attr.ib takes a repr argument that may be a bool or callable.
To override how the attribute value is formatted, pass a callable that takes a single value and returns a string. Note that the resulting string is used as-is, i.e. it will be used directly instead of calling
repr()(the default).
That does not appear to be the case when passing datetime.datetime.fromisoformat():
import datetime
import attr
@attr.s
class Test(object):
foo: str = attr.ib()
dt: datetime.datetime = attr.ib(repr=datetime.datetime.isoformat)
Result:
>>> t = Test(foo='bar', dt=datetime.datetime.utcnow())
>>> t
Test(foo='bar', dt=datetime.datetime(2019, 10, 31, 17, 59, 34, 603971))
Expected:
Test(foo='bar', dt='2019-10-31T17:59:34.603971')
In fact, it looks like any parameter is ignored. Here is a slight modification of the example from the attr docs:
>>> @attr.s
... class C(object):
... user = attr.ib()
... password = attr.ib(repr=lambda value: value[:2])
...
>>> C("me", "s3kr3t")
C(user='me', password='s3kr3t')
What am I missing here?
Your attrs installation must be too old. Your code works perfectly with attrs 19.3:
>>> import datetime
>>> import attr
>>>
>>> @attr.s
... class Test(object):
... foo: str = attr.ib()
... dt: datetime.datetime = attr.ib(repr=datetime.datetime.isoformat)
...
>>> Test(foo='bar', dt=datetime.datetime.utcnow())
Test(foo='bar', dt=2019-11-01T05:40:59.745157)
You can check you attrs's version using:
>>> attr.__version__
'19.3.0'
The repr feature has been added in 19.2.0 that was released only a month ago on 2019-10-01.