Move optimization deeper in the call stack

Author: rtlee9

doc/source/whatsnew/v2.0.0.rst

@@ -158,7 +158,7 @@ Performance improvements
 - Performance improvements to :func:`read_sas` (:issue:`47403`, :issue:`47405`, :issue:`47656`, :issue:`48502`)
 - Memory improvement in :meth:`RangeIndex.sort_values` (:issue:`48801`)
 - Performance improvement in :class:`DataFrameGroupBy` and :class:`SeriesGroupBy` when ``by`` is a categorical type and ``sort=False`` (:issue:`48976`)
-- Performance improvement for :meth:`NDFrame.to_csv` when data frame is sparse (:issue:`41023`)
+- Performance improvement for saving to CSV with :class:`CSVFormatter` when data frame is sparse (:issue:`41023`)
 
 .. ---------------------------------------------------------------------------
 .. _whatsnew_200.bug_fixes:

pandas/core/generic.py

@@ -3717,8 +3717,6 @@ def to_csv(
         >>> df.to_csv('folder/subfolder/out.csv')  # doctest: +SKIP
         """
         df = self if isinstance(self, ABCDataFrame) else self.to_frame()
-        if hasattr(df, "sparse"):
-            df = df.sparse.to_dense()  # fixes 41023
 
         formatter = DataFrameFormatter(
             frame=df,

pandas/io/formats/csvs.py

@@ -69,7 +69,10 @@ def __init__(
     ) -> None:
         self.fmt = formatter
 
-        self.obj = self.fmt.frame
+        if hasattr(self.fmt.frame, "sparse"):
+            self.obj = self.fmt.frame.sparse.to_dense()
+        else:
+            self.obj = self.fmt.frame
 
         self.filepath_or_buffer = path_or_buf
         self.encoding = encoding