Improve the way to import optional modules

[seisman]

For optional modules, currently we do like this:

try:
    import contextily
except ImportError:
    contextily = None

but Mypy doesn't like it:

Incompatible types in assignment (expression has type "None", variable has type Module)

In this PR, we use another way to import optional modules:

try:
    import contextily

    _HAS_CONTEXTILY = True
except ImportError:
    _HAS_CONTEXTILY = False

See https://adamj.eu/tech/2021/12/29/python-type-hints-optional-imports/ for more detailed explanations.

Sometimes, modules like IPython are not directly used, then ruff reports F401 and recommends using importlib.util.find_spec.

Changes in this PR:

• 5deebf8: Use a bool variable to indicate whether an optional module is available
• c98c103: Use upper-case for constant name (Fix C0103)
• b1c40b0: Use importlib.util.find_spec for modules that are not used (ruff F401)

[weiji14]

Another option looking at https://stackoverflow.com/questions/61384752/how-to-type-hint-with-an-optional-import/77341843#77341843 seems to be something like:

IPython: Any = None
try:
    import IPython
except ImportError:
    pass

Though not sure if this is recommended.

[seisman]

It's unclear what the recommended way is for importing an optional module. It seems sometimes pandas and xarray use the has_xxx bool variable:

https://github.com/pandas-dev/pandas/blob/feca574525b2c37fac8d1d26bf82d428b95630c8/pandas/compat/compressors.py#L11

https://github.com/pandas-dev/pandas/blob/feca574525b2c37fac8d1d26bf82d428b95630c8/pandas/io/formats/style.py#L84

https://github.com/pydata/xarray/blob/49bd63a8332c1930a866724a2968b2d880dae25e/xarray/core/ops.py#L15

but sometimes they also use the old way, which mypy should complain, e.g.,
https://github.com/pydata/xarray/blob/49bd63a8332c1930a866724a2968b2d880dae25e/xarray/coding/times.py#L31

[weiji14]

Looking at mypy (e.g. python/mypy#1297 (comment)), they also seem to recommend a try-except-else pattern? Something like:

try:
    import IPython as _IPython
except ImportError:
    IPython = None
else:
    IPython = _IPython

[seisman]

I don't think it's officially recommended and it's also more difficult to understand than IPython = None and _HAS_IPYTHON = False.

[weiji14]

Ok, let's go with _HAS_IPython unless there's a better recommended way 🙂

That said, we should probably decide first if adding mypy is something we want (#2808), otherwise the changes here aren't really needed.

[seisman]

Since we're going to add type hints (#2794, #2812), we definitely need a static type checker. There are many static type checkers, like mypy, pyright, and pytype, in which mypy is the official one, so it's a good option to start.