Try to call Override less

CHANGES.txt

@@ -13,11 +13,16 @@ NOTE: Since SCons 4.9.0, Python 3.7.0 or above is required.
 RELEASE  VERSION/DATE TO BE FILLED IN LATER
 
   From John Doe:
-
-        - Whatever John Doe did.
+    - Whatever John Doe did.
 
   From William Deegan:
-        - Fix SCons Docbook schema to work with lxml > 5
+    - Fix SCons Docbook schema to work with lxml > 5
+
+  From Mats Wichmann:
+    - Reduce unneeded computation of overrides. The Mkdir builder used an
+      unknown argument ('explain') on creation, causing it to be considered
+      an override. Also, if override dict is empty, don't even call the
+      Override factory function.
 
 
 RELEASE 4.10.1 - Sun, 16 Nov 2025 10:51:57 -0700

RELEASE.txt

@@ -41,6 +41,11 @@ IMPROVEMENTS
   documentation:  performance improvements (describe the circumstances
   under which they would be observed), or major code cleanups
 
+- Reduce unneeded computation of overrides. The Mkdir builder used an
+  unknown argument ('explain') on creation, causing it to be considered
+  an override. Also, if override dict is empty, don't even call the
+  Override factory function.
+
 PACKAGING
 ---------
 

SCons/Builder.py

@@ -670,8 +670,8 @@ def prependDirIfRelative(f, srcdir=kw['srcdir']):
         else:
             env_kw = self.overrides
 
-        # TODO if env_kw: then the following line. there's no purpose in calling if no overrides.
-        env = env.Override(env_kw)
+        if env_kw:  # there's no purpose in calling if no overrides.
+            env = env.Override(env_kw)
         return self._execute(env, target, source, OverrideWarner(kw), ekw)
 
     def adjust_suffix(self, suff):

SCons/Environment.py

@@ -859,9 +859,12 @@ def Override(self, overrides):
            A proxy environment of type :class:`OverrideEnvironment`.
            or the current environment if *overrides* is empty.
         """
-        if not overrides: return self
+        # belt-and-suspenders - main callers should already have checked:
+        if not overrides:
+            return self
         o = copy_non_reserved_keywords(overrides)
-        if not o: return self
+        if not o:
+            return self
         overrides = {}
         merges = None
         for key, value in o.items():

SCons/Executor.py

@@ -168,21 +168,18 @@ class Executor(metaclass=NoSlotsPyPy):
                  '_do_execute',
                  '_execute_str')
 
-    def __init__(self, action, env=None, overridelist=[{}],
-                 targets=[], sources=[], builder_kw={}) -> None:
+    def __init__(self, action, env=None, overridelist=None,
+                 targets=[], sources=[], builder_kw=None) -> None:
         if SCons.Debug.track_instances: logInstanceCreation(self, 'Executor.Executor')
         self.set_action_list(action)
         self.pre_actions = []
         self.post_actions = []
         self.env = env
-        self.overridelist = overridelist
-        if targets or sources:
-            self.batches = [Batch(targets[:], sources[:])]
-        else:
-            self.batches = []
-        self.builder_kw = builder_kw
-        self._do_execute = 1
-        self._execute_str = 1
+        self.overridelist = [{}] if overridelist is None else overridelist
+        self.batches = [Batch(targets[:], sources[:])] if targets or sources else []
+        self.builder_kw = {} if builder_kw is None else builder_kw
+        self._do_execute: int = 1  # map key
+        self._execute_str: int = 1  # map key
         self._memo = {}
 
     def get_lvars(self):
@@ -358,7 +355,7 @@ def get_build_env(self):
 
         import SCons.Defaults
         env = self.env or SCons.Defaults.DefaultEnvironment()
-        build_env = env.Override(overrides)
+        build_env = env.Override(overrides) if overrides else env
 
         self._memo['get_build_env'] = build_env
 

SCons/Node/FS.py

@@ -381,7 +381,6 @@ def get_MkdirBuilder():
         # calling SCons.Defaults.DefaultEnvironment() when necessary.
         MkdirBuilder = SCons.Builder.Builder(action = Mkdir,
                                              env = None,
-                                             explain = None,
                                              is_explicit = None,
                                              target_scanner = SCons.Defaults.DirEntryScanner,
                                              name = "MkdirBuilder")