Change pybind11_abseil `absl::Time` to-python conversion to return a datetime…

Change pybind11_abseil `absl::Time` to-python conversion to return a datetime object with UTC timezone.

The original motivation for this change was to achieve compatibility with absl/python behavior (currently only used Google-internally), but it is a good change in its own right: `absl::Time` does not carry any timezone information, using the local timezone in the to-python conversion is far more complicated than using the UTC timezone, in particular because

1. it makes the to-python conversion dependent on zoneinfo or equivalent.

2. potentially (and confusingly) 3 instead of 2 timezones can appear in a from-python - to-python conversion roundtrip, e.g. starting with a non-local timezone in Python, implicit UTC timezone in C++, then the local timezone when converted back to Python.

If this CL causes a breakage
----------------------------

The most likely root cause is a latent inconistency/bug in handling timezones, most notably due to a "naive" datetime object in the mix:

* https://docs.python.org/3/library/datetime.html#aware-and-naive-objects

In such a case, callers need to be updated to pass "aware" datetime objects instead. In the general case, this is the only way to ensure system-wide consistency. As a side-effect, such fixes are likely to prevent or resolve issues during DST transitions.

PiperOrigin-RevId: 538240189

pybind11_abseil/absl_casters.h

@@ -278,31 +278,25 @@ struct type_caster<absl::Time> {
 
   // Conversion part 2 (C++ -> Python)
   static handle cast(const absl::Time& src, return_value_policy, handle) {
+    // As early as possible to avoid mid-process surprises.
+    internal::EnsurePyDateTime_IMPORT();
     // This function truncates fractional microseconds as the python datetime
     // objects cannot support a resolution higher than this.
-    auto py_datetime_t = module::import("datetime").attr("datetime");
     if (src == absl::InfiniteFuture()) {
-      // For compatibility with absl/python/time.cc
-      return replace_tzinfo_utc(py_datetime_t(9999, 12, 31, 23, 59, 59, 999999))
-          .release();
+      return PyDateTimeAPI->DateTime_FromDateAndTime(
+          9999, 12, 31, 23, 59, 59, 999999, PyDateTimeAPI->TimeZone_UTC,
+          PyDateTimeAPI->DateTimeType);
     }
     if (src == absl::InfinitePast()) {
-      // For compatibility with absl/python/time.cc
-      return replace_tzinfo_utc(py_datetime_t(1, 1, 1, 0, 0, 0, 0)).release();
-    }
-    auto py_from_timestamp = py_datetime_t.attr("fromtimestamp");
-    auto py_timezone_t = module::import("dateutil.tz").attr("gettz");
-    auto py_timezone = py_timezone_t(absl::LocalTimeZone().name());
-    double as_seconds = static_cast<double>(absl::ToUnixMicros(src)) / 1e6;
-    auto py_datetime = py_from_timestamp(as_seconds, "tz"_a = py_timezone);
-    return py_datetime.release();
-  }
-
- private:
-  static object replace_tzinfo_utc(handle dt) {
-    auto py_timezone_utc =
-        module::import("datetime").attr("timezone").attr("utc");
-    return dt.attr("replace")(arg("tzinfo") = py_timezone_utc);
+      return PyDateTimeAPI->DateTime_FromDateAndTime(
+          1, 1, 1, 0, 0, 0, 0, PyDateTimeAPI->TimeZone_UTC,
+          PyDateTimeAPI->DateTimeType);
+    }
+    absl::Time::Breakdown t = src.In(absl::UTCTimeZone());
+    return PyDateTimeAPI->DateTime_FromDateAndTime(
+        t.year, t.month, t.day, t.hour, t.minute, t.second,
+        static_cast<int>(t.subsecond / absl::Microseconds(1)),
+        PyDateTimeAPI->TimeZone_UTC, PyDateTimeAPI->DateTimeType);
   }
 };
 

pybind11_abseil/tests/absl_test.py

@@ -133,6 +133,7 @@ class AbslTimeTest(parameterized.TestCase):
     for time in (time_utc, time_local_aware, time_local_naive):
       self.assertTrue(absl_example.check_datetime(time, secs))
       self.assertEqual(int(absl_example.roundtrip_time(time).timestamp()), secs)
+    self.assertEqual(absl_example.roundtrip_time(time_utc), time_utc)
 
   def test_pass_datetime_pre_unix_epoch(self):
     dt = datetime.datetime(1969, 7, 16, 10, 56, 7, microsecond=140)