Don't convert data with time units to timedeltas by default

[ocefpaf]

Don't convert data with time units to timedeltas by default. Most of the time this behavior is not desirable (e.g: wave period data).

@shoyer suggest:

possibly we should add an explicit toggle for decoding timedeltas vs datetimes.

xref: #842 (comment)

[ocefpaf]

@shoyer is this what you have in mid? Replacing decode_time for decode_datetimes and decode_timedelta and then:

    if decode_datetimes and 'units' in attributes and 'since' in attributes['units']:
        units = pop_to(attributes, encoding, 'units')
        calendar = pop_to(attributes, encoding, 'calendar')
        data = DecodedCFDatetimeArray(data, units, calendar)

    if decode_timedelta and attributes['units'] in TIME_UNITS:
        units = pop_to(attributes, encoding, 'units')
        data = DecodedCFTimedeltaArray(data, units)

To be honest I cannot see why someone might want to convert time data to a timedelta and I would be inclined to remove decode_timedelta instead. However, that may be my very biased view since such transformation does not make sense with my data 😄

Do you have any advice? (I would like to try this before you release v0.8.0)

[shoyer]

It's too late for v0.8.0, I'm afraid! I already tagged/uploaded the release and am about to send out the announcement.

[ocefpaf]

No problem. I want to solve this anyways b/c it is quite awkward to keep converting back from timedeltas to time data. I will have a PR soon.

[shoyer]

To answer your original question: yes, separate decode_timedeltasand decode_datetimes options seems like a sane way to handle this.

[ocefpaf]

Prototype is ready http://nbviewer.jupyter.org/gist/ocefpaf/cabdcb27a5ef7da1b6110327a5f03e17

I will polish this and send a PR soon.

[ocefpaf]

@shoyer the more I think about this the more I don't like the addition of extra keywords. Even though I would like this behavior to be the default one I really do not like the complexity I added here.

Closing this...