BUG: repr of np.datetime64('NaT') in Series/DataFrame with dtype object

Author: Matt Roeschke

doc/source/whatsnew/v0.25.0.rst

@@ -233,7 +233,7 @@ Sparse
 Other
 ^^^^^
 
--
+- Bug in :class:`Series` and :class:`DataFrame` repr where ``np.datetime64('NaT')`` and ``np.timedelta64('NaT')`` with ``dtype=object`` would be represented as ``NaN`` (:issue:``)
 -
 -
 

pandas/_libs/tslibs/nattype.pyx

@@ -39,6 +39,18 @@ _nat_scalar_rules[Py_GE] = False
 # ----------------------------------------------------------------------
 
 
+cpdef bint is_np_nat(x):
+    """Compat check for np.datetime('NaT')"""
+    try:
+        return np.isnat(x)
+    except AttributeError:
+        # numpy 1.12 compat
+        return str(x) == 'NaT'
+    except TypeError:
+        # np.isnat only defined for datetime, timedelta
+        return False
+
+
 def _make_nan_func(func_name, doc):
     def f(*args, **kwargs):
         return np.nan

pandas/io/formats/format.py

@@ -13,6 +13,7 @@
 from pandas._libs import lib
 from pandas._libs.tslib import format_array_from_datetime
 from pandas._libs.tslibs import NaT, Timedelta, Timestamp, iNaT
+from pandas._libs.tslibs.nattype import is_np_nat
 from pandas.compat import StringIO, lzip, map, u, zip
 
 from pandas.core.dtypes.common import (
@@ -942,20 +943,11 @@ def _format_strings(self):
             self.formatter if self.formatter is not None else
             (lambda x: pprint_thing(x, escape_chars=('\t', '\r', '\n'))))
 
-        def is_nat(x):
-            # Compat for numpy 1.12. Replace with np.isnat once min dep is 1.13
-            try:
-                return np.isnat(x)
-            except AttributeError:
-                return str(x) == 'NaT'
-            except TypeError:
-                return False
-
         def _format(x):
             if self.na_rep is not None and is_scalar(x) and isna(x):
                 if x is None:
                     return 'None'
-                elif x is NaT or is_nat(x):
+                elif x is NaT or is_np_nat(x):
                     return 'NaT'
                 return self.na_rep
             elif isinstance(x, PandasObject):

pandas/tests/frame/test_repr_info.py

@@ -521,3 +521,11 @@ def test_repr_categorical_dates_periods(self):
 
         df = DataFrame({'dt': Categorical(dt), 'p': Categorical(p)})
         assert repr(df) == exp
+
+    @pytest.mark.parametrize('arg', [np.datetime64, np.timedelta64])
+    @pytest.mark.parametrize('box, expected', [
+        [Series, '0    NaT\ndtype: object'],
+        [DataFrame, '     0\n0  NaT']])
+    def test_repr_np_nat_with_object(self, arg, box, expected):
+        result = repr(box([arg('NaT')], dtype=object))
+        assert result == expected