SCons uses extensive automated tests to ensure quality. The primary goal is that users 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.

There are three types of SCons tests:

End-to-End Tests

End-to-end tests of SCons are Python scripts (*.py) underneath the test/ subdirectory. They use the test infrastructure modules in the testing/framework subdirectory. They build set up complete projects and call scons to execute them, checking that the behavior is as expected.

Unit Tests

Unit tests for individual SCons modules live underneath the SCons/ subdirectory and are the same base name as the module to be tested, with Tests appended to the basename. For example, the unit tests for the Builder.py module are in the BuilderTests.py script. Unit tests tend to be based on assertions.

External Tests

For the support of external Tools (in the form of packages, preferably), the testing framework is extended so it can run in standalone mode. You can start it from the top-level directory 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 directory named test, like for the SCons core.

In general, functionality with end-to-end tests should be considered a hardened part of the public interface (that is, 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.)

End-to-end tests are by their nature harder to debug. You can drop straight into the Python debugger on the unit test scripts by using the runtest.py --pdb option, but the end-to-end tests treat an SCons invocation as a "black box" and just look for external effects; simple methods like inserting print statements in the SCons code itself can disrupt those external effects. See Debugging end-to-end tests for some more thoughts.

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

   Feature.py

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

   Feature-x.py

   for the test of a specified feature using option x

3. The command line option tests take the form

   option-x.py

   for a lower-case single-letter option

   option--X.py

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

   option--lo.py

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

The standard set of SCons tests are run from the top-level source directory by the runtest.py script.

Help is available through the -h option:

$ python runtest.py -h

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

$ python runtest.py -a

By default, runtest.py 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 runtest.py SCons/BuilderTests.py
$ python runtest.py test/option-j.py test/Program.py

Folder names are allowed in the test list as well, so you can do:

$ python runtest.py test/SWIG

to run all SWIG tests only.

You can also use the -f option to execute just the tests listed in a test list file:

$ cat testlist.txt
test/option-j.py
test/Program.py
$ python runtest.py -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 runtest.py script prints a summary of how many tests passed, failed, or yielded no result, and lists any unsuccessful tests.

The above invocations all test against the scons files underneath the src/ subdirectory, and do not require that a packaging build of SCons be performed first. This is the most common mode: make some changes, and test the effects in place. The runtest.py 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, call the runtest.py script in external (stand-alone) mode:

$ python ~/scons/runtest.py -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 as each test is run, it is executed in a temporary directory created just for that test, which is by default removed when the test is complete. This ensures that your source directories don't get clobbered with temporary files and changes from the test runs. If the test itself needs to know the directory, it can be obtained as test.workdir, or more commonly by calling test.workpath(), a function which takes a path-component argument and returns the path to that path-component in the testing directory.

The use of an ephemeral test directory means that you can't simply change into a directory 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 to use fixures below.

If you simply want to check which tests would get executed, you can call the runtest.py script with the -l option combined with whichever test finding options (see below) you intend to use. Example:

$ python runtest.py -l test/scons-time

runtest.py also has a -n option, which prints the command line for each test which would have been run, but doesn't actually run them:

$ python runtest.py -n -a

When started in standard mode:

$ python runtest.py -a

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

*Test.py

for the src directory (unit tests)

*.py

for the test directory (end-to-end tests)

When using fixtures, you may end up in a situation where you have supporting Python script files in a subdirectory which shouldn't be picked up as test scripts. There are two options here:

1. Add a file with the name sconstest.skip to your subdirectory. This tells runtest.py to skip the contents of the directory completely.
2. Create a file .exclude_tests in each directory in question, and in it list line-by-line the files to exclude from testing.

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

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

#!python
import TestSCons

test = TestSCons.TestSCons()

test.write('SConstruct', """\
Program('hello.c')
""")

test.write('hello.c', """\
#include <stdio.h>

int
main(int argc, char *argv[])
{
    printf("Hello, world!\\n");
    exit (0);
}
""")

test.run()

test.run(program='./hello', stdout="Hello, world!\n")

test.pass_test()

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.
• The test does 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-quoted string for the contents of the SConstruct file (and see the next section for an alternative approach).

test.write('hello.c', ...)

This line creates an hello.c file in the temporary directory. Note that we have to escape the newline in the "Hello, world!\\n" string so that it ends up as a single backslash in the hello.c file on disk.

test.run()

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.

test.run(program='./hello', stdout="Hello, world!\n")

This shows use of the TestSCons.run() method to execute a program other than scons, in this case the hello program we just built. The stdout= keyword argument also tells the TestSCons.run() method to fail if the program output does not match the expected string "Hello, world!\n". Like the previous test.run() line, it will also fail the test if the exit status is non-zero, or there is any error output.

test.pass_test()

This is always the last line in a test script. If we get to this line, it means we haven't bailed out on a failure or skip, so the result was good. 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.

