Guide to Testing Bazaar


Testing Bazaar

The Importance of Testing

Reliability is a critical success factor for any Version Control System. We want Bazaar to be highly reliable across multiple platforms while evolving over time to meet the needs of its community.

In a nutshell, this is what we expect and encourage:

  • New functionality should have test cases. Preferably write the test before writing the code.

    In general, you can test at either the command-line level or the internal API level. See Writing tests below for more detail.

  • Try to practice Test-Driven Development: before fixing a bug, write a test case so that it does not regress. Similarly for adding a new feature: write a test case for a small version of the new feature before starting on the code itself. Check the test fails on the old code, then add the feature or fix and check it passes.

By doing these things, the Bazaar team gets increased confidence that changes do what they claim to do, whether provided by the core team or by community members. Equally importantly, we can be surer that changes down the track do not break new features or bug fixes that you are contributing today.

As of May 2008, Bazaar ships with a test suite containing over 12000 tests and growing. We are proud of it and want to remain so. As community members, we all benefit from it. Would you trust version control on your project to a product without a test suite like Bazaar has?

Running the Test Suite

Currently, bzr selftest is used to invoke tests. You can provide a pattern argument to run a subset. For example, to run just the blackbox tests, run:

./bzr selftest -v blackbox

To skip a particular test (or set of tests), use the --exclude option (shorthand -x) like so:

./bzr selftest -v -x blackbox

To ensure that all tests are being run and succeeding, you can use the --strict option which will fail if there are any missing features or known failures, like so:

./bzr selftest --strict

To list tests without running them, use the --list-only option like so:

./bzr selftest --list-only

This option can be combined with other selftest options (like -x) and filter patterns to understand their effect.

Once you understand how to create a list of tests, you can use the --load-list option to run only a restricted set of tests that you kept in a file, one test id by line. Keep in mind that this will never be sufficient to validate your modifications, you still need to run the full test suite for that, but using it can help in some cases (like running only the failed tests for some time):

./bzr selftest -- load-list my_failing_tests

This option can also be combined with other selftest options, including patterns. It has some drawbacks though, the list can become out of date pretty quick when doing Test Driven Development.

To address this concern, there is another way to run a restricted set of tests: the --starting-with option will run only the tests whose name starts with the specified string. It will also avoid loading the other tests and as a consequence starts running your tests quicker:

./bzr selftest --starting-with bzrlib.blackbox

This option can be combined with all the other selftest options including --load-list. The later is rarely used but allows to run a subset of a list of failing tests for example.

Test suite debug flags

Similar to the global -Dfoo debug options, bzr selftest accepts -E=foo debug flags. These flags are:

allow_debug:do not clear the global debug flags when running a test. This can provide useful logging to help debug test failures when used with e.g. bzr -Dhpss selftest -E=allow_debug

Writing Tests

Where should I put a new test?

Bzrlib's tests are organised by the type of test. Most of the tests in bzr's test suite belong to one of these categories:

  • Unit tests
  • Blackbox (UI) tests
  • Per-implementation tests
  • Doctests

A quick description of these test types and where they belong in bzrlib's source follows. Not all tests fall neatly into one of these categories; in those cases use your judgement.

Unit tests

Unit tests make up the bulk of our test suite. These are tests that are focused on exercising a single, specific unit of the code as directly as possible. Each unit test is generally fairly short and runs very quickly.

They are found in bzrlib/tests/test_*.py. So in general tests should be placed in a file named where FOO is the logical thing under test.

For example, tests for merge3 in bzrlib belong in bzrlib/tests/ See bzrlib/tests/ for a template test script.

Blackbox (UI) tests

Tests can be written for the UI or for individual areas of the library. Choose whichever is appropriate: if adding a new command, or a new command option, then you should be writing a UI test. If you are both adding UI functionality and library functionality, you will want to write tests for both the UI and the core behaviours. We call UI tests 'blackbox' tests and they belong in bzrlib/tests/blackbox/*.py.

When writing blackbox tests please honour the following conventions:

  1. Place the tests for the command 'name' in bzrlib/tests/blackbox/ This makes it easy for developers to locate the test script for a faulty command.
  2. Use the 'self.run_bzr("name")' utility function to invoke the command rather than running bzr in a subprocess or invoking the method directly. This is a lot faster than subprocesses and generates the same logging output as running it in a subprocess (which invoking the method directly does not).
  3. Only test the one command in a single test script. Use the bzrlib library when setting up tests and when evaluating the side-effects of the command. We do this so that the library api has continual pressure on it to be as functional as the command line in a simple manner, and to isolate knock-on effects throughout the blackbox test suite when a command changes its name or signature. Ideally only the tests for a given command are affected when a given command is changed.
  4. If you have a test which does actually require running bzr in a subprocess you can use run_bzr_subprocess. By default the spawned process will not load plugins unless --allow-plugins is supplied.

