Try to call Override less

Eliminate a kwarg 'explain' from the call to Builder() for Mkdir.  Arg is
not used (maybe it was in the past, though have not found the evidence
for this), which meant we had an override dict in this case. Suprisingly,
this ended up in the override logic a *lot* in a build of a large sample
project - over 2000 extra calls to scons_subst_once that weren't needed.

Builder and Executor both have places where they call Override().
While the Override factory indeed returns quickly if the override dict
is empty, avoid the function call entirely in this case by checking
(there was an old TODO comment to this effect in Builder.__init__.py).

The Executor init method declared multiple paramters with mutable
defaults, which means they "persist". While there's no evidence this
is calling a real problem, eliminate part of this footgun by defaulting
overridelist and builder_kw to None and checking for that (targets and
sources still default to a mutable empty list, a TBD perhaps).

Signed-off-by: Mats Wichmann <[email protected]>

Author: mwichmann

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")