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Avoiding Temptation: Notes on using 'unittest' effectively

Notes on getting the most mileage out of tests written with the Python unittest module

Created by tseaver. Last modified 2011-02-17 12:37:54.

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Avoiding Temptation: Notes on using 'unittest' effectively

Goals

The goals of the kind of testing outlined here are simplicity, loose or no coupling, and speed:

  • Tests should be as simple as possible, while exercising the application- under-test (AUT) completely.
  • Tests should run as quickly as possible, to encourage running them frequently.
  • Tests should avoid coupling with other tests, or with parts of the AUT which they are not responsible for testing.

Developers write such tests to verify that the AUT is abiding by the contracts the developer specifies. While an instance this type of test case may be illustrative of the contract it tests, such test cases do not take the place of either API documentation or of narrative / "theory of operations" documentation. Still less are they intended for end-user documentation.

Rule: Never import the module-under-test at test module scope.

Import failures in the module-under-test (MUT) should cause individual test cases to fail: they should never prevent those tests from being run. Depending on the testrunner, import problems may be much harder to distinguish at a glance than normal test failures.

For example, rather than the following:

# test the foo module
import unittest

from package.foo import FooClass

class FooClassTests(unittest.TestCase):

    def test_bar(self):
        foo = FooClass('Bar')
        self.assertEqual(foo.bar(), 'Bar')

prefer:

# test the foo module
import unittest

class FooClassTests(unittest.TestCase):

    def _getTargetClass(self):
        from package.foo import FooClass
        return FooClass

    def _makeOne(self, *args, **kw):
        return self._getTargetClass()(*args, **kw)

    def test_bar(self):
        foo = self._makeOne('Bar')
        self.assertEqual(foo.bar(), 'Bar')

Guideline: Minimize module-scope dependencies.

Unit tests need to be runnable even in an enviornment which is missing some required features: in that case, one or more of the testcase methods (TCMs) will fail. Defer imports of any needed library modules as late as possible.

For instance, this example generates no test failures at all if the 'qux' module is not importable:

# test the foo module
import unittest
import qux

class FooClassTests(unittest.TestCase):

    def _getTargetClass(self):
        from package.foo import FooClass
        return FooClass

    def _makeOne(self, *args, **kw):
        return self._getTargetClass()(*args, **kw)

    def test_bar(self):
        foo = self._makeOne(qux.Qux('Bar'))

while this example raises failures for each TCM which uses the missing module:

# test the foo module
import unittest

class FooClassTests(unittest.TestCase):

    def _getTargetClass(self):
        from package.foo import FooClass
        return FooClass

    def _makeOne(self, *args, **kw):
        return self._getTargetClass()(*args, **kw)

    def test_bar(self):
        import qux
        foo = self._makeOne(qux.Qux('Bar'))

It may be a reasonable tradeoff in some cases to import a module (but not the MUT!) which is used widely within the test cases. Such a tradeoff should probably occur late in the life of the TCM, after the pattern of usage is clearly understood.

Rule: Make each test case method test Just One Thing.

Avoid the temptation to write fewer, bigger tests. Ideally, each TCM will exercise one set of preconditions for one method or function. For instance, the following test case tries to exercise far too much:

def test_bound_used_container(self):
    from AccessControl.SecurityManagement import newSecurityManager
    from AccessControl import Unauthorized
    newSecurityManager(None, UnderprivilegedUser())
    root = self._makeTree()
    guarded = root._getOb('guarded')

    ps = guarded._getOb('bound_used_container_ps')
    self.assertRaises(Unauthorized, ps)

    ps = guarded._getOb('container_str_ps')
    self.assertRaises(Unauthorized, ps)

    ps = guarded._getOb('container_ps')
    container = ps()
    self.assertRaises(Unauthorized, container)
    self.assertRaises(Unauthorized, container.index_html)
    try:
        str(container)
    except Unauthorized:
        pass
    else:
        self.fail("str(container) didn't raise Unauthorized!")

    ps = guarded._getOb('bound_used_container_ps')
    ps._proxy_roles = ( 'Manager', )
    ps()

    ps = guarded._getOb('container_str_ps')
    ps._proxy_roles = ( 'Manager', )
    ps()

This test has a couple of faults, but the critical one is that it tests too many things (eight different cases).

In general, the prolog of the TCM should establish the one set of preconditions by setting up fixtures / mock objects / static values, and then instantiate the class or import the FUT. The TCM should then call the method / function. The epilog should test the outcomes, typically by examining either the return value or the state of one or more fixtures / mock objects.

Thinking about the separate sets of preconditions for each function or method being tested helps clarify the contract, and may inspire a simpler / cleaner / faster implementation.

Rule: Name TCMs to indicate what they test.

The name of the test should be the first, most useful clue when looking at a failure report: don't make the reader (yourself, most likely) grep the test module to figure out what was being tested.

