Make netcdftime.datetime immutable and hashable. Fixes #255.

[ddasilva]

I went ahead and put together a pull request for the change approved in #255. The ticket approves making netcdftime.datetime immutable so it can be used as a dictionary key. This is my first contribution, so let me of any areas for improvement.

Best,
Daniel

[shoyer]

Awesome! I'm glad you're able to work on this.

The main complication is that netcdftime.datetime objects can be equal to datetime.datetime objects, so they need to have the same hash value. Otherwise, dictionaries won't work properly.

[shoyer]

The cleaner way to write this is:

with self.assertRaises(AttributeError):
    d1.year = 1999

[ddasilva]

The immutability hack before was to avoid doing a lot repetitive properties, but I agree doing properties is cleaner and more intuitive. I updated the code to use that approach.

Also, the hash function is now coded to use the hash from a datetime.datetime that is equal to it. I added test code to check that a netcdftime.datetime and datetime.datetime that are equal have the same hash value.

[shoyer]

The last remaining wrinkle would be hashing dates that can't be represented with datetime.datetime -- the entire reason why netcdftime.datetime is necessary (e.g., February 30 with a 360 day calendar). My not so elegant solution would to be use try... except ValueError to catch cases where converting to a real datetime fails.

Finally, this would be good time to bump the version in netcdftime.__version__ (it is versioned separately from the rest of the netCDF4 package, for reasons I'm not entirely sure of).

[ddasilva]

Sounds reasonable. I implemented the hashing as you suggested for dates that can't be represented with datetime. There is a unit test for that now too. I bumped the version up one minor number from 1.1 to 1.2.

[shoyer]

Looks good to go to me! 👍

[ddasilva]

Great! Let me know of anything else that needs to be done to merge.

[shoyer]

It's up to @jswhit now.