Repository: pandas-dev/pandas

Issue:
PERF: DataFrame.from_records with DataFrame input
Speculative: DataFrame.from_records if passed a DataFrame goes through internals.construction.to_arrays and ends up splitting the frame into individual columns.  This can result in slower subsequent operations if that splitting turned out to be unnecessary.  Can we handle DataFrame input inside from_records and avoid this splitting?

This is also the only way we get to to_arrays with a DataFrame input, so the function could be simplified if that case could be ruled out.



Discussion:
Can we simply deprecate a DataFrame input? I might be missing something, but there seems to be no use-case that can't be achieved through calling other functions? e.g. set_index, reindex and drop
Agreed it seems like a really weird use case.

Patch:
diff --git a/doc/source/whatsnew/v2.1.0.rst b/doc/source/whatsnew/v2.1.0.rst
index c0ca5b2320338..2ac20e97409e7 100644
--- a/doc/source/whatsnew/v2.1.0.rst
+++ b/doc/source/whatsnew/v2.1.0.rst
@@ -93,6 +93,7 @@ Other API changes
 Deprecations
 ~~~~~~~~~~~~
 - Deprecating pinning ``group.name`` to each group in :meth:`SeriesGroupBy.aggregate` aggregations; if your operation requires utilizing the groupby keys, iterate over the groupby object instead (:issue:`41090`)
+- Deprecated passing a :class:`DataFrame` to :meth:`DataFrame.from_records`, use :meth:`DataFrame.set_index` or :meth:`DataFrame.drop` instead (:issue:`51353`)
 - Deprecated accepting slices in :meth:`DataFrame.take`, call ``obj[slicer]`` or pass a sequence of integers instead (:issue:`51539`)
 -
 
diff --git a/pandas/core/frame.py b/pandas/core/frame.py
index e6af875ab1c23..aa3b10df742a2 100644
--- a/pandas/core/frame.py
+++ b/pandas/core/frame.py
@@ -2109,6 +2109,9 @@ def from_records(
         ----------
         data : structured ndarray, sequence of tuples or dicts, or DataFrame
             Structured input data.
+
+            .. deprecated:: 2.1.0
+                Passing a DataFrame is deprecated.
         index : str, list of fields, array-like
             Field of array to use as the index, alternately a specific set of
             input labels to use.
@@ -2171,6 +2174,23 @@ def from_records(
         2      1     c
         3      0     d
         """
+        if isinstance(data, DataFrame):
+            warnings.warn(
+                "Passing a DataFrame to DataFrame.from_records is deprecated. Use "
+                "set_index and/or drop to modify the DataFrame instead.",
+                FutureWarning,
+                stacklevel=find_stack_level(),
+            )
+            if columns is not None:
+                if is_scalar(columns):
+                    columns = [columns]
+                data = data[columns]
+            if index is not None:
+                data = data.set_index(index)
+            if exclude is not None:
+                data = data.drop(columns=exclude)
+            return data
+
         result_index = None
 
         # Make a copy of the input columns so we can modify it
@@ -2238,7 +2258,7 @@ def maybe_reorder(
                     arrays, arr_columns, columns, index
                 )
 
-        elif isinstance(data, (np.ndarray, DataFrame)):
+        elif isinstance(data, np.ndarray):
             arrays, columns = to_arrays(data, columns)
             arr_columns = columns
         else:
diff --git a/pandas/core/internals/construction.py b/pandas/core/internals/construction.py
index 005c166bb1f2a..ce14145e18f06 100644
--- a/pandas/core/internals/construction.py
+++ b/pandas/core/internals/construction.py
@@ -769,19 +769,6 @@ def to_arrays(
     -----
     Ensures that len(result_arrays) == len(result_index).
     """
-    if isinstance(data, ABCDataFrame):
-        # see test_from_records_with_index_data, test_from_records_bad_index_column
-        if columns is not None:
-            arrays = [
-                data._ixs(i, axis=1)._values
-                for i, col in enumerate(data.columns)
-                if col in columns
-            ]
-        else:
-            columns = data.columns
-            arrays = [data._ixs(i, axis=1)._values for i in range(len(columns))]
-
-        return arrays, columns
 
     if not len(data):
         if isinstance(data, np.ndarray):


Test patch:
diff --git a/pandas/tests/frame/constructors/test_from_records.py b/pandas/tests/frame/constructors/test_from_records.py
index 18bf633d60186..0ba4f6d249e6d 100644
--- a/pandas/tests/frame/constructors/test_from_records.py
+++ b/pandas/tests/frame/constructors/test_from_records.py
@@ -25,7 +25,8 @@ def test_from_records_dt64tz_frame(self):
         # GH#51162 don't lose tz when calling from_records with DataFrame input
         dti = date_range("2016-01-01", periods=10, tz="US/Pacific")
         df = DataFrame({i: dti for i in range(4)})
-        res = DataFrame.from_records(df)
+        with tm.assert_produces_warning(FutureWarning):
+            res = DataFrame.from_records(df)
         tm.assert_frame_equal(res, df)
 
     def test_from_records_with_datetimes(self):
@@ -177,29 +178,34 @@ def test_from_records_with_index_data(self):
         df = DataFrame(np.random.randn(10, 3), columns=["A", "B", "C"])
 
         data = np.random.randn(10)
-        df1 = DataFrame.from_records(df, index=data)
+        with tm.assert_produces_warning(FutureWarning):
+            df1 = DataFrame.from_records(df, index=data)
         tm.assert_index_equal(df1.index, Index(data))
 
     def test_from_records_bad_index_column(self):
         df = DataFrame(np.random.randn(10, 3), columns=["A", "B", "C"])
 
         # should pass
-        df1 = DataFrame.from_records(df, index=["C"])
+        with tm.assert_produces_warning(FutureWarning):
+            df1 = DataFrame.from_records(df, index=["C"])
         tm.assert_index_equal(df1.index, Index(df.C))
 
-        df1 = DataFrame.from_records(df, index="C")
+        with tm.assert_produces_warning(FutureWarning):
+            df1 = DataFrame.from_records(df, index="C")
         tm.assert_index_equal(df1.index, Index(df.C))
 
         # should fail
         msg = "|".join(
             [
-                r"Length of values \(10\) does not match length of index \(1\)",
+                r"'None of \[2\] are in the columns'",
             ]
         )
-        with pytest.raises(ValueError, match=msg):
-            DataFrame.from_records(df, index=[2])
-        with pytest.raises(KeyError, match=r"^2$"):
-            DataFrame.from_records(df, index=2)
+        with pytest.raises(KeyError, match=msg):
+            with tm.assert_produces_warning(FutureWarning):
+                DataFrame.from_records(df, index=[2])
+        with pytest.raises(KeyError, match=msg):
+            with tm.assert_produces_warning(FutureWarning):
+                DataFrame.from_records(df, index=2)
 
     def test_from_records_non_tuple(self):
         class Record: