DevGuide TestingMethodology - SCons/scons GitHub Wiki

SCons uses extensive automated tests to ensure quality, so users can safely upgrade from version to version without any surprise changes in behavior.

Changes to SCons code, whether fix or new feature, need testing support before being merged. Sometimes this might mean writing tests for an area that didn't have adequate, or any, testing before. Having tests isn't an iron-clad policy, but exceptions need to be okayed by the core team - it's good to discuss ahead of time. When in doubt, test it!

This guide should provide enough information to get started. Use it together with existing tests to "see how it's done". Note if you're coding in an IDE, you may need to add the path testing/framework to the known paths, or it won't recognize some symbols.

There are three categories of SCons tests:

Unit Tests

Unit tests for individual SCons modules live underneath the SCons/ subdirectory (that is alongside the code they are testing) and use the Python unittest module to perform assertion-based testing of individual routines, classes and methods in the SCons code base.

End-to-End Tests

End-to-end tests of SCons are small self-contained projects that are run by calling scons (or in a few cases, sconsign or scons-time) to execute them, and thus simulate the results of running all the way through building a project - thus "end to end". These tests need a Python script to drive them - setup, issuing the correct command line, checking and recording results; these operations are aided by using the test infrastructure modules in the testing/framework subdirectory. The scripts need a test runner to find end execute them (the runner specifically requires a .py filename suffix on these). The standard tests are located in the test/ subdirectory,

External Tests

For the support of external Tools (in the form of packages, preferably), the testing framework can also 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.

End-to-end tests exist to verify hardened parts of the public interface: features documented in the reference manual that are available to use in a project. They accomplish this by themselves being complete (though usually very small) SCons projects. Unit tests are more for testing internal interfaces which may not themselves directly have public API guarantees. They let you hone in on the point of error much more precisely. As an example, an end-to-end test verifies that variable contents added by env.Append actually appear correctly in issued command lines, while unit tests verify that internal routine SCons.Environment._add_cppdefines properly handles the complexities of combining different types of arguments and still produce the expected contents of the CPPDEFINES variable being added to by the env.Append interface..

If you're pursuing a fix and it can be tested by adding a new case to a unit test, that is often simpler and more direct. However, bug reports tend describe end-to-end type behavior ("I put this in my SConscripts, and something went wrong"), so it's often useful to code an e2e test to match a bug report, and develop a solution that turns that to a pass. So pick the testing strategy that makes sense (sometimes both do) - and if it's possible to reduce an end-to-end test to a unit test, please do

End-to-end tests are by their nature harder to debug. For the unit tests, you're running a test program directly, so you can drop straight into the Python debugger by calling runtest.py with the -d / --debug option, set some breakpoints and otherwise step through examining internal state and how it changes as the test is running. The end-to-end tests are each small SCons projects executed by an instance of SCons in a subprocess, so the Python debugger is harder to bring into play in this context. There's a separate section devoted to debugging ideas: Debugging end-to-end tests.

The unit tests have the same base name as the module to be tested, with Tests appended. For packages, it's conventional to put the test file in the package directory, and there's typically only one test file for the whole package directory, but feel free to use multiples if it makes more sense. For example, there's an SCons/Platform/PlatformTests.py which covers most of the Platform code, but also SCons/Platform/virtualenvTests.py for testing the virtualenv (sub-)module.

The end-to-end tests, more or less, follow this naming convention:

• Tests are Python scripts, so they end with a .py suffix.

• In the General form we use

  Feature.py

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

  Feature-x.py

  for the test of a specified feature using option x

  Feature-live.py

  for a test which explicitly uses an external tool to operate.

• 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; you can abbreviate the long option name to a few characters (the abbreviation must be unique, of course).

• Use a suitably named subdirectory if there's a whole group of

  related test files, this allows easier selection.

The test framework provides a lot of useful functions for use within a test program. This includes test setup, parameterization, running tests, examining results and reporting outcomes. You can run a particular unit test directly by making sure the Python interpreter can find the framework:

$ PYTHON_PATH=testing/framework python SCons/ActionTests.py

The framework does not provide facilities for test selection, or otherwise dealing with collections of tests. For that, SCons provides a runner script runtest.py. Help is available through the -h option:

$ python runtest.py -h

You run tests from the top-level source directory. To simply run all the tests, use the -a option:

$ python runtest.py -a

You may specifically list one or more tests to be run. runtest considers all arguments it doesn't recognize as options to be part of the test list:

$ python runtest.py SCons/BuilderTests.py
$ python runtest.py -t test/option/option-j.py test/option/option-p.py

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

$ python runtest.py test/SWIG

to run all SWIG tests (and no others).

