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SCons Testing Framework

SCons uses extensive automated tests to try to ensure quality. The primary goal is that users should be able to upgrade from version to version without any surprise changes in behavior.

In general, no change goes into SCons unless it has one or more new or modified tests that demonstrably exercise the bug being fixed or the feature being added. There are exceptions to this guideline, but they should be just that, ''exceptions''. When in doubt, make sure it's tested.

Test Organization

There are three types of SCons tests:

End-to-End Tests
End-to-end tests of SCons are all Python scripts (*.py) underneath the test/ subdirectory. They use the test infrastructure modules in the QMTest subdirectory.
Unit Tests
Unit tests for individual SCons modules live underneath the src/engine/ subdirectory and are the same base name as the module with appended--for example, the unit tests for the module are in the script.
External Tests
For the support of external Tools (in the form of packages, preferably), the testing framework got extended, such that it can run in standalone mode. You can start it from the top-level folder of your Tool's source tree, where it then finds all Python scripts (*.py) underneath the local test/ directory. This implies that Tool tests have to be kept in a folder named test, like for the SCons core.

Contrasting End-to-End and Unit Tests

In general, anything that we've put into an end-to-end test script should be considered a hardened part of the interface (that is, it's something that a user might do) and should not be broken. Unit tests are now considered more malleable, more for testing internal interfaces that can change so long as we don't break users' SConscript files. (This wasn't always the case, and there's a lot of meaty code in many of the unit test scripts that does, in fact, capture external interface behavior. In general, we should try to move those things to end-to-end scripts as we find them.)

It's more difficult to debug end-to-end tests. You can actually go straight into the Python debugger on the unit test scripts by using the --pdb option, but the end-to-end tests treat an SCons invocation as a "black box" and just look for external effects. Simple print statements within the SCons code itself often don't help debug end-to-end because they end up in SCons output that gets compared against expected output and cause a test failure. Probably the most effective technique is to use the internal SCons.Debug.Trace() function, which prints output to /dev/tty on Linux/UNIX systems and con on Windows systems, so you can see what's going on.

Naming conventions

The end-to-end tests, more or less, stick to the following naming conventions:

  1. All tests end with a .py suffix.

  2. In the General form we use

    for the test of a specified feature; try to keep this description reasonably short

    for the test of a specified feature using option x

  3. The command line option tests take the form

    for a lower-case single-letter option

    upper-case single-letter option (with an extra hyphen, so the file names will be unique on case-insensitive systems)

    long option; abbreviate the long option name to a few characters

Running Tests

The standard set of SCons tests are run from the top-level source directory by the script. There is a --qmtest option that checks whether the QMTest package is installed on your system. If it can be found, then the script will use it to carry out the tests.

Help is available through the -h option:

$ python -h

To simply run all the tests, use the -a option:

$ python -a

By default, prints a count and percentage message for each test case, along with the name of the test file. If you need the output to be more silent, have a look at the -q, -s and -k options.

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

$ python src/engine/SCons/
$ python test/ test/

Folder names are allowed arguments as well, so you can do a

$ python test/SWIG

to run all SWIG tests only.

You can 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 (the intent being to allow 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).

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 src/ subdirectory, and do not require that a packaging build be performed first. The script supports additional options to run tests against unpacked packages in the build/test-*/ subdirectories.

If you are testing a separate Tool outside of the SCons source tree, you have to call the script in external (stand-alone) mode:

$ python ~/scons/ -e -a

. This ensures that the testing framework doesn't try to access SCons classes needed for some of the internal test cases.

Note, that the actual tests are carried out in a temporary folder each, which gets deleted afterwards. This ensures that your source directories don't get clobbered with temporary files from the test runs. It also means that you can't simply change into a folder to "debug things" after a test has gone wrong. For a way around this, check out the PRESERVE environment variable. It can be seen in action in How to convert old tests below.

Not Running Tests

If you simply want to check which tests would get executed, you can call the script with the -l option:

$ python -l

Then there is also the -n option, which prints the command line for each single test, but doesn't actually execute them:

$ python -n

Finding Tests

When started in standard mode

$ python -a

, assumes that it is run from the SCons top-level source directory. It then dives into the src and test folders, where it tries to find filenames

for the src directory, and
for the test folder.

When using fixtures, you may quickly end up in a position where you have supporting Python script files in a subfolder, but they shouldn't get picked up as test scripts. In this case you have two options:

  1. Add a file with the name sconstest.skip to your subfolder. This lets skip the contents of the directory completely.
  2. Create a file .exclude_tests in each folder in question, and in it list line-by-line the files to get excluded from testing.

The same rules apply when testing external Tools by using the -e option.

"Hello, world!" SCons Test Script

To illustrate how the end-to-end test scripts work, let's walk through a simple "Hello, world!" example:

import TestSCons

test = TestSCons.TestSCons()

test.write('SConstruct', """\

test.write('hello.c', """\
main(int argc, char *argv[])
      printf("Hello, world!\\n");
      exit (0);
""")'./hello', stdout="Hello, world!\n")

import TestSCons
Imports the main infrastructure for writing SCons tests. This is normally the only part of the infrastructure that needs importing. Sometimes other Python modules are necessary or helpful, and get imported before this line.
test = TestSCons.TestSCons()

This initializes an object for testing. A fair amount happens under the covers when the object is created, including:

  • A temporary directory is created for all the in-line files that will get created.
  • The temporary directory's removal is arranged for when the test is finished.
  • We os.chdir() to the temporary directory.
test.write('SConstruct', ...
This line creates an SConstruct file in the temporary directory, to be used as input to the scons run(s) that we're testing. Note the use of the Python triple-quote syntax for the contents of the SConstruct file. Because input files for tests are all created from in-line data like this, the tests can sometimes get a little confusing to read, because some of the Python code is found
test.write('hello.c', ...
This lines creates an hello.c file in the temporary directory. Note that we have to escape the \\n in the "Hello, world!\\n" string so that it ends up as a single backslash in the hello.c file on disk.

