A new SCons release, 4.5.0, is now available on the SCons download page:


Here is a summary of the changes since 4.4.0:

NOTE: If you build with Python 3.10.0 and then rebuild with 3.10.1 (or higher), you may
see unexpected rebuilds. This is due to Python internals changing which changed the signature of a Python Action Function.
NOTE: If you use a dictionary to specify your CPPDEFINES, you may see an unexpected rebuild.
The insertion order of dictionary keys is now preserved when generating the command line. Previously these were sorted alphabecially. This change to the command line, while generating identical set of CPPDEFINES can change order and cause a rebuild.


  • Added ValidateOptions() which will check that all command line options are in either those specified by SCons itself, or by AddOption() in SConstruct/SConscript. It should not be called until all AddOption() calls are completed. Resolves Issue #4187
  • Added --experimental=tm_v2, which enables Andrew Morrow's NewParallel Job implementation. This should scale much better for highly parallel builds. You can also enable this via SetOption().
  • Added FILE_ENCODING, to allow explicitly setting the text encoding for files written by the Textfile() and Substfile() builders. If not specified, Textfile() and Substfile() builders will write files as UTF-8.


  • The qt tool has been renamed qt3.


  • Added -fsanitize support to ParseFlags(). This will propagate to CCFLAGS and LINKFLAGS.
  • Calling EnsureSConsVersion() and EnsurePythonVersion() won't initialize DefaultEnvironment anymore.
  • The console message from the Chmod() action function now displays octal modes using the modern Python syntax (0o755 rather than 0755).
  • Migrated logging logic for --taskmastertrace to use Python's logging module. Added logging to NewParallel Job class (Andrew Morrow's new parallel job implementation)
  • Preliminary support for Python 3.12.
  • Run LaTeX after biber/bibtex only if necessary
  • Configure context methods CheckLib and CheckLibWithHeader now expose two additional keyword arguments: 'append', which controls whether to append (the default) or prepend discovered libraries to $LIBS, and 'unique', which controls whether to add the library if it is already in the $LIBS list. This brings the library-adding functionality in Configure in line with the regular Append, AppendUnique, Prepend and PrependUnique methods.
  • CPPDEFINES values added via a dictionary type are longer sorted by key. This used to be required to maintain a consistent order of commandline arguments between SCons runs, but meant macros were not always emitted in the order entered. Sorting is no longer required after Python interpreter improvements. There might be a one-time rebuild of targets that involved such sorted keys in their actions.
  • Renamed the 'qt' tools to 'qt3' since the logic in that tool is only for QT version 3. Renamed all env vars which affect qt3 from QT_ to QT3_. If you are still using SCons to build QT 3 code, you'll need to update your SConscripts. Note that using 'qt' tool has been deprecated for some time.


  • Added missing newline to generated compilation database (compile_commands.json)
  • A list argument as the source to the Copy() action function is now handled. Both the implementation and the strfunction which prints the progress message were adjusted.
  • The Java Scanner processing of JAVACLASSPATH for dependencies (behavior that was introduced in SCons 4.4.0) is adjusted to split on the system's search path separator instead of on a space. The previous behavior meant that a path containing spaces (e.g. r"C:somepathMy Classes") would lead to unexpected errors. If the split-on-space behavior is desired, pre-split the value: instead of: env["JAVACLASSPATH"] = "foo bar baz" use: env["JAVACLASSPATH"] = env.Split("foo bar baz") There is no change in how JAVACLASSPATH gets turned into the -classpath argument passed to the JDK tools.
  • Ninja: Fix execution environment sanitation for launching ninja. Previously if you set an execution environment variable set to a python list it would crash. Now it will create a string joining the list with os.pathsep
  • Fixed command line argument --diskcheck: previously a value of 'none' was ignored. SetOption('diskcheck','none') is unaffected, as it did not have the problem.
  • Fixed Issue #4275 - when outputting compilation db and TEMPFILE was in use, the compilation db would have command lines using the generated tempfile for long command lines, instead of the full command line for the compilation step for the source/target pair.
  • A refactor in the caching logic for version 4.4.0 left Java inner classes failing with an exception when a CacheDir was enabled. This is now corrected.
  • When using the gfortran tool (the default on most platforms as long as a GNU toolchain is installed), the user setting of the "dialect" compilers (F77, F90, F03 and F09, as well as the shared-library equivalents SHF77, SHF90, SHF03, SHF09) is now honored; previously the tool overwrote the settings to 'gfortran', which made it difficult reference a cross-compile version for dialects.
  • Fix issue #2757, where Configure checks that perform a check which reads a modified source (including program, source or header file(s)) would incorrectly mark that file "up to date" so the actual build would not see the file as modified. Leading to incorrect incremental builds. Now configure checks now clear node info for non conftest nodes, so they will be re-evaluated for the real taskmaster run when the build commences.
  • Inconsistent behavior of adding values to the CPPDEFINES construction variable has been cleaned up (described in issue #4254). Now a tuple, whether provided by itself or as a member of a list, is consistently interpreted as a macro with replacement value. When adding a list, individual members are added in sequence without interpretation. A string value containing spaces is split if it is the initial value or added, but not if it is given as a member of an added list. Detection of duplicate macros now works for both valued and unvalued forms.
  • Handling of CPPDEFINES macros via Prepend and PrependUnique now works (previously this was special-cased only for Append and AppendUnique).


