PERF: Use shallow copies/remove unnecessary copies in reshaping (#58959)

Author: mroeschke

pandas/core/reshape/melt.py

@@ -202,9 +202,9 @@ def melt(
         if value_vars_was_not_none:
             frame = frame.iloc[:, algos.unique(idx)]
         else:
-            frame = frame.copy()
+            frame = frame.copy(deep=False)
     else:
-        frame = frame.copy()
+        frame = frame.copy(deep=False)
 
     if col_level is not None:  # allow list or other?
         # frame is a copy

pandas/core/reshape/pivot.py

@@ -557,8 +557,6 @@ def _all_key(key):
                 piece = piece.T
                 all_key = _all_key(key)
 
-                # we are going to mutate this, so need to copy!
-                piece = piece.copy()
                 piece[all_key] = margin[key]
 
                 table_pieces.append(piece)
@@ -842,11 +840,11 @@ def pivot(
     # If columns is None we will create a MultiIndex level with None as name
     # which might cause duplicated names because None is the default for
     # level names
-    data = data.copy(deep=False)
-    data.index = data.index.copy()
-    data.index.names = [
-        name if name is not None else lib.no_default for name in data.index.names
-    ]
+    if any(name is None for name in data.index.names):
+        data = data.copy(deep=False)
+        data.index.names = [
+            name if name is not None else lib.no_default for name in data.index.names
+        ]
 
     indexed: DataFrame | Series
     if values is lib.no_default:

pandas/core/reshape/reshape.py

@@ -461,7 +461,7 @@ def _unstack_multiple(
         )
 
     if isinstance(data, Series):
-        dummy = data.copy()
+        dummy = data.copy(deep=False)
         dummy.index = dummy_index
 
         unstacked = dummy.unstack("__placeholder__", fill_value=fill_value, sort=sort)
@@ -1025,7 +1025,7 @@ def stack_reshape(
     buf = []
     for idx in stack_cols.unique():
         if len(frame.columns) == 1:
-            data = frame.copy()
+            data = frame.copy(deep=False)
         else:
             if not isinstance(frame.columns, MultiIndex) and not isinstance(idx, tuple):
                 # GH#57750 - if the frame is an Index with tuples, .loc below will fail