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

$ cat testlist.txt
test/option/option-j.py
test/option/option-p.py
$ python runtest.py -f testlist.txt

List one test file per line. Lines that begin with the comment mark # will be ignored (this lets you quickly change the test list by commenting out a few tests in the testlist file).

If more than one test is run, the runtest.py script prints a summary and count of tests that failed or yielded no result (skips). Skipped tests do not count towards considering the overall run to have failed, unless the --no-ignore-skips option is used. Passed tests can be listed using the --passed option, though this tends to make the result section at the end quite noisy, which is why it's off by default. Also by default, runtest.py prints a running count and completion percentage message for each test case as it finishes, along with the name of the test file. You can quiet this output: have a look at the -q, -s and -k options.

Since a test run can produce a lot of output that you may want to examine later, there is an option -o FILE to save the same output that went to the screen to a file named by FILE. There is also an option to save the results in a custom XML format.

The above invocations all test against the SCons files in the current directory (that is, in ./SCons), 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 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 fixtures 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 selection 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 SCons and test directories, where it tries to find filenames

*Test.py

for the SCons 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 of their own. There are two options here:

• Add a file with the name sconstest.skip to your subdirectory. This tells runtest.py to skip the contents of the directory completely.
• Create a file .exclude_tests in your subdirectory, and in it list line-by-line the files to exclude from testing - the rest will still be picked up as long as they meet the selection criteria.

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 SCons tests. This is normally the only part of the infrastructure that needs importing. If you need Python standard library modules in your code, the convention it to import those before the framework.

test = TestSCons.TestSCons()

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, plus 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 for 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 escaping, using raw strings, and other fiddly details. These embedded files are then tricky to maintain - they're not recognized as code by editors, static checkers, or formatters. Readability is further hurt if the test script grows large - lots of files-in-strings obscure the flow of the actual testing logic.

In testing parlance, a fixture is a repeatable test setup. The SCons test harness allows the use of saved files as well as collections of files in named directories to be used in that sense: the fixture for this test is foo. Since these setups can be reused 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 Selecting tests.

The test 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 optional dstdir is used as a destination path under the temporary working directory. dstdir is created automatically if it does not already exist. The srcdir and dstdir parameters may each be a list, which will be concatenated into a path string.

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 given by srcdir.

A short example showing the syntax:

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

This copies all files and subdirectories from the local fixture directory, or if not found, from a fixture located in one of the fixture dirs, to the temporary directory for the current test, before running the test.

To see an example in action, refer to the test named test/packaging/convenience-functions/convenience-functions.py.

The test 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 string.

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.

As an example, 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 Qt5 Tool at https://github.com/SCons/scons-contrib/tree/master/sconscontrib/SCons/Tool/qt5. There are many other tools in the contrib repository, and you can 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 via a trick: you can capture the test directory as it is created, which will contain the files in their final form. To do this, set the environment variable PRESERVE to a non-zero value when calling runtest.py to run the test, and it will preserve the directory rather than getting rid of it, and report the path. A thing to keep in mind is some tests rewrite files while running - for example some tests create an SConstruct`, then write a new one for another part of the test, then possibly do so again - "preserving" will only keep the state of the test as it exits. For this and debugging reasons, it is preferred not to have tests replace the contents of key files during a run.

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 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 suffix to distinguish them), and then be successively 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

As mentioned earlier, this isn't really ideal and it's preferred in such cases to keep the separate names in the test directory, and instead vary how the tests are executed, like:

test.run(arguments=['-f', 'SConstruct.part1'])
test.run(arguments=['-f', 'SConstruct.part2'])

Static test files are well suited to fixtures, you just copy them over. Files with dynamically created content - usually to interpolate information discovered during test setup, are more problematic. Here's an example of a rather common pattern:

import TestSCons
_python_ = TestSCons._python_

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

Here the value of _python_ (the path to the Python executable actually in use for the test) is obtained by the test program from the framework, and pasted in via f-string formatting in setting up the string that will make up the contents of the SConstruct. A simple fixture isn't useful here because the value of _python_ isn't known until runtime (also note that as it will be an absolute pathname, it is entered using Python raw string notation to avoid interpretation problems on Windows, where the path separator is a backslash).