In the simple example above, the files to set up the test are created on the fly by the test program. We give a filename to the TestSCons.write() method, and a string holding its contents, and it gets written to the test directory right before starting..

This simple technique can be seen throughout most of the end-to-end tests as it was the original technique provided to test developers, but it is no longer the preferred way to write a new test. To develop this way, you first need to create the necessary files and get them to work, then convert them to an embedded string form, which may involve lots of extra escaping. These embedded files are then tricky to maintain. As a test grows multiple steps, it becomes less easy to read, since many if the embedded strings aren't quite the final files, and the volume of test code obscures the flow of the testing steps. Additionally, as SCons moves more to the use of automated code checkers and formatters to detect problems and keep a standard coding style for better readability, note that such tools don't look inside strings for code, so the effect is lost on them.

In testing parlance, a fixture is a repeatable test setup. The SCons test harness allows the use of saved files or directories to be used in that sense: "the fixture for this test is foo", instead of writing a whole bunch of strings to create files. Since these setups can be reusable across multiple tests, the fixture terminology applies well.

Note: fixtures must not be treated by SCons as runnable tests. To exclude them, see instructions in the above section named "Finding Tests".

The test harness method dir_fixture(srcdir, [dstdir]) copies the contents of the specified directory 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 into a path first. The optional dstdir is used as a destination path under the temporary working directory. distdir is created automatically, if it does not already exist.

If srcdir represents an absolute path, it is used as-is. Otherwise, if the harness was invoked with the environment variable FIXTURE_DIRS set (which runtest.py does by default), the test instance will present that list of directories to search as self.fixture_dirs, each of these are additionally searched for a directory with the name of srcdir.

A short syntax example:

test = TestSCons.TestSCons()
test.dir_fixture('image')
test.run()

would copy all files and subdirectories from the local image directory to the temporary directory for the current test, then run it.

To see a real example for this in action, refer to the test named test/packaging/convenience-functions/convenience-functions.py.

The method file_fixture(srcfile, [dstfile]) copies the file srcfile from the directory of the called script to the temporary test directory. The optional dstfile is used as a destination file name under the temporary working directory, unless it is an absolute path name. If dstfile includes directory elements, they are created automatically if they don't already exist. The srcfile and dstfile parameters may each be a list, which will be concatenated into a path.

If srcfile represents an absolute path, it is used as-is. Otherwise, any passed in fixture directories are used as additional places to search for the fixture file, as for the dir_fixture case.

With the following code:

test = TestSCons.TestSCons()
test.file_fixture('SConstruct')
test.file_fixture(['src', 'main.cpp'], ['src', 'main.cpp'])
test.run()

The files SConstruct and src/main.cpp are copied to the temporary test directory. Notice the second file_fixture call preserves the path of the original, otherwise main.cpp would have been placed in the top level of the test directory.

Again, a reference example can be found in the current revision of SCons, see test/packaging/sandbox-test/sandbox-test.py.

For even more examples you should check out one of the external Tools, e.g. the Qt4 Tool at https://bitbucket.org/dirkbaechle/scons_qt4. Also visit the SCons Tools Index at https://github.com/SCons/scons/wiki/ToolsIndex for a complete list of available Tools, though not all may have tests yet.

Tests using the inline TestSCons.write() method can fairly easily be converted to the fixture based approach. For this, we need to get at the files as they are written to each temporary test directory, which we can do by taking advantage of an existing debugging aid, namely that runtest.py checks for the existence of an environment variable named PRESERVE. If it is set to a non-zero value, the testing framework preserves the test directory instead of deleting it, and prints a message about its name to the screen.

So, you should be able to give the commands:

$ PRESERVE=1 python runtest.py test/packaging/sandbox-test.py

assuming Linux and a bash-like shell. For a Windows cmd shell, use set PRESERVE=1 (that will leave it set for the duration of the cmd session, unless manually cleared).

The output will then look something like this:

1/1 (100.00%) /usr/bin/python test/packaging/sandbox-test.py
PASSED
preserved directory /tmp/testcmd.4060.twlYNI

You can now copy the files from that directory to your new fixture directory. Then, in the test script you simply remove all the tedious TestSCons.write() statements and replace them with a single TestSCons.dir_fixture() call.

