Review using datetime related metadata keys

[sandorkertesz]

Current situation

• Field has the datetime() method. It returns a dict with the base_date and valid_date items (both are datetime.datetime objects)
• there is the valid_datetime generated metadata key. Field.metadata("valid_datetime") returns an ISO date string.
• the Metadata object has the base_datetime() and valid_datetime() methods. They return datetime.datetime.

f.datetime()
>>> {'base_time': datetime.datetime(2018, 8, 1, 12, 0),
 'valid_time': datetime.datetime(2018, 8, 2, 0, 0)}

f.metadata("valid_datetime")
>>> '2018-08-02T:00:00:00'

f.metadata().valid_datetime()
>>> datetime.datetime(2018, 8, 2, 0, 0)

f.metadata().base_datetime()
>>> datetime.datetime(2018, 8, 1, 12, 0)

Problems

• inconsistent naming: "valid_time" vs. "valid_datetime"
• inconsitent values: datetime.datetime vs. ISO date string
• datetime() always returns two values even if we only want to use one of them. Can be a waste of resources when metadata access is expensive.

Suggestions

A:

• use base_datetime and valid_datetime keys in datetime().
• add a base_datetime metadata key
• consider changing the value of the base_datetime and valid_datetime metadata keys to datetime.datetime

B:

• add the valid_datetime() and base_datetime() methods to Field.

Obviously, some of these his would not be backward compatible.