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

There's another approach that can be used in this case, letting you still use a fixture file: instead of using string interpolation at setup time, consider passing values at run-time via command-line arguments. In the example above, you can replace the substitution of _python_ at file-writing time with a check for a variable from the command line, and substitute that at run-time, so instead of the above sequence in the test script, put this in a new file fixture/SConstruct:

python = ARGUMENTS.get('PYTHON', 'python')
cc = Environment().Dictionary('CC')
env = Environment(
    LINK=rf'{python} mylink.py',
    LINKFLAGS=[],
    CC=rf'{python} mycc.py',
    CXX=cc,
    CXXFLAGS=[],
)
env.Program(target='test1', source='test1.c')

Read this in as a file fixture:

test.file_fixture(srcfile='fixture/SConstruct')

For this to work, you have to supply PYTHON in the argument list, so it appears in ARGUMENTS at run-time:

test.run(arguments=rf'PYTHON={_python_}')

An end-to-end tests is a hand-crafted SCons project, so testing involves building (or cleaning) that project with suitable arguments to control the behavior. The tests treat the SCons invocation as a black box, usually looking for external effects of the test - targets are created, generated files have expected contents, files are properly removed on clean, etc. They often also look for the flow of messages from SCons, which is unfortunately a bit fragile (many a test has been broken by a new Python version changing the precise format of an exception message, for example). Some tests do have test code inside the generated files, and based on the result emit special known strings that the test can look for.

Simple tricks like inserting print statements in the SCons code itself don't really help as they end up disrupting expected output. For example, test.run(stdout="Some text") expects a simple string on the standard output stream, but the presence of a print in the code means that appears in the output, too, and the string matching will fail the test.

Even more irritatingly, added text can cause other tests to fail and obscure the error you're looking for. Say you have three different tests in a script exercising different code paths for the same feature, and the third one is unexpectedly failing. You add some debug prints to the affected part of SCons, and now the first test of the three starts failing, aborting the test run before it even gets to the third test - the one 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 levels of verbosity 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, but do not contribute to the captured stdout/stderr and mess up the test expectations.

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. Or write directly to a trace file of your choosing.

Part of the technique discussed in the section How to convert old tests to use fixtures 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 scons manually (with appropriate arguments matching what the test did) to see the results without the presence of the test infrastructure which would otherwise consume output you may be interested in. In this case, adding debug prints may be more useful.

There are related variables PRESERVE_PASS, PRESERVE_FAIL and PRESERVE_NORESULT that preserve the directory only if the test result was the indicated one, which is helpful if you're trying to work with multiple tests showing an unusual result.

From a Windows cmd shell, you will have to set the environment variable first, it doesn't work on a single line like the example above for POSIX-style shells.

The main end-to-end test API is defined in the TestSCons class. TestSCons is a subclass of TestCommon, which is a subclass of TestCmd. TestCmd provides facilities for generically "running a command". TestCommon wraps this with features for result and error handling. TestSCons specializes that into support for specifically running the command scons (there are related classes TestSConsign for runing sconsign and TestSCons_time for running timing tests using bin/scons_time.py. 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()).

The unit tests do not run a separate instance of SCons, but instead import the SCons module that they intend to test. Those tests can usually use the TestCmd class for testing infrastructure (temporary directory, file creation, etc.), while the test classes themselves normally derive from unittest.TestCase.

The match functions work like this:

TestSCons.match_re

match each line with a regular expression.

• 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 regular expression.

• 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, ...)

Often you want to supply arguments to SCons when it is invoked to run a test, which you can do using an arguments parameter:

test.run(arguments="-O -v FOO=BAR")

One caveat here: the way the parameter is processed is unavoidably different from typing on the command line - if you need it not to be split on spaces, pre-split it yourself, and pass the list, like:

test.run(arguments=["-f", "SConstruct2", "FOO=Two Words"])

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 compiling 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'].

test.where_is() can be used to look for programs that do not have tool specifications (or you just don't want to involve a specific tool). 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.

There's an additional consideration for e2e tests: when a project developer needs a tool that requires some unique setup (in particular, the path to find an external executable), they can just adjust their build to make it work in their environment. It's not practical to change a bunch of tests in the test suite to do a similar thing. Calling test.where_is() might return a positive response based on searching the shell's PATH environment variable (which it checks if no specific paths to search are given), but that does not guarantee the copy of SCons launched to run the actual testcase will find it, so it may be necessary to pass the path to the test, perhaps via an argument to test.run().

