The site_scons directories give you a place to put Python modules and packages that you can import into your SConscript files (at the top level) add-on tools that can integrate into SCons (in a site_tools subdirectory), and a site_scons/site_init.py file that gets read before any SConstruct or SConscript file, allowing you to change SCons's default behavior.

Each system type (Windows, Mac, Linux, etc.) searches a canonical set of directories for site_scons; see the man page for details. The top-level SConstruct's site_scons dir (that is, the one in the project) is always searched last, and its dir is placed first in the tool path so it overrides all others.

If you get a tool from somewhere (the SCons wiki or a third party, for instance) and you'd like to use it in your project, a site_scons dir is the simplest place to put it. Tools come in two flavors; either a Python function that operates on an Environment or a Python module or package containing two functions, exists() and generate().

A single-function Tool can just be included in your site_scons/site_init.py file where it will be parsed and made available for use. For instance, you could have a site_scons/site_init.py file like this:

def TOOL_ADD_HEADER(env):
    """A Tool to add a header from $HEADER to the source file"""
    add_header = Builder(
        action=['echo "$HEADER" > $TARGET', 'cat $SOURCE >> $TARGET']
    )
    env.Append(BUILDERS={'AddHeader': add_header})
    env['HEADER'] = ''  # set default value
      

and a SConstruct like this:

# Use TOOL_ADD_HEADER from site_scons/site_init.py
env=Environment(tools=['default', TOOL_ADD_HEADER], HEADER="=====")
env.AddHeader('tgt', 'src')
    

The TOOL_ADD_HEADER tool method will be called to add the AddHeader tool to the environment.

A more full-fledged tool with exists() and generate() methods can be installed either as a module in the file site_scons/site_tools/toolname.py or as a package in the directory site_scons/site_tools/toolname. In the case of using a package, the exists() and generate() are in the file site_scons/site_tools/toolname/__init__.py. (In all the above case toolname is replaced by the name of the tool.) Since site_scons/site_tools is automatically added to the head of the tool search path, any tool found there will be available to all environments. Furthermore, a tool found there will override a built-in tool of the same name, so if you need to change the behavior of a built-in tool, site_scons gives you the hook you need.

Many people have a collection of utility Python functions they'd like to include in their SConscript files: just put them in site_scons/my_utils.py or any valid Python module name of your choice. For instance you can do something like this in site_scons/my_utils.py to add build_id and MakeWorkDir functions:

from SCons.Script import *  # for Execute and Mkdir

def build_id():
    """Return a build ID (stub version)"""
    return "100"

def MakeWorkDir(workdir):
    """Create the specified dir immediately"""
    Execute(Mkdir(workdir))
      

And then in your SConscript or any sub-SConscript anywhere in your build, you can import my_utils and use it:

import my_utils
print("build_id=" + my_utils.build_id())
my_utils.MakeWorkDir('/tmp/work')
    

You can put this collection in its own module in a site_scons and import it as in the example, or you can include it in site_scons/site_init.py, which is automatically imported (unless you disable site directories). Note that in order to refer to objects in the SCons namespace such as Environment or Mkdir or Execute in any file other than a SConstruct or SConscript you always need to do

from SCons.Script import *
    

This is true of modules in site_scons such as site_scons/site_init.py as well.

You can use any of the user- or machine-wide site directories such as ~/.scons/site_scons instead of ./site_scons, or use the --site-dir option to point to your own directory. site_init.py and site_tools will be located under that directory. To avoid using a site_scons directory at all, even if it exists, use the --no-site-dir option.