add type hints

In GitLab by @visr on May 20, 2019, 11:30

PEP 484 and PEP 526 added optional type hints to Python. We can use this in combination with mypy. I believe this can help catching bugs and also make the code easier to read. We don't have to add this everywhere at once, but can do so gradually. All code stays valid Python so there is no need to make use of mypy.

Right now often I check the docstrings to see which types arguments are supposed to be. Of course this is not necessarily correct. If we use the sphinx-autodoc-typehints extension we can make sure the type hints only have to be specified once, can be checked, and still end up in the documentation.

xarray added support for mypy recently as well: https://github.com/pydata/xarray/pull/2655

Start by reviewing the Python code and its existing docstrings to identify where argument and return types are documented. Define a manageable first scope for PEP 484 and PEP 526 annotations, then assess mypy and sphinx-autodoc-typehints; done means the selected code has type hints that can be checked and reflected in the documentation.