fix interpreting int as second--> nano (#24694)

doc/source/whatsnew/v0.24.0.rst

@@ -1309,6 +1309,7 @@ Deprecations
 - In :meth:`Series.where` with Categorical data, providing an ``other`` that is not present in the categories is deprecated. Convert the categorical to a different dtype or add the ``other`` to the categories first (:issue:`24077`).
 - :meth:`Series.clip_lower`, :meth:`Series.clip_upper`, :meth:`DataFrame.clip_lower` and :meth:`DataFrame.clip_upper` are deprecated and will be removed in a future version. Use ``Series.clip(lower=threshold)``, ``Series.clip(upper=threshold)`` and the equivalent ``DataFrame`` methods (:issue:`24203`)
 - :meth:`Series.nonzero` is deprecated and will be removed in a future version (:issue:`18262`)
+- Passing an integer to :meth:`Series.fillna` and :meth:`DataFrame.fillna` with ``timedelta64[ns]`` dtypes is deprecated, will raise ``TypeError`` in a future version.  Use ``obj.fillna(pd.Timedelta(...))` instead (:issue:`24694`)
 
 .. _whatsnew_0240.deprecations.datetimelike_int_ops:
 

pandas/core/internals/blocks.py

@@ -2499,8 +2499,14 @@ def _can_hold_element(self, element):
     def fillna(self, value, **kwargs):
 
         # allow filling with integers to be
-        # interpreted as seconds
+        # interpreted as nanoseconds
         if is_integer(value) and not isinstance(value, np.timedelta64):
+            # Deprecation GH#24694, GH#19233
+            warnings.warn("Passing integers to fillna is deprecated, will "
+                          "raise a TypeError in a future version.  To retain "
+                          "the old behavior, pass pd.Timedelta(seconds=n) "
+                          "instead.",
+                          FutureWarning, stacklevel=6)
             value = Timedelta(value, unit='s')
         return super(TimeDeltaBlock, self).fillna(value, **kwargs)
 

pandas/tests/series/test_missing.py

@@ -65,14 +65,17 @@ def test_timedelta_fillna(self):
         td = s.diff()
 
         # reg fillna
-        result = td.fillna(0)
+        with tm.assert_produces_warning(FutureWarning):
+            result = td.fillna(0)
         expected = Series([timedelta(0), timedelta(0), timedelta(1),
                            timedelta(days=1, seconds=9 * 3600 + 60 + 1)])
         assert_series_equal(result, expected)
 
-        # interprested as seconds
-        result = td.fillna(1)
-        expected = Series([timedelta(seconds=1), timedelta(0), timedelta(1),
+        # interpreted as seconds, deprecated
+        with tm.assert_produces_warning(FutureWarning):
+            result = td.fillna(1)
+        expected = Series([timedelta(seconds=1),
+                           timedelta(0), timedelta(1),
                            timedelta(days=1, seconds=9 * 3600 + 60 + 1)])
         assert_series_equal(result, expected)
 
@@ -96,14 +99,16 @@ def test_timedelta_fillna(self):
         # ffill
         td[2] = np.nan
         result = td.ffill()
-        expected = td.fillna(0)
+        with tm.assert_produces_warning(FutureWarning):
+            expected = td.fillna(0)
         expected[0] = np.nan
         assert_series_equal(result, expected)
 
         # bfill
         td[2] = np.nan
         result = td.bfill()
-        expected = td.fillna(0)
+        with tm.assert_produces_warning(FutureWarning):
+            expected = td.fillna(0)
         expected[2] = timedelta(days=1, seconds=9 * 3600 + 60 + 1)
         assert_series_equal(result, expected)