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SCons - a software construction tool

Welcome to the SCons development tree. The real purpose of this tree is to package SCons for production distribution in a variety of formats, not just to hack SCons code.

If all you want to do is install and run SCons, it will be easier for you to download and install the scons-{version}.tar.gz or scons-{version}.zip package rather than to work with the packaging logic in this tree.

To the extent that this tree is about building SCons packages, the full development cycle is not just to test the code directly, but to package SCons, unpack the package, "install" SCons in a test subdirectory, and then to run the tests against the unpacked and installed software. This helps eliminate problems caused by, for example, failure to update the list of files to be packaged.

For just working on making an individual change to the SCons source, however, you don't actually need to build or install SCons; you can actually edit and execute SCons in-place. See the following sections below for more information:

Making Changes
How to edit and execute SCons in-place.
Tips for debugging problems in SCons.
How to use the automated regression tests.
Development Workflow
An example of how to put the edit/execute/test pieces together in a reasonable development workflow.

Latest Version

Before going further, you can check that the package you have is the latest version at the SCons download page:

Execution Requirements

Running SCons requires Python 3.5 or higher. There should be no other dependencies or requirements to run scons.

As of SCons 4.2.0 support for Python 3.5 is deprecated and will be removed with the next major release.

The default SCons configuration assumes use of the Microsoft Visual C++ compiler suite on Win32 systems, and assumes a C compiler named 'cc', a C++ compiler named 'c++', and a Fortran compiler named 'gfortran' (such as found in the GNU C compiler suite) on any other type of system. You may, of course, override these default values by appropriate configuration of Environment construction variables.

By default, SCons knows how to search for available programming tools on various systems--see the SCons man page for details. You may, of course, override the default SCons choices made by appropriate configuration of Environment construction variables.

Installation Requirements

Nothing special.

Executing SCons Without Installing

You can execute the local SCons directly from the SCons subdirectory by first setting the SCONS_LIB_DIR environment variable to the local SCons subdirectory, and then executing the local scripts/ script to populate the build/scons/ subdirectory. You would do this as follows on a Linux or UNIX system (using sh or a derivative like bash or ksh):

$ setenv MYSCONS=`pwd`
$ python $MYSCONS/scripts/ [arguments]

Or on Windows:

C:\scons>set MYSCONS=%cd%
C:\scons>python %MYSCONS%\scripts\ [arguments]

An alternative approach is to skip the above and use:

$ python scripts/ [arguments]

You can use the -C option to have SCons change directory to another location where you already have a build configuration set up:

$ python scripts/ -C /some/other/location [arguments]

For simplicity in the following examples, we will only show the approach.


Note: You don't need to build SCons packages or install SCons if you just want to work on developing a patch. See the sections about Making Changes and Testing below if you just want to submit a bug fix or some new functionality.

Assuming your system satisfies the installation requirements in the previous section, install SCons from this package by first populating the build/scons/ subdirectory. (For an easier way to install SCons, without having to populate this directory, use the scons-{version}.tar.gz or scons-{version}.zip package.)

Install the built SCons files

Any of the above commands will populate the build/scons/ directory with the necessary files and directory structure to use the Python-standard setup script as follows on Linux or UNIX:

# python install

Or on Windows:

C:\scons>python install

By default, the above commands will do the following:

  • Install scripts named "scons" and "sconsign" scripts in the default system script directory (/usr/bin or C:\Python*\Scripts, for example).
  • Install "scons-3.1.2.exe" and "scons.exe" executables in the Python prefix directory on Windows (C:\Python*, for example).
  • Install the SCons build engine (a Python module) in the standard Python library directory (/usr/lib/python*/site-packages or C:\Python*\Lib\site-packages).

Making Changes

Because SCons is implemented in a scripting language, you don't need to build it in order to make changes and test them.

Virtually all of the SCons functionality exists in the "build engine," the SCons subdirectory hierarchy that contains all of the modules that make up SCons. The scripts/ wrapper script exists mainly to find the appropriate build engine library and then execute it.

In order to make your own changes locally and test them by hand, simply edit modules in the local SCons subdirectory tree and then running (see the section above about Executing SCons Without Installing):

$ python scripts/ [arguments]

If you want to be able to just execute your modified version of SCons from the command line, you can make it executable and add its directory to your $PATH like so:

$ chmod 755 scripts/
$ export PATH=$PATH:`pwd`/scripts

You should then be able to run this version of SCons by just typing "" at your UNIX or Linux command line.

Note that the regular SCons development process makes heavy use of automated testing. See the Testing and Development Workflow sections below for more information about the automated regression tests and how they can be used in a development cycle to validate that your changes don't break existing functionality.


Python comes with a good interactive debugger. When debugging changes by hand (i.e., when not using the automated tests), you can invoke SCons under control of the Python debugger by specifying the --debug=pdb option:

$ scons --debug=pdb [arguments]
> /home/knight/scons/SCons/Script/
-> default_warnings = [ SCons.Warnings.CorruptSConsignWarning,

Once in the debugger, you can set breakpoints at lines in files in the build engine modules by providing the path name of the file relative to the top directory (that is, including the SCons/ as the first directory component):

(Pdb) b SCons/Tool/

The debugger also supports single stepping, stepping into functions, printing variables, etc.