  • Changed the Taskmaster trace logic to use python's logging module.
  • Add cache-debug messages for push failures.
  • Added error message to handle the case when SCons attempts to retrieve all the targets for a specified builder from the CacheDir, fails to do so, and then runs into an error when deleting the files which were retrieved. Previously if this happened there were no errors or warnings.


  • SCons now has three requirements files: requirements.txt describes requirements to run scons; requirements-dev.txt requirements to develop it - mainly things needed to run the testsuite; requirements_pkg.txt are the requirements to do a full build (including docs build) with an intent to create the packages.
  • Moved rpm and debian directories under packaging
  • Added logic to help packagers enable reproducible builds into packaging/etc/. Please read packaging/etc/README.txt if you are interested.
  • A zipapp of scons-local is now also built.


  • Updated the --hash-format manpage entry.
  • EnsureSConsVersion, EnsurePythonVersion, Exit, GetLaunchDir and SConscriptChdir are now listed as Global functions only.
  • Updated the docs for Glob.
  • Updated SHELL_ENV_GENERATORS description and added versionadded indicator.
  • JAVABOOTCLASSPATH, JAVACLASSPATH and JAVASOURCEPATH better document the acceptable syntax for values, and how they will be interpreted, including that JAVACLASSPATH will be scanned for dependencies. Added note on the possibly surprising feature that SCons always passes -sourcepath when calling javac, which affects how the class path is used when finding sources.
  • Updated the User Guide chapter on installation: modernized the notes on Python installs, SCons installs, and having multiple SCons versions present on a single system.
  • Updated the User Guide chapter on variant directories with more explanation, and the introduction of terms like "out of tree" that may help in forming a mental model.
  • Additional explanations for MSVSProject and MSVSSolution builders.
  • Updated MSVC documentation - adds "version added" annotations on recently added construction variables and provides a version-mapping table.
  • Added further details in the documentation of Append and related methods on the special handling of CPPDEFINES.


  • Refactored SCons/Taskmaster into a package. Moved SCons/Jobs.py into that package. NOTE: If you hook into SCons.Jobs, you'll have to change that to use SCons.Taskmaster.Jobs
  • Refactored SCons.Util, which had grown quite large, to be a package, not a single-file module. Change should be transparent: the same import of SCons.Util and public symbols from it will continue to work, however code which reaches directly in to grab private symbols (starting with underscore) which moved to a submodule, that code will have to be adjusted, as those are not imported to the package level (new SCons.Util.hashes has some of these, which are used by existing unit tests).
  • Added overrides argument to SCons.Subst.scons_subst(), subst_list(), subst(), and Action's process(), strfunction(). This allows passing a dictionary of envvars to override when evaluating subst()'d strings/lists

Thanks to the following contributors listed below for their contributions to this release.

git shortlog --no-merges -ns 4.4.0..HEAD
93  Mats Wichmann
85  William Deegan
 9  Anatoli Babenia
 8  Daniel Moody
 4  Andrew Morrow
 3  Ryan Saunders
 2  Flaviu Tamas
 1  Dan Mezhiborsky
 1  Lukas Schrangl
 1  Nickolai Korshunov
 1  TZe
 1  Thad Guidry
 1  djh