relativedelta.months > 12 raise an AssertionError

[mutetella]

An calculation for more than 12 months forward or backward will raise an AssertionError. This error occurs with dateutils 1.5 and 2.0, for previous versions I don't know.

Currently I solved this as follow:

    if delta.months > 12:
        delta.years += delta.months / 12
        delta.months = delta.months % 12

mutetella

[mutetella (mutetella)]

I must substantiate my Bug Description:

>>> datetime.date(2012, 9, 4) + dateutil.relativedelta.relativedelta(months=+13)
datetime.date(2013, 10, 4)

>>> d = dateutil.relativedelta.relativedelta()
>>> d.months = 13
>>> datetime.date(2012, 9, 4) + d
AssertionError

>>> d = dateutil.relativedelta.relativedelta()
>>> d.days = 40
>>> datetime.date(2012, 9, 4) + d
datetime.date(2012, 10, 14)

The AssertionError only occurs with 'relativedelta.months' > 12 but not with 'days'- or 'years'-attributes. With 'months' > 12 passed as an __init__-attribute it works proper.