my.time.tz: implement different policies for localizing

[karlicoss]

related: #17

also follow up for #96 (comment)

[karlicoss]

not sure what the default should be, 'keep' or 'convert'? @seanbreckenridge what do you think?
I'm not sure because I'm expecting to use this in 'error' mode most of the time to fail fast :)
An upside of convert would be is that you basically put trust into the tz module, which is ultimately good. A downside is that it might be too coarse (e.g. if tz fell back on the home locations), in that case the original timezone might be better.

[seanbreckenridge]

I assume this will will be configurable through my.config. I think it makes sense to have the default be keep, forcing the user to be aware of the tz module and configure it before it possibly makes changes to the data they may not be expecting.

[karlicoss]

Not sure about Literal, haven't really used it much before.. But in my experience having real enums for such things just makes it a bit annoying (e.g. you have to import it first, etc). I guess mypy gives enough safety while keeping it flexible.
Either way should be easy to switch in the future, so maybe good to give it a go anyway.

[seanbreckenridge]

wonderful 👍

In [1]: from more_itertools import last

In [2]: from my.time.tz.main import localize

In [3]: from my.body import weight

In [4]: localize(last(weight()).when)
Out[4]: datetime.datetime(2020, 10, 31, 15, 18, 57, tzinfo=<DstTzInfo 'America/Los_Angeles' PDT-1 day, 17:00:00 DST>)

Not sure if you want to add this as a utility function to HPI directly, but I'll probably add a hacky function like:

X = TypeVar("X")
def localize_iterable(it: Iterator[X]) -> Iterator[X]:
	"""
	If policy == convert, localize all datetimes on some iterator
	"""
	if policy != 'convert':
		yield from it
		return
	for item in it:
		if hasattr(it, 'dt'):
			# tz.localize and yield
		elif hasattr(it, 'at'):
			# tz.localize and yield
		elif hasattr(it, 'when'):
			# tz.localize and yield
		else:
			warnings.warn(f"Couldnt find datetime attr {it}")
			yield tz

Seeing as I have multiple attributes that match datetimes, and I prefer 'at'/'when' to 'dt' sometimes.

May be easier to standarize 'dt' across all modules (or use @property wrappers), or check the definition of 'X' to see if the constructor has a typed attribute with the type datetime, unsure.

[karlicoss]

Oh, nice! Good idea, I'll think about it too.

May be easier to standarize 'dt' across all modules (or use @Property wrappers), or check the definition of 'X' to see if the constructor has a typed attribute with the type datetime, unsure.

Yeah, or a special decorator, or type alias (which acts like datetime, but indicates that it's an 'event time'). Or all of these :)
In principle, it would be very nice to apply them uniformly without having to modify the data source code, would be simpler and better decomposition.

Also this feels like it's begging for lenses :) Haven't really used them in other languages before either, but found something for Python https://github.com/ingolemo/python-lenses