cloud-init has both unit tests and integration tests. Unit tests can be found in-tree alongside the source code, as well as at tests/unittests. Integration tests can be found at tests/integration_tests. Documentation specifically for integration tests can be found on the Integration Testing page, but the guidelines specified below apply to both types of tests.

cloud-init uses pytest to run its tests, and has tests written both as unittest.TestCase sub-classes and as un-subclassed pytest tests.


The following guidelines should be followed.

Test Layout

  • For ease of organisation and greater accessibility for developers not familiar with pytest, all cloud-init unit tests must be contained within test classes
    • Put another way, module-level test functions should not be used
  • As all tests are contained within classes, it is acceptable to mix TestCase test classes and pytest test classes within the same test file

pytest Tests

  • pytest test classes should use pytest fixtures to share functionality instead of inheritance
  • pytest tests should use bare assert statements, to take advantage of pytest’s assertion introspection
    • For == and other commutative assertions, the expected value should be placed before the value under test: assert expected_value == function_under_test()

pytest Version Gotchas

As we still support Ubuntu 16.04 (Xenial Xerus), we can only use pytest features that are available in v2.8.7. This is an inexhaustive list of ways in which this may catch you out:

  • Support for using yield in pytest.fixture functions was only introduced in pytest 3.0. Such functions must instead use the pytest.yield_fixture decorator.
  • Only the following built-in fixtures are available [1]:
    • cache
    • capfd
    • caplog (provided by python3-pytest-catchlog on xenial)
    • capsys
    • monkeypatch
    • pytestconfig
    • record_xml_property
    • recwarn
    • tmpdir_factory
    • tmpdir
  • On xenial, the objects returned by the tmpdir fixture cannot be used where paths are required; they are rejected as invalid paths. You must instead use their .strpath attribute.
    • For example, instead of util.write_file(tmpdir.join("some_file"), ...), you should write util.write_file(tmpdir.join("some_file").strpath, ...).
  • The pytest.param function cannot be used. It was introduced in pytest 3.1, which means it is not available on xenial. The more limited mechanism it replaced was removed in pytest 4.0, so is not available in focal or later. The only available alternatives are to write mark-requiring test instances as completely separate tests, without utilising parameterisation, or to apply the mark to the entire parameterized test (and therefore every test instance).

Mocking and Assertions

  • Variables/parameter names for Mock or MagicMock instances should start with m_ to clearly distinguish them from non-mock variables

    • For example, m_readurl (which would be a mock for readurl)
  • The assert_* methods that are available on Mock and MagicMock objects should be avoided, as typos in these method names may not raise AttributeError (and so can cause tests to silently pass). An important exception: if a Mock is autospecced then misspelled assertion methods will raise an AttributeError, so these assertion methods may be used on autospecced Mock objects.

    For non-autospecced Mock s, these substitutions can be used (m is assumed to be a Mock):

    • m.assert_any_call(*args, **kwargs) => assert mock.call(*args, **kwargs) in m.call_args_list
    • m.assert_called() => assert 0 != m.call_count
    • m.assert_called_once() => assert 1 == m.call_count
    • m.assert_called_once_with(*args, **kwargs) => assert [mock.call(*args, **kwargs)] == m.call_args_list
    • m.assert_called_with(*args, **kwargs) => assert mock.call(*args, **kwargs) == m.call_args_list[-1]
    • m.assert_has_calls(call_list, any_order=True) => for call in call_list: assert call in m.call_args_list
      • m.assert_has_calls(...) and m.assert_has_calls(..., any_order=False) are not easily replicated in a single statement, so their use when appropriate is acceptable.
    • m.assert_not_called() => assert 0 == m.call_count
  • When there are multiple patch calls in a test file for the module it is testing, it may be desirable to capture the shared string prefix for these patch calls in a module-level variable. If used, such variables should be named M_PATH or, for datasource tests, DS_PATH.

Test Argument Ordering

  • Test arguments should be ordered as follows:
    • mock.patch arguments. When used as a decorator, mock.patch partially applies its generated Mock object as the first argument, so these arguments must go first.
    • pytest.mark.parametrize arguments, in the order specified to the parametrize decorator. These arguments are also provided by a decorator, so it’s natural that they sit next to the mock.patch arguments.
    • Fixture arguments, alphabetically. These are not provided by a decorator, so they are last, and their order has no defined meaning, so we default to alphabetical.
  • It follows from this ordering of test arguments (so that we retain the property that arguments left-to-right correspond to decorators bottom-to-top) that test decorators should be ordered as follows:
    • pytest.mark.parametrize
    • mock.patch

This list of fixtures (with markup) can be reproduced by running:

py.test-3 --fixtures -q | grep "^[^ -]" | grep -v '\(no\|capturelog\)' | sort | sed 's/.*/* ``\0``/'

in a xenial lxd container with python3-pytest-catchlog installed.