Trying to debug problems found by running the automated tests (see the Testing section, below) is more difficult, because the test automation harness re-invokes SCons and captures output. Consequently, there isn't an easy way to invoke the Python debugger in a useful way on any particular SCons call within a test script.

The most effective technique for debugging problems that occur during an automated test is to use the good old tried-and-true technique of adding statements to print tracing information. But note that you can't just use the "print" function, or even "sys.stdout.write()" because those change the SCons output, and the automated tests usually look for matches of specific output strings to decide if a given SCons invocation passes the test - so these additions may cause apparent failures different than the one you are trying to debug.

To deal with this, SCons supports a Trace() function that (by default) will print messages to your console screen ("/dev/tty" on UNIX or Linux, "con" on Windows). By adding Trace() calls to the SCons source code:

def sample_method(self, value):
    from SCons.Debug import Trace
    Trace('called sample_method(%s, %s)\n' % (self, value))

You can then run automated tests that print any arbitrary information you wish about what's going on inside SCons, without interfering with the test automation.

The Trace() function can also redirect its output to a file, rather than the screen:

def sample_method(self, value):
    from SCons.Debug import Trace
    Trace('called sample_method(%s, %s)\n' % (self, value),

Where the Trace() function sends its output is stateful: once you use the "file=" argument, all subsequent calls to Trace() send their output to the same file, until another call with a "file=" argument is reached.


Tests are run by the script in this directory.

There are two types of tests in this package:

  1. Unit tests for individual SCons modules live underneath the SCons subdirectory and have the same base name as the module with "" appended--for example, the unit test for the module is the script.
  2. End-to-end tests of SCons live in the test/ subdirectory.

You may specifically list one or more tests to be run:

$ python SCons/

$ python test/ test/

You also use the -f option to execute just the tests listed in a specified text file:

$ cat testlist.txt
$ python -f testlist.txt

One test must be listed per line, and any lines that begin with '#' will be ignored (allowing you, for example, to comment out tests that are currently passing and then uncomment all of the tests in the file for a final validation run).

The script also takes a -a option that searches the tree for all of the tests and runs them:

$ python -a

If more than one test is run, the script prints a summary of how many tests passed, failed, or yielded no result, and lists any unsuccessful tests.

The above invocations all test directly the files underneath the SCons/ subdirectory, and do not require that a build be performed first.

Development Workflow

Caveat: The point of this section isn't to describe one dogmatic workflow. Just running the test suite can be time-consuming, and getting a patch to pass all of the tests can be more so. If you're genuinely blocked, it may make more sense to submit a patch with a note about which tests still fail, and how. Someone else may be able to take your "initial draft" and figure out how to improve it to fix the rest of the tests. So there's plenty of room for use of good judgement.

The various techniques described in the above sections can be combined to create simple and effective workflows that allow you to validate that patches you submit to SCons don't break existing functionality and have adequate testing, thereby increasing the speed with which they can be integrated.

For example, suppose your project's SCons configuration is blocked by an SCons bug, and you decide you want to fix it and submit the patch. Here's one possible way to go about doing that (using UNIX/Linux as the development platform, Windows users can translate as appropriate)):

  • Change to the top of your checked-out SCons tree.

  • Confirm that the bug still exists in this version of SCons by using the -C

    option to run the broken build:

    $ python scripts/ -C /home/me/broken_project .
  • Fix the bug in SCons by editing appropriate module files underneath SCons.

  • Confirm that you've fixed the bug affecting your project:

    $ python scripts/ -C /home/me/broken_project .
  • Test to see if your fix had any unintended side effects that break existing functionality:

    $ python -a -o test.log

    Be patient, there are more than 1100 test scripts in the whole suite. If you are on UNIX/Linux, you can use:

    $ python -a | tee test.log

    instead so you can monitor progress from your terminal.

    If any test scripts fail, they will be listed in a summary at the end of the log file. Some test scripts may also report NO RESULT because (for example) your local system is the wrong type or doesn't have some installed utilities necessary to run the script. In general, you can ignore the NO RESULT list, beyond having checked once that the tests that matter to your change are actually being executed on your test system!

  • Cut-and-paste the list of failed tests into a file:

    $ cat > failed.txt
  • Now debug the test failures and fix them, either by changing SCons, or by making necessary changes to the tests (if, for example, you have a strong reason to change functionality, or if you find that the bug really is in the test script itself). After each change, use the -f option to examine the effects of the change on the subset of tests that originally failed:

    $ [edit]
    $ python -f failed.txt

    Repeat this until all of the tests that originally failed now pass.

  • Now you need to go back and validate that any changes you made while getting the tests to pass didn't break the fix you originally put in, and didn't introduce any additional unintended side effects that broke other tests:

    $ python scripts/ -C /home/me/broken_project .
    $ python -a -o test.log

    If you find any newly-broken tests, add them to your "failed.txt" file and go back to the previous step.

