From 0285ae83d245406eb72754926e7f79278416fd9b Mon Sep 17 00:00:00 2001
From: Marco Gorelli <33491632+MarcoGorelli@users.noreply.github.com>
Date: Wed, 13 Nov 2024 09:08:00 +0000
Subject: [PATCH 1/2] perf: concatenate less in pandas-like with_columns

---
 narwhals/_pandas_like/dataframe.py | 36 +++++++++++++++++-------------
 1 file changed, 20 insertions(+), 16 deletions(-)

diff --git a/narwhals/_pandas_like/dataframe.py b/narwhals/_pandas_like/dataframe.py
index 3bf60f845..1893bb14e 100644
--- a/narwhals/_pandas_like/dataframe.py
+++ b/narwhals/_pandas_like/dataframe.py
@@ -422,27 +422,31 @@ def with_columns(
 
         if fast_path:
             new_column_name_to_new_column_map = {s.name: s for s in new_columns}
-            to_concat = []
-            # Make sure to preserve column order
-            for name in self._native_frame.columns:
-                if name in new_column_name_to_new_column_map:
-                    to_concat.append(
-                        validate_dataframe_comparand(
-                            index, new_column_name_to_new_column_map.pop(name)
-                        )
-                    )
-                else:
-                    to_concat.append(self._native_frame[name])
-            to_concat.extend(
-                validate_dataframe_comparand(index, new_column_name_to_new_column_map[s])
-                for s in new_column_name_to_new_column_map
+            columns_that_stay_the_same = list(
+                set(self.columns).difference(new_column_name_to_new_column_map)
             )
-
+            frame = select_columns_by_name(
+                self._native_frame,
+                columns_that_stay_the_same,
+                self._backend_version,
+                self._implementation,
+            )
+            other = [
+                validate_dataframe_comparand(index, value).rename(key, copy=False)
+                for key, value in new_column_name_to_new_column_map.items()
+                if key not in frame.columns
+            ]
             df = horizontal_concat(
-                to_concat,
+                [frame, *other],
                 implementation=self._implementation,
                 backend_version=self._backend_version,
             )
+            order = self.columns + [
+                x for x in new_column_name_to_new_column_map if x not in self.columns
+            ]
+            df = select_columns_by_name(
+                df, order, self._backend_version, self._implementation
+            )
         else:
             # This is the logic in pandas' DataFrame.assign
             if self._backend_version < (2,):

From fd5aca432fc00bed7b301edaabf0e2ec9b0927ab Mon Sep 17 00:00:00 2001
From: Marco Gorelli <33491632+MarcoGorelli@users.noreply.github.com>
Date: Wed, 13 Nov 2024 09:13:44 +0000
Subject: [PATCH 2/2] simplify

---
 narwhals/_pandas_like/dataframe.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/narwhals/_pandas_like/dataframe.py b/narwhals/_pandas_like/dataframe.py
index 1893bb14e..d28dfb416 100644
--- a/narwhals/_pandas_like/dataframe.py
+++ b/narwhals/_pandas_like/dataframe.py
@@ -432,7 +432,7 @@ def with_columns(
                 self._implementation,
             )
             other = [
-                validate_dataframe_comparand(index, value).rename(key, copy=False)
+                validate_dataframe_comparand(index, value)
                 for key, value in new_column_name_to_new_column_map.items()
                 if key not in frame.columns
             ]