Tuesday, April 29, 2014

Writing SCons Plugin: Discovery

UP201410: Add version info and link to Parts concept.

SCons is a build tool written completely in Python, so that you can just put it into your repository, extend for your own purposes and run directly from checkout without installing anything. SCons documentation doesn't use word plugin to refer to extensions. It calls them Tools. If you're not familiar with SCons concepts, check out this wiki page.

Plugin Discovery

SCons tries its best to avoid magical behavior. That's why it ignores paths and options set in system environment and requires that you specify everything explicitly. The same is true for plugins. Example from SCons man page:
env = Environment(tools = ['default', 'foo'], toolpath = ['tooldir'])
This creates build environment, initializes default tools, sets lookup path for plugins to tooldir/ and enables tool named foo, which is located in tooldir/foo.py. foo.py should have two functions - generate(env) and exists(env).

To test that your tool is found correctly, check env['TOOLS']:
If filename is not there - SCons was unable to find it. If it was a problem with contents of foo.py - SCons would fail with exception.

SCons has an automatic tool discovery mechanism - if you don't like to specify toolpath directly - you may place your tool in one of several locations that SCons scans before executing SConstruct. See description of the --site-dir=dir option for details.

Version compatibility and future

Described behavior is true for SCons 2.3.4 and earlier versions. It may be not very intuitive, so things may change in future, especially if we find resources to integrate Parts.