Lazy-load dateutil submodules?

[pganssle]

It is a fairly common problem (see, e.g. issue #770) that people assume that import dateutil will automatically import all of dateutil's sub-modules. Currently we don't do this because we want to be as lazy as possible about imports.

We'll need to profile it, but I think there is a "best of both worlds" option here, where in dateutil.__init__ we define lazy-loaded modules, so that the first time you invoke, e.g. dateutil.tz dateutil.tz will be imported in the background, and subsequent queries come from a cache.

For Python 3.7+, I think the best / fastest way to do this is to use module-level __getattr__ and __dir__ implementations, but for pre-Python 3.7, we may need to do something fancy and module sys.modules or something to implement the equivalent to a module-level __getattr__.

[kirit93]

@pganssle have you considered using lazy_import for this?
I added

import lazy_import
tz  = lazy_import.lazy_module("dateutil.tz")

to __init__.py and I was able to do this

>>> import dateutil
>>> tz = dateutil.tz
>>> tz.gettz()
tzfile('/etc/localtime')
>>> 

lazy_import is compatible with Python ≥ 2.7 or ≥ 3.4.

[pganssle]

I'm not willing to add any new dependencies. It shouldn't be too difficult to implement, though.

[pganssle]

Looks like lazy_import is GPL, so it's also an incompatible license. Our implementation should be a "clean room" implementation not based on theirs.

[mariocj89]

Done via #1007