Add Python datetime interop

Fixes #185

src/epoch/python.rs

@@ -17,7 +17,7 @@ use core::str::FromStr;
 use crate::epoch::leap_seconds_file::LeapSecondsFile;
 use pyo3::prelude::*;
 use pyo3::pyclass::CompareOp;
-use pyo3::types::PyType;
+use pyo3::types::{PyDateAccess, PyDateTime, PyTimeAccess, PyType, PyTzInfoAccess};
 
 #[pymethods]
 impl Epoch {
@@ -502,4 +502,48 @@ impl Epoch {
             CompareOp::Ge => *self >= other,
         }
     }
+
+    /// Returns a Python datetime object from this Epoch (truncating the nanoseconds away)
+    fn todatetime<'py>(&self, py: Python<'py>) -> Result<Bound<'py, PyDateTime>, PyErr> {
+        let (y, mm, dd, hh, min, s, nanos) =
+            Epoch::compute_gregorian(self.duration, TimeScale::UTC);
+
+        let datetime = PyDateTime::new_bound(py, y, mm, dd, hh, min, s, nanos / 1_000, None)?;
+
+        Ok(datetime)
+    }
+
+    /// Builds an Epoch in UTC from the provided datetime after timezone correction if any is present.
+    #[classmethod]
+    fn fromdatetime(
+        _cls: &Bound<'_, PyType>,
+        dt: &Bound<'_, PyAny>,
+    ) -> Result<Self, HifitimeError> {
+        let dt = dt
+            .downcast::<PyDateTime>()
+            .map_err(|e| HifitimeError::PythonError {
+                reason: e.to_string(),
+            })?;
+
+        // If the user tries to convert a timezone aware datetime into a naive one,
+        // we return a hard error. We could silently remove tzinfo, or assume local timezone
+        // and do a conversion, but better leave this decision to the user of the library.
+        let has_tzinfo = dt.get_tzinfo_bound().is_some();
+        if has_tzinfo {
+            return Err(HifitimeError::PythonError {
+                reason: "expected a datetime without tzinfo, call my_datetime.replace(tzinfo=None)"
+                    .to_string(),
+            });
+        }
+
+        Epoch::maybe_from_gregorian_utc(
+            dt.get_year(),
+            dt.get_month().into(),
+            dt.get_day().into(),
+            dt.get_hour().into(),
+            dt.get_minute().into(),
+            dt.get_second().into(),
+            dt.get_microsecond() * 1_000,
+        )
+    }
 }

src/errors.rs

@@ -38,6 +38,11 @@ pub enum HifitimeError {
     Duration {
         source: DurationError,
     },
+    #[cfg(feature = "python")]
+    #[snafu(display("python interop error: {reason}"))]
+    PythonError {
+        reason: String,
+    },
 }
 
 #[cfg_attr(kani, derive(kani::Arbitrary))]

tests/python/test_epoch.py

@@ -1,5 +1,5 @@
 from hifitime import Duration, Epoch, HifitimeError, ParsingError, TimeScale, TimeSeries, Unit
-from datetime import datetime
+from datetime import datetime, timezone
 import pickle
 
 
@@ -108,4 +108,20 @@ def test_regression_gh249():
     e = Epoch.init_from_gregorian(year=2022, month=3, day=1, hour=1, minute=1, second=59, nanos=1, time_scale=TimeScale.GPST)
     assert e.strftime("%Y %m %d %H %M %S %f %T") == "2022 03 01 01 01 59 000000001 GPST"
     e = Epoch.init_from_gregorian(year=2022, month=3, day=1, hour=1, minute=1, second=59, nanos=1, time_scale=TimeScale.UTC)
-    assert e.strftime("%Y %m %d %H %M %S %f %T") == "2022 03 01 01 01 59 000000001 UTC"
+    assert e.strftime("%Y %m %d %H %M %S %f %T") == "2022 03 01 01 01 59 000000001 UTC"
+
+def test_interop():
+    hifinow = Epoch.system_now()
+    lofinow = hifinow.todatetime()
+    hifirtn = Epoch.fromdatetime(lofinow)
+    assert hifirtn.timedelta(hifinow).abs() < Unit.Microsecond * 1
+    # Now test with timezone, expect an error
+    tz_datetime = datetime(2023, 10, 8, 15, 30, tzinfo=timezone.utc)
+    try:
+        Epoch.fromdatetime(tz_datetime)
+    except Exception as e:
+        print(e)
+    else:
+        assert False, "tz aware dt did not fail"
+    # Repeat after the strip
+    assert Epoch.fromdatetime(tz_datetime.replace(tzinfo=None)) == Epoch("2023-10-08 15:30:00")