Rather than adding a comment:

class FooClassTests(unittest.TestCase):

   def test_some_random_blather(self):
       # test the 'bar' method in the case where 'baz' is not set.

prefer to use the TCM name to indicate its purpose:

class FooClassTests(unittest.TestCase):

   def test_getBar_wo_baz(self):
       #...

Guideline: Share setup via helper methods, not via attributes of 'self'.

Doing unneeded work in the 'setUp' method of a testcase class sharply increases coupling between TCMs, which is a Bad Thing. For instance, suppose the class-under-test (CUT) takes a context as an argument to its constructor. Rather than instantiating the context in 'setUp':

class FooClassTests(unittest.TestCase):

   def setUp(self):
       self.context = DummyContext()

   # ...

   def test_bar(self):
       foo = self._makeOne(self.context)

add a helper method to instantiate the context, and keep it as a local:

class FooClassTests(unittest.TestCase):

   def _makeContext(self, *args, **kw):
       return DummyContext(*args, **kw)

   def test_bar(self):
       context = self.
       foo = self._makeOne(self.context)

This practice allows different tests to create the mock context differently, avoiding coupling. It also makes the tests run faster, as the tests which don't need the context don't pay for creating it.

Guideline: Make fixtures as simple as possible.

When writing a mock object, start off with an empty class, e.g.:

class DummyContext:
    pass

Run the tests, adding methods only enough to the mock object to make the dependent tests pass. Avoid giving the mock object any behavior which is not necessary to make one or more tests pass.

Guideline: Use hooks and registries judiciously.

If the application already allows registering plugins or components, take advantage of the fact to insert your mock objects. Don't forget to cleanup after each test!

It may be acceptable to add hook methods to the application, purely to allow for simplicity of testing. For instance, code which normally sets datetime attributes to "now" could be tweaked to use a module-scope function, rather than calling 'datetime.now()' directly. Tests can then replace that function with one which returns a known value (as long as they put back the original version after they run).

Guideline: Use mock objects to clarify dependent contracts

Keeping the contracts on which the AUT dependes as simple as possible makes the AUT easier to write, and more resilient to changes. Writing mock objects which supply only the simplest possible implementation of such contracts keeps the AUT from acquiring "dependency creep."

For example, in a relational application, the the SQL queries used by the application can be mocked up as a dummy implementation which takes keyword parameters and returns lists of dictionaires:

class DummySQL:

    def __init__(self, results):
        # results should be a list of lists of dictionaries
        self.called_with = []
        self.results = results

    def __call__(self, **kw):
        self.called_with.append(kw.copy())
        return results.pop(0)

In addition to keeping the dependent contract simple (in this case, the SQL object should return a list of mappings, one per row), the mock object allows for easy testing of how it is used by the AUT:

class FooTest(unittest.TestCase):

   def test_barflies_returns_names_from_SQL(self):
       from foo.sqlregistry import registerSQL
       RESULTS = [[{'name': 'Chuck', 'drink': 'Guiness'},
                   {'name': 'Bob', 'drink': 'Knob Creek'},
                  ]]
       query = DummySQL(RESULTS[:])
       registerSQL('list_barflies', query)
       foo = self._makeOne('Dog and Whistle')

       names = foo.barflies()

       self.assertEqual(len(names), len(RESULTS))
       self.failUnless('NAME1' in names)
       self.failUnless('NAME2' in names)

       self.assertEqual(query.called_with, {'bar', 'Dog and Whistle'})

Rule: Don't share text fixtures between test modules.

The temptation here is to save typing by borrowing mock objects or fixture code from another test module. Once indulged, one often ends up moving such "generic" fixtures to shared modules.

The rationale for this prohibition is simplicity: unit tests need to exercise the AUT, while remaining as clear and simple as possible.

  • Because they are not in the module which uses them, shared mock objects and fixtures makes impose a lookup burden on the reader.
  • Because they have to support APIs used by multiple clients, shared fixtures tend grow to grow APIs / data structures needed only by one client: in the degenerate case, become as complicated as the application they are supposed to stand in for!

In some cases, it may be cleaner to avoid sharing fixtures even among test case methods (TCMs) within the same module / class.

Conclusion

Tests which conform to these rules and guidelines have the following properties:

  • The tests are straightforward to write.
  • The tests yield excellent coverage of the AUT.
  • They reward the developer through predictable feedback (e.g., the growing list of dots for passed tests).
  • They run quickly, and thus encourage the developer to run them frequently.
  • Expected failures confirm missing / incomplete implementations.
  • Unexpected failures are easy to diagnose and repair.
  • When used as regression tests, failures help pinpoint the exact source of the regression (a changed contract, for instance, or an underspecified constraint).
  • Writing such tests clarifies thinking about the contracts of the code they test, as well as the dependencies of that code.