intersphinx looks for implementation modules

[crusaderky]

This is a widespread issue caused by the pattern of defining objects in private module and then exposing them to the final user by importing them in the top-level __init__.py, vs. how intersphinx works.

Exact same issue in different projects:

• Can't make intersphinx work aio-libs/aiohttp#3714
• https://jira.mongodb.org/browse/MOTOR-338
• Intersphinx looks for implementation modules tkem/cachetools#178
• https://github.com/AmphoraInc/xarray_mongodb/pull/22
• intersphinx looks for implementation modules jonathanslenders/asyncio-redis#143

If a project

1. uses xarray, intersphinx, and autodoc
2. subclasses any of the classes exposed by xarray/__init__.py and documents the new class with the :show-inheritance: flag
3. Starting from Sphinx 3, has any of the above classes anywhere in a type annotation

Then Sphinx emits a warning and fails to create a hyperlink, because intersphinx uses the __module__ attribute to look up the object in objects.inv, but __module__ points to the implementation module while objects.inv points to the top-level xarray module.

Workaround

In conf.py:

import xarray
xarray.DataArray.__module__ = "xarray"

Solution

Put the above hack in xarray/__init__.py