Support freezegun fake types

[nhairs]

freezegun is a utility library commonly used in testing to control the time. In doing so it causes the datetime library to produce fake types.

Although these types correctly identify themselves as instances of their relevant type, msgspec does not identify them as such leading to errors.

Although it would be possible to use the default argument, this may not always be feasible for the calling library. It would be great if msgspec could natively support these.

>>> import datetime
>>> import freezegun.api
>>> isinstance(freezegun.api.FakeDate(2024,5,5), datetime.date)
True
>>> isinstance(freezegun.api.FakeDatetime(2024, 5, 5), datetime.datetime)
True

>>> import datetime
>>> import freezegun.api
>>> import msgspec.json
>>> encoder = msgspec.json.Encoder()
>>> now = datetime.datetime.now()
>>> encoder.encode(now)
b'"2024-05-05T15:18:26.690496"'
>>> encoder.encode(freezegun.api.datetime_to_fakedatetime(now))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: Encoding objects of type FakeDatetime is unsupported

[gsakkis]

Same issue with convert:

from dataclasses import dataclass
from datetime import datetime

import msgspec
from freezegun import freeze_time

@dataclass
class Token:
    exp: datetime


def test_datetime():
    exp = datetime(2022, 2, 3)
    token = msgspec.convert({"exp": exp}, Token, strict=False)
    assert token.exp == exp


@freeze_time("2022-02-03")
def test_fake_datetime():
    exp = datetime.now()
    token = msgspec.convert({"exp": exp}, Token, strict=False)
    assert token.exp == exp

Output:

test_msgspec.py::test_datetime PASSED
test_msgspec.py::test_fake_datetime FAILED

FAILED test_msgspec.py::test_fake_datetime - msgspec.ValidationError: Expected `datetime`, got `FakeDatetime` - at `$.exp`