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.

We know that needing to write tests makes the job of contributing code to SCons more cumbersome. But as noted in the introduction, the testing strategy is extremely important to SCons, it has allowed the project to serve many users with very few nasty surprises (won't lie and say there has never been a surprise!) for over two decades. We suggest thinking in terms of test-driven development for your contribution: you'll need something to show that your change actually works anyway. If it's a bug report, this may be the minimum viable reproducer; if it's a new feature, you still want to show how something that couldn't be done before can now be done. Code that up first, and document your expectations (for yourself as much as anyone), and use it when developing the fix/feature. Often that code can be converted into a test that will fit into the SCons testuite without doing too much extra work. If that work looks too daunting, please ask for help - tips, advice, and coding help may all be available.

• DO group tests by topic area. This makes selection easier, for example the tests specific to the yacc tool can be run using runtest.py test/YACC
• DO keep tests simple. Some tests are by their nature complex, but narrowing in on a specific feature makes for easier debugging - more simpler test files is easier on future maintainers than a huge compilcated one.
• DON'T gang too many things together in one file (related to the previous item). It's cleaner if they're split into different files unless they share complex infrastructure. This helps avoid the problem of "fail fast": a test aborts when it detects a failure condition, and the other tests in the same file don't ever run, which may keep you from seeing a pattern exposed when several related tests all fail.
• DON'T require the use of an external executable "unless necessary". Usually the SCons behavior is the thing we want to test, not the behavior of the external tool. Unless necessary is intentionally vague, use your judgement. If it's a ton of work to mock an executable's behavior, perhaps in the combinations of different flags, don't. However, if you don't actually need the output (files or stderr/stdout) of an external, try to avoid.
• DO be prepared to skip a test using an external tool if it is unavailable. We want the tests to be runnable in many configurations, and not produce tons of fails jut because that configuration didn't install some things. Yes, we know tons of things will fail if you don't have a findable C compiler - sorry!
• DON'T combine tests that need an external tool with ones that do not, split them into separate test files. e2e tests can't do a partial skip, so if you successfully complete seven of eight tests, and then come to a conditional "skip if tool missing" or "skip if on Windows", then the whole test file ends up marked as a skip. On the other side, if you have a platform or tool-specific condition that does not issue a skip_test, then part of your test may be skipped and you'll see no indication of that in the output log. Splitting gives you a more complete picture.
• DO leave hints when a test requires external executables. The current convention is to use the word "live" in the test name, either as an ending ( (e.g. test/AS/as-live.py) or use it as the entire name of the test (e.g. test/SWIG/live.py).
• DO use test fixtures where it makes sense. Real files are easier to read than strings embedded in a test script used to create those files - not just by humans, but by editors, checkers and formatters. And in particular, try to make use of shareable mocked tools, which, by getting lots of use, will be better debugged than single-use ones (e.g., try to avoid each Fortran test containing its own mock compiler myfortan.py - all those copies will have to be maintained).

• DO test at an appropriate level. A "unit" of SCons behavior is something with predictable outcomes, which has multiple consumers. A convenience function used by only one other function may not need its own tests, as long as the caller is suitably tested.

• DO keep tests independent. This is just standard testing practice - a test in one function should not depend on the results of an earlier test in the same function. Make sure they have independent setup, either by repeating the setup, or splitting into a separate function.

• DO consider whether "fail fast" is appropriate. Tests within a test function can be made independent by using the unittest module's subTest method - if one subtest fails, results will be collected and execution continues, which may be more helpful in some cases. This is a comparatively recent addition to unittest (Python 3.4), so much of SCons' body of unit tests was written without the advantage of that feature.

• DO make use of helpful unittest features. In particular, using basic assert statements leaves you responsible for the output if the test fails. Even in simple cases this tends to look awkward. The various assert methods, on the other hand, provide decent formatting of output on failure, often showing where two complex elements differ, and you only need to add something for output if it needs specialization. Compare:

  assert out, "expected string", out
  self.assertEqual(out, "expected string")
  

  There is an assert method for checking that an exception happens (self.assertRaises), which is more readable than hand-coding something with a try block to check the exception was raised. Please use this!