🐛 Series.diff was always setting the dtype to object

Author: Marco Gorelli

doc/source/whatsnew/v1.0.0.rst

@@ -1146,6 +1146,7 @@ ExtensionArray
 - Bug in :class:`arrays.PandasArray` when setting a scalar string (:issue:`28118`, :issue:`28150`).
 - Bug where nullable integers could not be compared to strings (:issue:`28930`)
 - Bug where :class:`DataFrame` constructor raised ``ValueError`` with list-like data and ``dtype`` specified (:issue:`30280`)
+- Bug in :meth:`Series.diff` was always setting the ``dtype`` to ``Object`` (:issue:`30889`)
 
 
 Other

pandas/core/series.py

@@ -34,6 +34,7 @@
     is_bool,
     is_categorical_dtype,
     is_datetime64_dtype,
+    is_datetime64tz_dtype,
     is_dict_like,
     is_extension_array_dtype,
     is_integer,
@@ -2316,6 +2317,12 @@ def diff(self, periods=1) -> "Series":
         dtype: float64
         """
         result = algorithms.diff(com.values_from_object(self), periods)
+        if is_extension_array_dtype(self.dtype) and not is_datetime64tz_dtype(
+            self.dtype
+        ):
+            return self._constructor(
+                result, index=self.index, dtype=self.dtype
+            ).__finalize__(self)
         return self._constructor(result, index=self.index).__finalize__(self)
 
     def autocorr(self, lag=1) -> float:

pandas/tests/series/methods/test_diff.py

@@ -75,3 +75,9 @@ def test_diff_object_dtype(self):
         result = s.diff()
         expected = s - s.shift(1)
         tm.assert_series_equal(result, expected)
+
+    def test_nullable_integer(self, any_nullable_int_dtype):
+        # GH 30889
+        dtype = any_nullable_int_dtype
+        result = Series([1, 2, 3], dtype=dtype).diff().dtype
+        assert result == dtype