deprecation and rather document get_plugin_manager() as official API (in plugins.txt i guess)

[nicoddemus]

Removed the deprecation warning and added a reference to get_plugin_manager in writing_plugins.txt, as it looked like the better fit for this particular function.

While writing the docs for get_plugin_manager I realized there are no reference in the docs to pluggy.PluginManager or PytestPluginManager, so I added some to see how they would look like. My question is, should I continue to add more docs, or that API is not stable yet so it would be better to not even mentioning them at all, leaving only the reference to the get_plugin_manager function?

[nicoddemus]

Decided to merge, we can decide on improving the documentation later. 😄