Merge pull request #6465 from shoyer/datetime64-NaT-casting

BUG: fix casting rules for generic datetime64/timedelta64 dtypes

2 files changed, 35 insertions, 8 deletions

diff --git a/numpy/core/src/multiarray/datetime.c b/numpy/core/src/multiarray/datetime.c
index 9e4e00e9c..264178d30 100644
--- a/numpy/core/src/multiarray/datetime.c
+++ b/numpy/core/src/multiarray/datetime.c
@@ -1232,12 +1232,18 @@ datetime_metadata_divides(
 {
     npy_uint64 num1, num2;
 
-    /* Generic units divide into anything */
-    if (divisor->base == NPY_FR_GENERIC) {
+    /*
+     * Any unit can always divide into generic units. In other words, we
+     * should be able to convert generic units into any more specific unit.
+     */
+    if (dividend->base == NPY_FR_GENERIC) {
         return 1;
     }
-    /* Non-generic units never divide into generic units */
-    else if (dividend->base == NPY_FR_GENERIC) {
+    /*
+     * However, generic units cannot always divide into more specific units.
+     * We cannot safely convert datetimes with units back into generic units.
+     */
+    else if (divisor->base == NPY_FR_GENERIC) {
         return 0;
     }
 
@@ -1330,7 +1336,7 @@ can_cast_datetime64_units(NPY_DATETIMEUNIT src_unit,
          */
         case NPY_SAME_KIND_CASTING:
             if (src_unit == NPY_FR_GENERIC || dst_unit == NPY_FR_GENERIC) {
-                return src_unit == dst_unit;
+                return src_unit == NPY_FR_GENERIC;
             }
             else {
                 return (src_unit <= NPY_FR_D && dst_unit <= NPY_FR_D) ||
@@ -1344,7 +1350,7 @@ can_cast_datetime64_units(NPY_DATETIMEUNIT src_unit,
          */
         case NPY_SAFE_CASTING:
             if (src_unit == NPY_FR_GENERIC || dst_unit == NPY_FR_GENERIC) {
-                return src_unit == dst_unit;
+                return src_unit == NPY_FR_GENERIC;
             }
             else {
                 return (src_unit <= dst_unit) &&
@@ -1380,7 +1386,7 @@ can_cast_timedelta64_units(NPY_DATETIMEUNIT src_unit,
          */
         case NPY_SAME_KIND_CASTING:
             if (src_unit == NPY_FR_GENERIC || dst_unit == NPY_FR_GENERIC) {
-                return src_unit == dst_unit;
+                return src_unit == NPY_FR_GENERIC;
             }
             else {
                 return (src_unit <= NPY_FR_M && dst_unit <= NPY_FR_M) ||
@@ -1394,7 +1400,7 @@ can_cast_timedelta64_units(NPY_DATETIMEUNIT src_unit,
          */
         case NPY_SAFE_CASTING:
             if (src_unit == NPY_FR_GENERIC || dst_unit == NPY_FR_GENERIC) {
-                return src_unit == dst_unit;
+                return src_unit == NPY_FR_GENERIC;
             }
             else {
                 return (src_unit <= dst_unit) &&
diff --git a/numpy/core/tests/test_datetime.py b/numpy/core/tests/test_datetime.py
index 8a8eafee8..360463d38 100644
--- a/numpy/core/tests/test_datetime.py
+++ b/numpy/core/tests/test_datetime.py
@@ -114,6 +114,27 @@ class TestDateTime(TestCase):
         # Can cast safely if the integer multiplier does divide
         assert_(np.can_cast('M8[6h]', 'M8[3h]', casting='safe'))
 
+        # We can always cast types with generic units (corresponding to NaT) to
+        # more specific types
+        assert_(np.can_cast('m8', 'm8[h]', casting='same_kind'))
+        assert_(np.can_cast('m8', 'm8[h]', casting='safe'))
+        assert_(np.can_cast('M8', 'M8[h]', casting='same_kind'))
+        assert_(np.can_cast('M8', 'M8[h]', casting='safe'))
+        # but not the other way around
+        assert_(not np.can_cast('m8[h]', 'm8', casting='same_kind'))
+        assert_(not np.can_cast('m8[h]', 'm8', casting='safe'))
+        assert_(not np.can_cast('M8[h]', 'M8', casting='same_kind'))
+        assert_(not np.can_cast('M8[h]', 'M8', casting='safe'))
+
+    def test_compare_generic_nat(self):
+        # regression tests for GH6452
+        assert_equal(np.datetime64('NaT'),
+                     np.datetime64('2000') + np.timedelta64('NaT'))
+        # nb. we may want to make NaT != NaT true in the future; this test
+        # verifies the existing behavior (and that it should not warn)
+        assert_(np.datetime64('NaT') == np.datetime64('NaT', 'us'))
+        assert_(np.datetime64('NaT', 'us') == np.datetime64('NaT'))
+
     def test_datetime_scalar_construction(self):
         # Construct with different units
         assert_equal(np.datetime64('1950-03-12', 'D'),