Per-implementation tests

Per-implementation tests are tests that are defined once and then run against multiple implementations of an interface. For example, defines tests that all Transport implementations (local filesystem, HTTP, and so on) must pass.

They are found in bzrlib/tests/*_implementations/test_*.py, bzrlib/tests/per_*/*.py, and bzrlib/tests/test_*

These are really a sub-category of unit tests, but an important one.


We make selective use of doctests. In general they should provide examples within the API documentation which can incidentally be tested. We don't try to test every important case using doctests — regular Python tests are generally a better solution. That is, we just use doctests to make our documentation testable, rather than as a way to make tests.

Most of these are in bzrlib/doc/api. More additions are welcome.

Skipping tests

In our enhancements to unittest we allow for some addition results beyond just success or failure.

If a test can't be run, it can say that it's skipped by raising a special exception. This is typically used in parameterized tests — for example if a transport doesn't support setting permissions, we'll skip the tests that relating to that.

    return self.branch_format.initialize(repo.bzrdir)
except errors.UninitializableFormat:
    raise tests.TestSkipped('Uninitializable branch format')

Raising TestSkipped is a good idea when you want to make it clear that the test was not run, rather than just returning which makes it look as if it was run and passed.

Several different cases are distinguished:

Generic skip; the only type that was present up to bzr 0.18.
The test doesn't apply to the parameters with which it was run. This is typically used when the test is being applied to all implementations of an interface, but some aspects of the interface are optional and not present in particular concrete implementations. (Some tests that should raise this currently either silently return or raise TestSkipped.) Another option is to use more precise parameterization to avoid generating the test at all.

The test can't be run because a dependency (typically a Python library) is not available in the test environment. These are in general things that the person running the test could fix by installing the library. It's OK if some of these occur when an end user runs the tests or if we're specifically testing in a limited environment, but a full test should never see them.

See Test feature dependencies below.


The test exists but is known to fail, for example this might be appropriate to raise if you've committed a test for a bug but not the fix for it, or if something works on Unix but not on Windows.

Raising this allows you to distinguish these failures from the ones that are not expected to fail. If the test would fail because of something we don't expect or intend to fix, KnownFailure is not appropriate, and TestNotApplicable might be better.

KnownFailure should be used with care as we don't want a proliferation of quietly broken tests.

We plan to support three modes for running the test suite to control the interpretation of these results. Strict mode is for use in situations like merges to the mainline and releases where we want to make sure that everything that can be tested has been tested. Lax mode is for use by developers who want to temporarily tolerate some known failures. The default behaviour is obtained by bzr selftest with no options, and also (if possible) by running under another unittest harness.

result strict default lax
TestSkipped pass pass pass
TestNotApplicable pass pass pass
UnavailableFeature fail pass pass
KnownFailure fail pass pass

Test feature dependencies

Writing tests that require a feature

Rather than manually checking the environment in each test, a test class can declare its dependence on some test features. The feature objects are checked only once for each run of the whole test suite.

(For historical reasons, as of May 2007 many cases that should depend on features currently raise TestSkipped.)

For example:

class TestStrace(TestCaseWithTransport):

    _test_needs_features = [StraceFeature]

This means all tests in this class need the feature. If the feature is not available the test will be skipped using UnavailableFeature.

Individual tests can also require a feature using the requireFeature method:


Features already defined in bzrlib.tests include:

  • SymlinkFeature,
  • HardlinkFeature,
  • OsFifoFeature,
  • UnicodeFilenameFeature,
  • FTPServerFeature, and
  • CaseInsensitiveFilesystemFeature.

Defining a new feature that tests can require

New features for use with _test_needs_features or requireFeature are defined by subclassing bzrlib.tests.Feature and overriding the _probe and feature_name methods. For example:

class _SymlinkFeature(Feature):

    def _probe(self):
        return osutils.has_symlinks()

    def feature_name(self):
        return 'symlinks'

SymlinkFeature = _SymlinkFeature()

Testing exceptions and errors

It's important to test handling of errors and exceptions. Because this code is often not hit in ad-hoc testing it can often have hidden bugs -- it's particularly common to get NameError because the exception code references a variable that has since been renamed.

In general we want to test errors at two levels:

  1. A test in checking that when the exception object is constructed with known parameters it produces an expected string form. This guards against mistakes in writing the format string, or in the str representations of its parameters. There should be one for each exception class.
  2. Tests that when an api is called in a particular situation, it raises an error of the expected class. You should typically use assertRaises, which in the Bazaar test suite returns the exception object to allow you to examine its parameters.

In some cases blackbox tests will also want to check error reporting. But it can be difficult to provoke every error through the commandline interface, so those tests are only done as needed — eg in response to a particular bug or if the error is reported in an unusual way(?) Blackbox tests should mostly be testing how the command-line interface works, so should only test errors if there is something particular to the cli in how they're displayed or handled.

