Trigger errors when coordinates don't match about esmlab

[matt-long]

cc @klindsay28

[matt-long]

@andersy005, I am reopening this issue. I am concerned that the implementation we chose in #54 sets xarray options globally, so that on import of esmlab, a user will get different behavior from xarray than its defaults. While I think the xarray settings are not the best choice for defaults, I don't think we should be mucking around with these settings on the user's behalf, but rather confine our application of the non-default to within esmlab.

How can we do this without wrapping every computation in a context manager?

One option is to define a function decorator:

from contextlib import ContextDecorator

class xarray_setoptions(ContextDecorator):
    def __init__(self, **kwargs):
        self.old = xr.set_options(**kwargs).old
    def __enter__(self):
        return self
    def __exit__(self, *exc):
        xr.set_options(**self.old)
        return

@xarray_setoptions(arithmetic_join="exact")
def compute_mon_climatology(dset, time_coord_name=None):
    # computation

This seems like a nice way to implement this feature at the function level, but it's not Python 2 compatible. We could put

xr_settings_old = xr.set_options(arithmetic_join="exact").old

at the top of every function, and

xr.set_options(**self.old)

at the bottom.

@mnlevy1981, we should discuss plans for marbl-diags; at present the esmlab dependencies are modest, so we might consider dropping py2 support here sooner rather than later.

[mnlevy1981]

@matt-long I think marbl-diags will need to use an updated esmlab if that's where we decide to put the zonal-averaging code... otherwise I'm happy to stick with an older release until the post-processing scripts are py3-compatible. What happens with the code snippet above in python 2? Does it throw an error, or does it just ignore the function decorator?

[matt-long]

I don't think ContextDecorator is defined in python 2, so we could protect the class definition from a python 2 invocation. Not sure what it would do with the decorator...but we would not get the desired
settings.

[mnlevy1981]

If we could protect the class definition from python 2, I'd be okay with having marbl-diags define xarray settings before calling esmlab (I think that would work without altering xarray behavior from other calls)

[matt-long]

@andersy005, I think Mike's suggestion is a good one, if the decorator is ignored. There would have to be a dummy object in py2 to import into the computational modules so as to keep these clean of version checks.

[andersy005]

@mnlevy1981, there's a contextlib backport module for python2 (https://github.com/jazzband/contextlib2). So I used the approach in #46 (comment) and this should work for both Python2 and Python3