Add guide on how to handle missing extras

Author: smheidrich

source/guides/handling-missing-extras-at-runtime.rst

@@ -0,0 +1,66 @@
+.. _handling-missing-extras-at-runtime:
+
+============================================================
+Handling missing optional dependencies ("extras") at runtime
+============================================================
+
+If your package has :ref:`optional dependencies ("extras")
+<metadata_provides_extra>` which the package consumer hasn't installed, the
+default outcome is an ordinary `ModuleNotFoundError` exception being raised at
+the first attempted import of a missing module.
+
+This can make for a bad user experience, because there is no guidance about why
+the module is missing - users might think they've found a bug. If you're not
+careful, it can even make your package unusable without the extras installed,
+e.g. if your package is a library that imports the affected modules from the
+top-level module or from the application entry point if it's an application.
+
+As of the time of writing, there is no *great* way to handle this issue in
+the Python packaging ecosystem, but there are a few options that might be
+better than nothing:
+
+
+Overall approach
+================
+
+TODO General guidance about how to isolate imports in question
+
+TODO Optimistic vs pessimistic handling?
+
+
+Handling failing imports
+========================
+
+TODO example
+
+TODO mention it doesn't check versions, so a bit dangerous
+
+
+Using ``pkg_resources``
+=======================
+
+The now-deprecated ``pkg_resources`` package (part of the ``setuptools``
+distribution) provides a ``require`` function that you can use to check if a
+given optional dependency of your package is installed or not:
+
+
+.. code-block:: python
+
+   from pkg_resources import require, DistributionNotFound, VersionConflict
+
+   try:
+     require(["your-package-name[your-extra]"])
+   except DistributionNotFound:
+     ...  # handle package(s) not being installed at all
+   except VersionConflict:
+     ...  # handle version mismatches
+
+Unfortunately, no replacement for this functionality exists in the
+``pkg_resources``'s successor packages yet
+(`packaging-problems #664 <packaging-problems #664>`_).
+
+
+------------------
+
+.. _packaging-problems-664: https://github.com/pypa/packaging-problems/issues/664
+