For more complex testing scenarios you can use file_fixture with the optional second argument (or the keyword arg dstfile) to assign a name to the file being copied. For example, some tests need to write multiple SConstruct files across the full run. These files can be given different names in the source (perhaps using a sufffix to distinguish them), and then be sucessively copied to the final name as needed:

test.file_fixture('fixture/SConstruct.part1', 'SConstruct')
# more setup, then run test
test.file_fixture('fixture/SConstruct.part2', 'SConstruct')
# run new test

Note that some files are not appropriate for use in a fixture as-is: fixture files should be static. If the creation of the file involves interpolating data discovered during the run of the test script, that process should stay in the script. Here is an example of this kind of usage that does not lend itself to a fixture:

import TestSCons
_python_ = TestSCons._python_

test.write('SConstruct', """
cc = Environment().Dictionary('CC')
env = Environment(
    LINK=r'%(_python_)s mylink.py',
    LINKFLAGS=[],
    CC=r'%(_python_)s mycc.py',
    CXX=cc,
    CXXFLAGS=[],
)
env.Program(target='test1', source='test1.c')
""" % locals())

Here the value of _python_ is picked out of the script's locals dictionary - which works because we've set it above -and interpolated using a mapping key into the string that will be written to SConstruct. A fixture would be hard to use here because we don't know the value of _python_ until runtime.

The other files created in this test may still be candidates for use as fixture files, however.

Most of the end to end tests have expectations for standard output and error embedded in the tests. The expectation could be either that there is nothing on that stream, or that it will contain very specific text which the test matches against. So adding print() calls, or sys.stderr.write() or similar will emit data that the tests do not expect, and thus cause further failures - possibly even obscuring the original error. Say you have three different tests in a script, and the third one is unexpectedly failing. You add some debug prints to the part of scons that is involved, and now the first test of the three starts failing, aborting the test run before it gets to the third test you were trying to debug.

Still, there are some techniques to help debugging.

The first step should be to run the tests so the harness emits more information, without forcing more information into the test stdout/stderr which will confuse result evaluation. runtest.py has several verbose levels which can be used for this purpose:

$ python runtest.py --verbose=2 test/foo.py

You can also 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.

If you do need to add informational messages in scons code to debug a problem, you can use logging and send the messages to a file instead, so they don't interrupt the test expectations.

Part of the technique discussed in the section How to Convert Old Tests to Use Fixures can also be helpful for debugging purposes. If you have a failing test, try:

$ PRESERVE=1 python runtest.py test/failing-test.py

You can now go to the save directory reported from this run and invoke the test manually to see what it is doing, without the presence of the test infrastructure which would otherwise "swallow" output you may be interested in. In this case, adding debug prints may be more useful.

The main test API is defined in the TestSCons class. TestSCons is a subclass of TestCommon, which is a subclass of TestCmd. All those classes are defined in Python files of the same name in testing/framework. Start in testing/framework/TestCmd.py for the base API definitions, like how to create files (test.write()) and run commands (test.run()).

Use TestSCons for the end-to-end tests in test, but use TestCmd for the unit tests in the src directory.

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:

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

or:

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

For many tests, if the tool being tested is backed by an external program which is not installed on the machine under test, it may not be worth proceeding with the test. For example, it's hard to test complilng code with a C compiler if no C compiler exists. In this case, the test should be skipped.

Here's a simple example for end-to-end tests:

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

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

The where_is() method can be used to look for programs that are do not have tool specifications. The existing test code will have many samples of using either or both of these to detect if it is worth even proceeding with a test.

For the unit tests, there are decorators for conditional skipping and other actions that will produce the correct output display and statistics in abnormal situations.

@unittest.skip(reason)

Unconditionally skip the decorated test. reason should describe why the test is being skipped.

@unittest.skipIf(condition, reason)

Skip the decorated test if condition is true.

@unittest.skipUnless(condition, reason)

Skip the decorated test unless condition is true.

@unittest.expectedFailure

Mark the test as an expected failure. If the test fails it will be considered a success. If the test passes, it will be considered a failure.

You can also directly call testcase.skipTest(reason).

Note that it is usually possible to test at least part of the operation of a tool without the underlying program. Tools are responsible for setting up construction variables and having the right builders, scanners and emitters plumbed into the environment. These things can be tested by mocking the behavior of the executable. Many examples of this can be found in the test directory. See for example test/subdivide.py.

This leads to a suggestion for E2E test organization because the framework doesn't have a way to indicate a partial skip - if you executed 200 lines of test, then found a condition which caused you to skip the last 20 lines, the whole test is marked as a skip; it also doesn't have a way to indicate a partial pass. To improve on this, keep tool tests which don't need the underlying program in separate files from ones which do -that way one can see in the test results that the "plumbing" tests worked even if the the ones using the underlying program maybe were skipped.