Testing warnings

The Python warnings module is used to indicate a non-fatal code problem. Code that's expected to raise a warning can be tested through callCatchWarnings.

The test suite can be run with -Werror to check no unexpected errors occur.

However, warnings should be used with discretion. It's not an appropriate way to give messages to the user, because the warning is normally shown only once per source line that causes the problem. You should also think about whether the warning is serious enought that it should be visible to users who may not be able to fix it.

Interface implementation testing and test scenarios

There are several cases in Bazaar of multiple implementations of a common conceptual interface. ("Conceptual" because it's not necessary for all the implementations to share a base class, though they often do.) Examples include transports and the working tree, branch and repository classes.

In these cases we want to make sure that every implementation correctly fulfils the interface requirements. For example, every Transport should support the has() and get() and clone() methods. We have a sub-suite of tests in test_transport_implementations. (Most per-implementation tests are in submodules of bzrlib.tests, but not the transport tests at the moment.)

These tests are repeated for each registered Transport, by generating a new TestCase instance for the cross product of test methods and transport implementations. As each test runs, it has transport_class and transport_server set to the class it should test. Most tests don't access these directly, but rather use self.get_transport which returns a transport of the appropriate type.

The goal is to run per-implementation only the tests that relate to that particular interface. Sometimes we discover a bug elsewhere that happens with only one particular transport. Once it's isolated, we can consider whether a test should be added for that particular implementation, or for all implementations of the interface.

The multiplication of tests for different implementations is normally accomplished by overriding the load_tests function used to load tests from a module. This function typically loads all the tests, then applies a TestProviderAdapter to them, which generates a longer suite containing all the test variations.

See also Per-implementation tests (above).

Test scenarios

Some utilities are provided for generating variations of tests. This can be used for per-implementation tests, or other cases where the same test code needs to run several times on different scenarios.

The general approach is to define a class that provides test methods, which depend on attributes of the test object being pre-set with the values to which the test should be applied. The test suite should then also provide a list of scenarios in which to run the tests.

Typically multiply_tests_from_modules should be called from the test module's load_tests function.

Test support

We have a rich collection of tools to support writing tests. Please use them in preference to ad-hoc solutions as they provide portability and performance benefits.

TestCase and its subclasses

The bzrlib.tests module defines many TestCase classes to help you write your tests.

A base TestCase that extends the Python standard library's TestCase in several ways. It adds more assertion methods (e.g. assertContainsRe), addCleanup, and other features (see its API docs for details). It also has a setUp that makes sure that global state like registered hooks and loggers won't interfere with your test. All tests should use this base class (whether directly or via a subclass).
Extends TestCase and adds methods like get_transport, make_branch and make_branch_builder. The files created are stored in a MemoryTransport that is discarded at the end of the test. This class is good for tests that need to make branches or use transports, but that don't require storing things on disk. All tests that create bzrdirs should use this base class (either directly or via a subclass) as it ensures that the test won't accidentally operate on real branches in your filesystem.
Extends TestCaseWithMemoryTransport. For tests that really do need files to be stored on disk, e.g. because a subprocess uses a file, or for testing functionality that accesses the filesystem directly rather than via the Transport layer (such as dirstate).
Extends TestCaseInTempDir. Provides get_url and get_readonly_url facilities. Subclasses can control the transports used by setting vfs_transport_factory, transport_server and/or transport_readonly_server.

See the API docs for more details.


When writing a test for a feature, it is often necessary to set up a branch with a certain history. The BranchBuilder interface allows the creation of test branches in a quick and easy manner. Here's a sample session:

builder = self.make_branch_builder('relpath')
branch = builder.get_branch()

make_branch_builder is a method of TestCaseWithMemoryTransport.

Note that many current tests create test branches by inheriting from TestCaseWithTransport and using the make_branch_and_tree helper to give them a WorkingTree that they can commit to. However, using the newer make_branch_builder helper is preferred, because it can build the changes in memory, rather than on disk. Tests that are explictly testing how we work with disk objects should, of course, use a real WorkingTree.

Please see bzrlib.branchbuilder for more details.

If you're going to examine the commit timestamps e.g. in a test for log output, you should set the timestamp on the tree, rather than using fuzzy matches in the test.


The TreeBuilder interface allows the construction of arbitrary trees with a declarative interface. A sample session might look like:

tree = self.make_branch_and_tree('path')
builder = TreeBuilder()
builder.start_tree(tree)['foo', "bar/", "bar/file"])
tree.commit('commit the tree')

Usually a test will create a tree using make_branch_and_memory_tree (a method of TestCaseWithMemoryTransport) or make_branch_and_tree (a method of TestCaseWithTransport).

Please see bzrlib.treebuilder for more details.