This actually runs SCons. Like the object initialization, things happen under the covers:

  • The exit status is verified; the test exits with a failure if the exit status is not zero.
  • The error output is examined, and the test exits with a failure if there is any'./hello', stdout="Hello, world!\n")
This shows use of the method to execute a program other than scons, in this case the hello program we just presumably built. The stdout= keyword argument also tells the method to fail if the program output does not match the expected string "Hello, world!\n". Like the previous line, it will also fail the test if the exit status is non-zero, or there is any error output.
This is always the last line in a test script. It prints PASSED on the screen and makes sure we exit with a 0 status to indicate the test passed. As a side effect of destroying the test object, the created temporary directory will be removed.

Working with fixtures

In the simple example above, we have seen how to create files in the temporary test directory. We give a filename to the TestSCons.write() method, together with its contents, and it gets written to the test folder right before its start.

This technique can still be seen throughout most of the end-to-end tests, but there is a better way. It's much easier to edit, create and maintain real files, instead of copy/pasting content to/from a Python script. If the test files get longer, the test script gets longer and is harder to read.

Against this, we now have the possibility to copy single files or the contents of a local folder to the test directory. Since we can reuse these files/folders to setup several tests, we call them fixtures in the following.

Directory fixtures

The function dir_fixture(self, srcdir, dstdir=None) in the TestCmd class copies the contents of the specified folder srcdir from the directory of the called test script, to the current temporary test directory. The srcdir name may be a list, in which case the elements are concatenated with the os.path.join() method. The dstdir is assumed to be under the temporary working directory, it gets created automatically, if it does not already exist.

A short syntax example:

test = TestSCons.TestSCons()

would copy all files and subfolders from the local image folder, to the temporary directory for the current test.

If you'd like to see a real example for this in action, refer to the test named test/packaging/convenience-functions/

File fixtures

Like for directory fixtures, file_fixture(self, srcfile, dstfile=None) copies the file srcfile from the directory of the called script, to the temporary test directory. The dstfile is assumed to be under the temporary working directory, unless it is an absolute path name. If dstfile is specified, its target directory gets created automatically if it doesn't already exist.

With a:

test = TestSCons.TestSCons()

you would copy the files SConstruct and src/main.cpp to the temporary test folder, prior to running the test itself.

Again, a reference example can be found in the current default revision of SCons, it is test/packaging/sandbox-test/

For even more examples you should check out one of the external Tools, e.g. the Qt4 Tool at Also visit the SCons Tools Index at for a complete list of available Tools, though not all may have tests yet.

How to convert old tests

We now show how to convert a test, still using the TestSCons.write() method, to the fixture based approach. For this, we need to get at the files as they are written to each temporary test folder.

Luckily, checks for the existence of an environment variable named PRESERVE. If it is set to a non-zero value, the testing framework doesn't delete the test folder as ususal, but prints its name to the screen.

So, you should be able to give the commands

$ export PRESERVE=1
$ python test/packaging/

, assuming Linux and a bash-like shell.

The output should then look something like this:

1/1 (100.00%) /usr/bin/python -tt test/packaging/
Preserved directory /tmp/testcmd.4060.twlYNI

and you see that the test files have been kept in the folder /tmp/testcmd.4060.twlYNI, where you can now copy them from to your new fixture folder. Then, in the test script you simply remove all the tedious TestSCons.write() statements and replace them by a single TestSCons.dir_fixture().

Finally, you shouldn't forget to clean up and remove the temporary test directory. ;)

Test Infrastructure

The test API is in QMTest/ TestSCons is a subclass of TestCommon, which is a subclass of TestCmd; all those python files are in QMTest. Start in QMTest/ for the base API definitions, like how to create files (test.write()) and run commands (

You want to use TestSCons for the end-to-end tests in test, but TestCmd for the unit tests in the src folder.

The match functions work like this:

TestSCons.match_re:: match each line with a RE
  • Splits the lines into a list (unless they already are)
  • splits the REs at newlines (unless already a list) and puts ^..$ around each
  • then each RE must match each line. This means there must be as many REs as lines.
TestSCons.match_re_dotall:: match all the lines against a single RE
  • Joins the lines with newline (unless already a string)
  • joins the REs with newline (unless it's a string) and puts ^..$ around the whole thing
  • then whole thing must match with python re.DOTALL.

Use them in a test like this:, match=TestSCons.match_re, ...)


test.must_match(..., match=TestSCons.match_re, ...)

Avoiding Tests based on Tool existence

Here's an easy sample:

intelc = test.detect_tool('intelc', prog='icpc')
if not intelc:
    test.skip_test("Could not load 'intelc' Tool; skipping test(s).\n")

See QMTest/ for the detect_tool method. It calls the tool's generate() method, and then looks for the given prog (tool name by default) in env['ENV']['PATH'].