Of course, the above is only one suggested workflow. In practice, there is a lot of room for judgment and experience to make things go quicker. For example, if you're making a change to just the Java support, you might start looking for regressions by just running the test/Java/*.py tests instead of running all of " -a".

Building Packages

We use SCons (version 3.1.2 or later) to build its own packages. If you already have an appropriate version of SCons installed on your system, you can build everything by simply running it:

$ scons

If you don't have SCons already installed on your system, you can use the supplied script (see the section above about Executing SCons Without Installing):

$ python scripts/ build/scons

Depending on the utilities installed on your system, any or all of the following packages will be built:


The SConstruct file is supposed to be smart enough to avoid trying to build packages for which you don't have the proper utilities installed.

If you receive a build error, please report it to the scons-devel mailing list and open a bug report on the SCons bug tracker.

Note that in addition to creating the above packages, the default build will also unpack one or more of the packages for testing.

Contents of this Package

Not guaranteed to be up-to-date (but better than nothing):

A subdirectory for benchmarking scripts, used to perform timing tests to decide what specific idioms are most efficient for various parts of the code base. We check these in so they're available in case we have to revisit any of these decisions in the future.

Miscellaneous utilities used in SCons development. Right now, some of the stuff here includes:

  • a script that runs pychecker on our source tree;
  • a script that counts source and test files and numbers of lines in each;
  • a prototype script for capturing sample SCons output in xml files;
  • a script that can profile and time a packaging build of SCons itself;
  • a copy of xml_export, which can retrieve project data from SourceForge; and
  • scripts and a Python module for translating the SCons home-brew XML documentation tags into DocBook and man page format
Obsolete packaging logic.
Files needed to construct a Debian package. The contents of this directory are dictated by the Debian Policy Manual ( The package will not be accepted into the Debian distribution unless the contents of this directory satisfy the relevant Debian policies.
SCons documentation. A variety of things here, in various stages of (in)completeness.
A copy of the copyright and terms under which SCons is distributed (the Open Source Initiative-approved MIT license).
A copy of the copyright and terms under which SCons is distributed for inclusion in the scons-local-{version} packages. This is the same as LICENSE with a preamble that specifies the licensing terms are for SCons itself, not any other package that includes SCons.
What you're looking at right now.
A README file for inclusion in the scons-local-{version} packages. Similar to this file, but stripped down and modified for people looking at including SCons in their shipped software.
Script for running SCons tests. By default, this will run a test against the code in the local SCons tree, so you don't have to do a build before testing your changes.

The file describing to SCons how to build the SCons distribution.

(It has been pointed out that it's hard to find the SCons API in this SConstruct file, and that it looks a lot more like a pure Python script than a build configuration file. That's mainly because all of the magick we have to perform to deal with all of the different packaging formats requires a lot of pure Python manipulation. In other words, don't look at this file for an example of how easy it is to use SCons to build "normal" software.)

Where the actual source code is kept, of course.
End-to-end tests of the SCons utility itself. These are separate from the individual module unit tests, which live side-by-side with the modules under SCons.
SCons testing framework.


See the RELEASE.txt file for notes about this specific release, including known problems. See the CHANGES.txt file for a list of changes since the previous release.

The doc/man/scons.1 man page is included in this package, and contains a section of small examples for getting started using SCons.

Additional documentation for SCons is available at:


SCons is distributed under the MIT license, a full copy of which is available in the LICENSE file.

Reporting Bugs

The SCons project welcomes bug reports and feature requests.

Please make sure you send email with the problem or feature request to the SCons users mailing list, which you can join via the link below:

Once you have discussed your issue on the users mailing list and the community has confirmed that it is either a new bug or a duplicate of an existing bug, then please follow the instructions the community provides to file a new bug or to add yourself to the CC list for an existing bug

You can explore the list of existing bugs, which may include workarounds for the problem you've run into on GitHub Issues:

Mailing Lists

An active mailing list for developers of SCons is available. You may send questions or comments to the list at:

You may subscribe to the developer's mailing list using form on this page:

Subscription to the developer's mailing list is by approval. In practice, no one is refused list membership, but we reserve the right to limit membership in the future and/or weed out lurkers.

There are other mailing lists available for SCons users, for notification of SCons code changes, and for notification of updated bug reports and project documents. Please see our mailing lists page for details.


If you find SCons helpful, please consider making a donation (of cash, software, or hardware) to support continued work on the project. Information is available at:


GitHub Sponsors button on

For More Information

Check the SCons web site at:

Author Info

SCons was originally written by Steven Knight, knight at baldmt dot com. Since around 2010 it has been maintained by the SCons development team, co-managed by Bill Deegan and Gary Oberbrunner, with many contributors, including but not at all limited to:

  • Chad Austin
  • Dirk Baechle
  • Charles Crain
  • William Deegan
  • Steve Leblanc
  • Rob Managan
  • Greg Noel
  • Gary Oberbrunner
  • Anthony Roach
  • Greg Spencer
  • Tom Tanner
  • Anatoly Techtonik
  • Christoph Wiedemann
  • Russel Winder
  • Mats Wichmann

... and many others.

Copyright (c) 2001 - 2021 The SCons Foundation

Source: README.rst, updated 2022-07-30