Reduce the time taken to import cf

[davidhassell]

Fixes #902

Should be considered after NCAS-CMS/cfdm#362 has been dealt with

[sadielbartholomew]

Hmm, the code changes all look fine and sensible for improving import time to some extent, but as for timings on my local system, I am not seeing much speed-up on this branch relative to main in terms of import time, at least judging it by the useful test command you shared in one of the original issues.

There is a bit, which is of course good, but nothing like the factor 6 you saw when you tested as per your comment on the Issue. For example, on first run in a fresh terminal tab, with the cfdm branch kept on the one just merged with its own import timing improvements:

$ git checkout main # after
...
import time:      1297 |     606844 | cf
$ git checkout david/import-speed-up # after
...
import time:      1160 |     602408 | cf

so remains ~0.6 s, only 1% faster.

I wonder if you are quoting the factor 6 improvement using timings with this branch in combination with the cfdm sister one NCAS-CMS/cfdm#362, relative to the cfdm main (before that was merged) - i.e. the timing improvement is mostly down to the cfdm-side changes? I do notice considerable speed-up on the cf-python side with the cfdm branch set to that relative to main before: 0.6 s relative to 1.3 s. But we can't attribute that to this PR.

So, unless I'm missing something this cf-side PR doesn't seem to produce that significant of a reduction in import time, but it does produce a small one which is good enough to be merge-worthy - so I am approving (but would be interesting to know more context to your own timings). Please merge after checking the small number of minor feedback items raised, plus this more general comment:

• I would prefer it if we don't change the import order in the recipes, since those were carefully placed to avoid the possibility for seg faulting due to bad matplotlib and esmpy lower-level code interactions (see NCAS-CMS/cf-plot#24) - however I will review that issue in terms of whether it can be avoided given the state of dependencies on our and those libraries now. I imagine the changes to anything in docs/source/recipes was due to linting and notably isort. Would you mind reverting those?

(Again it is a shame we can't use 'PEP 810 – Explicit lazy imports' now to avoid the movement of imports as per my notes on the cfdm PR. But should be good for the next several years until Python 3.15 is the standard minimum. 😆)

[sadielbartholomew]

Suggested change

	(https://github.com/NCAS-CMS/cfdm/issues/902)
	(https://github.com/NCAS-CMS/cf-python/issues/902)

[sadielbartholomew]

Ideally either the variable name (preferred) or comment here can be more descriptive e.g. changed to datetime_data or units_data, as neither says anything as-is.

[sadielbartholomew]

Left over during dev?

Suggested change

# regridding = _get_module_info("esmpy", alternative_name="ESMF", try_except=True)

[sadielbartholomew]

As for the other checks, same note as NCAS-CMS/cfdm#362 (comment) so might be worth noting on the related Issue you opened, NCAS-CMS/cfdm#364, to consider cf-python as well.