Merge pull request #1284 from BastianZim/add-empty

Add empty packages to docs

Author: beckermr

src/maintainer/knowledge_base.rst

@@ -846,6 +846,31 @@ To use this package in a build, put it in the host environment like so
         - pybind11-abi
 
 
+.. _knowledge:empty:
+
+Empty Python packages
+----------
+For some features introduced in later Python versions, the Python community creates backports, which makes these
+features available for earlier versions of Python as well.
+One example here is `dataclasses <https://www.python.org/dev/peps/pep-0557/>`__ which was introduced with
+Python3.7 but is available as a `backport <https://github.com/ericvsmith/dataclasses>`__ for Python3.6 too.
+Therefore, most upstream packages make those backports only mandatory for specific versions of Python and exclude them otherwise.
+
+Implementing this restriction in conda-forge is currently only possible through the use of ``skips``
+which restricts the corresponding conda-forge recipes from becoming ``noarch``.
+
+Therefore, some conda-forge recipes only create an actual package on specific Python versions and are otherwise an
+empty placeholder. This allows them to be safely installed under all Python versions and makes using ``skips`` unnecessary.
+
+Currently available packages:
+
++-------------+-------------------+--------------+
+| Name        | Available on:     | Empty on:    |
++=============+===================+==============+
+| dataclasses | python >=3.6,<3.7 | python >=3.7 |
++-------------+-------------------+--------------+
+
+
 Noarch builds
 =============