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

doc/source/whatsnew/v0.25.0.rst

@@ -395,7 +395,7 @@ 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:`25445`)
 -
 -
 

pandas/io/formats/format.py

@@ -941,10 +941,16 @@ def _format_strings(self):
 
         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:
-                    return 'NaT'
+                try:
+                    # try block for np.isnat specifically
+                    # determine na_rep if x is None or NaT-like
+                    if x is None:
+                        return 'None'
+                    elif x is NaT or np.isnat(x):
+                        return 'NaT'
+                except (TypeError, ValueError):
+                    # np.isnat only handles datetime or timedelta objects
+                    pass
                 return self.na_rep
             elif isinstance(x, PandasObject):
                 return '{x}'.format(x=x)

pandas/tests/frame/test_repr_info.py

@@ -512,3 +512,12 @@ 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):
+        # GH 25445
+        result = repr(box([arg('NaT')], dtype=object))
+        assert result == expected