DOC: Clarify output of diff (returned type and possible overflow)

pandas/core/frame.py

@@ -7019,40 +7019,14 @@ def melt(
     # ----------------------------------------------------------------------
     # Time series-related
 
-    def diff(self, periods: int = 1, axis: Axis = 0) -> "DataFrame":
-        """
-        First discrete difference of element.
-
-        Calculates the difference of a DataFrame element compared with another
-        element in the DataFrame (default is the element in the same column
-        of the previous row).
-
-        Parameters
-        ----------
-        periods : int, default 1
-            Periods to shift for calculating difference, accepts negative
-            values.
-        axis : {0 or 'index', 1 or 'columns'}, default 0
-            Take difference over rows (0) or columns (1).
-
-        Returns
-        -------
-        DataFrame
-
-        See Also
-        --------
-        Series.diff: First discrete difference for a Series.
-        DataFrame.pct_change: Percent change over given number of periods.
-        DataFrame.shift: Shift index by desired number of periods with an
-            optional time freq.
-
-        Notes
-        -----
-        For boolean dtypes, this uses :meth:`operator.xor` rather than
-        :meth:`operator.sub`.
-
-        Examples
-        --------
+    @doc(
+        Series.diff,
+        klass="Dataframe",
+        extra_params="axis : {0 or 'index', 1 or 'columns'}, default 0\n    "
+        "Take difference over rows (0) or columns (1).\n",
+        other_klass="Series",
+        examples=dedent(
+            """
         Difference with previous row
 
         >>> df = pd.DataFrame({'a': [1, 2, 3, 4, 5, 6],
@@ -7108,7 +7082,18 @@ def diff(self, periods: int = 1, axis: Axis = 0) -> "DataFrame":
         3 -1.0 -2.0  -9.0
         4 -1.0 -3.0 -11.0
         5  NaN  NaN   NaN
-        """
+
+        Overflow in input dtype
+
+        >>> df = pd.DataFrame({'a': [1, 0]}, dtype=np.uint8)
+        >>> df.diff()
+               a
+        0    NaN
+        1  255.0"""
+        ),
+    )
+    def diff(self, periods: int = 1, axis: Axis = 0) -> "DataFrame":
+
         bm_axis = self._get_block_manager_axis(axis)
         self._consolidate_inplace()
 

pandas/core/series.py

@@ -2293,38 +2293,12 @@ def cov(self, other, min_periods=None) -> float:
             return np.nan
         return nanops.nancov(this.values, other.values, min_periods=min_periods)
 
-    def diff(self, periods: int = 1) -> "Series":
-        """
-        First discrete difference of element.
-
-        Calculates the difference of a Series element compared with another
-        element in the Series (default is element in previous row).
-
-        Parameters
-        ----------
-        periods : int, default 1
-            Periods to shift for calculating difference, accepts negative
-            values.
-
-        Returns
-        -------
-        Series
-            First differences of the Series.
-
-        See Also
-        --------
-        Series.pct_change: Percent change over given number of periods.
-        Series.shift: Shift index by desired number of periods with an
-            optional time freq.
-        DataFrame.diff: First discrete difference of object.
-
-        Notes
-        -----
-        For boolean dtypes, this uses :meth:`operator.xor` rather than
-        :meth:`operator.sub`.
-
-        Examples
-        --------
+    @doc(
+        klass="Series",
+        extra_params="",
+        other_klass="DataFrame",
+        examples=dedent(
+            """
         Difference with previous row
 
         >>> s = pd.Series([1, 1, 2, 3, 5, 8])
@@ -2358,6 +2332,51 @@ def diff(self, periods: int = 1) -> "Series":
         4   -3.0
         5    NaN
         dtype: float64
+
+        Overflow in input dtype
+
+        >>> s = pd.Series([1, 0], dtype=np.uint8)
+        >>> s.diff()
+        0      NaN
+        1    255.0
+        dtype: float64"""
+        ),
+    )
+    def diff(self, periods: int = 1) -> "Series":
+        """
+        First discrete difference of element.
+
+        Calculates the difference of a {klass} element compared with another
+        element in the {klass} (default is element in previous row).
+
+        Parameters
+        ----------
+        periods : int, default 1
+            Periods to shift for calculating difference, accepts negative
+            values.
+        {extra_params}
+        Returns
+        -------
+        {klass}
+            First differences of the Series.
+
+        See Also
+        --------
+        {klass}.pct_change: Percent change over given number of periods.
+        {klass}.shift: Shift index by desired number of periods with an
+            optional time freq.
+        {other_klass}.diff: First discrete difference of object.
+
+        Notes
+        -----
+        For boolean dtypes, this uses :meth:`operator.xor` rather than
+        :meth:`operator.sub`.
+        The result is calculated according to current dtype in {klass},
+        however dtype of the result is always float64.
+
+        Examples
+        --------
+        {examples}
         """
         result = algorithms.diff(self.array, periods)
         return self._constructor(result, index=self.index).__finalize__(

pandas/core/sorting.py

@@ -385,7 +385,7 @@ def ensure_key_mapped(values, key: Optional[Callable], levels=None):
     from pandas.core.indexes.api import Index
 
     if not key:
-        return values.copy()
+        return values
 
     if isinstance(values, ABCMultiIndex):
         return ensure_key_mapped_multiindex(values, key, level=levels)