Default “scons -c” targets

As I mentioned in a previous post, the automatic “clean” target provided by SCons (scons -c) is very useful for cleaning out build files, without requiring much in the way of configuration. Anything that SCons generates when you run scons will be automatically cleaned when you run scons -c.

While useful, I’d like more control over the behavior of scons -c. Specifically, being a good TDD junkie, I have several test cases that I can run using scons test:

build_test = env.Program( ... )
env.Alias("build-tests", build_test)

run_test = env.Alias("test", [build_test],
                     ["@%s" % build_test[0].abspath])
env.AlwaysBuild(run_test)

By setting it up this way, the test programs aren’t built by default: you have to explicitly run scons build-tests (if you want to build the tests but not run them) or scons test (if you want to build and run them). Moreover, because of SCons’s dependency tracking, I can just use scons test as my usual build command during my Edit-Test-Debug loop. SCons will automatically rebuild any changed source files before running the tests.

All of this is great. So what’s the problem? As I mentioned above, scons -c only cleans the build files that are created by scons — and since I’ve explicitly set things up so that tests aren’t built by default, they’ll also not be cleaned by default. This means that to fully clean out my build targets, I have to run two commands:

$ scons -c
$ scons -c build-tests

Not ideal! I’d prefer if scons -c cleaned everything, just like make clean would in the Automake world.

The solution

So how to fix this? First we need to understand how SCons decides what to clean when you run scons -c. The answer is “exactly what’s built by scons”. And how does SCons decide what to build when you run scons? That’s what the Default command is for.

For instance, I could add

env.Default("build-tests")

to my SConstruct file. This would cause all of my tests to be built by default, and by extension, to have them all cleaned by default, as well.

This is close, since scons -c now does what we want, but this means that scons is now building more than we would like. What we need is a way to have a different list of default targets depending on whether we’re building or cleaning. Luckily, the GetOption function gives us exactly that:

if GetOption("clean"):
    env.Default("build-tests")

With this in our SConstruct file, the tests will be considered a default target when we’re cleaning, but not when we’re building. So now we have what we want: scons builds just the code, scons test builds and runs the tests, and scons -c cleans it all.