Rationale behind fields.DateTime().deserialize('1916') returning datetime(1916, 11, 30, 0, 0)?

I just noticed that when I deserialize a date expressed as a ‘YY’ or ‘YY-mm’ string, I get a datetime with month defaulting to 11 and day defaulting to 30 (except for february).

f = fields.DateTime()
f.deserialize('1916')
# datetime.datetime(1916, 11, 30, 0, 0)
f.deserialize('1916-2')
# datetime.datetime(1916, 2, 29, 0, 0)
f.deserialize('1916-3')
# datetime.datetime(1916, 3, 30, 0, 0)

Is this intended?

Shouldn’t this be either rejected or default to a documented value (could be january 1st)?

To prevent this seemingly non-deterministic behavior (it seems reproducible, but I don’t get the rationale), as a workaround, it is possible to set a format explicitly, which rejects such incomplete inputs, but this might not always be desired (be loose on inputs / strict on outputs).

f = fields.DateTime(format='%Y-%m-%d')
f.deserialize('1916-05-04')
# datetime.datetime(1916, 5, 4, 0, 0)
f.deserialize('1916')
# marshmallow.exceptions.ValidationError: Not a valid datetime.

[maximkulkin]

Exactly

[sloria]

@lnielsen See the discussion on https://github.com/marshmallow-code/marshmallow/pull/605 :

I’m sorry for any inconvenience this change may have caused you. This was, however, a bug fix–DateTime was accepting invalid input, and this fixes that.