DateTime and timezone awareness

I’m pulling my hair with datetime TZ awareness issues.

I initiate the connection to MongoDB with tz_aware = False. I’m no expert about this, and I had never thought much about it before now, but from what I gathered, it seems like a reasonable choice to make. Besides, it’s pymongo’s default (but flask-PyMongo’s hardcodes tz_aware to True).

When a document is pulled from the DB, its DateTimeField attribute’s TZ awareness depends only on MongoClient’s tz_aware parameter (no Marshmallow schema involved):

tz_aware=True -> pymongo provides an aware datetime -> umongo returns an aware datetime tz_aware=False -> pymongo provides a naive datetime -> umongo returns a naive datetime

This is one more reason to set tz_aware = False, because I’m passing a document/object to a lib that expects a naive timezone. (OAuth2 lib expects expires timestamp to be naive and compares it to datetime.utcnow() (see code).) I suppose I could alternatively modify my getters to make all datetimes naive before returning tokens/grants, but on some use cases, it could get cumbersome.

Marshmallow, however, returns every datetime as TZ aware (doc). For this reason, umongo’s DateTimeField’s _deserialize method returns a TZ aware datetime. Since I’m using it (via webargs) to parse the inputs to my API, I’m getting TZ aware datetimes. Likewise, calling load() on a document will use _deserialize and result in a TZ aware datetime.

So if I load a date, it becomes TZ aware. Therefore, to compare it to a date from the database, this one needs to be aware as well.

Should I understand that umongo is meant to be used with tz_aware=True, so that dates fetched from the database can be compared to dates loaded thought Marshmallow schemas?

Could there be a flag/meta allowing to specify if a DateTimeField should return a naive datetime?

I made a quick and dirty patch to DateTime’s _deserialize to remove the TZ from the returned output.

    dt = super()._deserialize(value, attr, data)
    return dt.replace(tzinfo=None)

This seems to work on my use case. However, the day we complete pure Marshmallow schema export, the exported schema I pass to webargs for API input parsing won’t have that feature. Unless this is added to Marshmallow as well. (I asked there about it there: https://github.com/marshmallow-code/marshmallow/issues/520.)

Feedback welcome. Those TZ issues are new to me, so I may be totally misguided.

[nedwill]

Yeah, that’s what I meant - date.astimezone returns a new date, it doesn’t have side effects/mutate the date.

[touilleMan]

#75 is merged, @nedwill thanks for pointing this 👍