As reported by Serhiy on https://bugs.python.org/issue37555#msg347733 there is a difference in __eq__ definition in datetime module's C and Python implementation for timedelta and time. When the other in __eq__ is not of the same type NotImplemented is not returned in Python implementation like C causing equality to behave differently.

Difference in C implementation and Python implementation for datetime.timedelta.__eq__

https://github.com/python/cpython/blob/c8e7146de257930ea8d0d4aa74b3a64fcaa79d4b/Modules/_datetimemodule.c#L2152

static PyObject *
delta_richcompare(PyObject *self, PyObject *other, int op)
{
    if (PyDelta_Check(other)) {
        int diff = delta_cmp(self, other);
        return diff_to_bool(diff, op);
    }
    else {
        Py_RETURN_NOTIMPLEMENTED;
    }
}


https://github.com/python/cpython/blob/c8e7146de257930ea8d0d4aa74b3a64fcaa79d4b/Lib/datetime.py#L732

def __eq__(self, other):
    if isinstance(other, timedelta):
        return self._cmp(other) == 0
    else:
        return False


I will add a PR for this with test.

Do you mind to fix also other similar cases if they are? __lt__ and others should have the same property.

New changeset e6b46aafad3427463d6264a68824df4797e682f1 by Paul Ganssle (Xtreak) in branch 'master':
bpo-37579: Improve equality behavior for pure Python datetime and time (GH-14726)
https://github.com/python/cpython/commit/e6b46aafad3427463d6264a68824df4797e682f1

New changeset 143672cf028740fc549e532c049559c522930c95 by Miss Islington (bot) in branch '3.8':
bpo-37579: Improve equality behavior for pure Python datetime and time (GH-14726)
https://github.com/python/cpython/commit/143672cf028740fc549e532c049559c522930c95

New changeset c6b31061997526b31961ec34328408ca421f51fc by Miss Islington (bot) (Xtreak) in branch '3.7':
[3.7] bpo-37579: Improve equality behavior for pure Python datetime and time (GH-14726) (GH-14745)
https://github.com/python/cpython/commit/c6b31061997526b31961ec34328408ca421f51fc

See issue37685.