# Organizing Test Fixtures into Layers

Note

New in version 0.4

Layers allow more flexible organization of test fixtures than test-, class- and module- level fixtures. Layers in nose2 are inspired by and aim to be compatible with the layers used by Zope’s testrunner.

Using layers, you can do things like:

- Implement package-level fixtures by sharing a layer among all test cases in the package.

- Share fixtures across tests in different modules without having them run multiple times.

- Create a fixture tree deeper than three levels (test, class and module).

- Make fixtures available for other packages or projects to use.

A layer is a new-style class that implements at least a `setUp` classmethod:

    class Layer(object):
       @classmethod
       def setUp(cls):
           # ...

It may also implement `tearDown`, `testSetUp` and `testTearDown`, all as classmethods.

To assign a layer to a test case, set the test case’s `layer` property:

    class Test(unittest.TestCase):
        layer = Layer

Note that the layer class is assigned, not an instance of the layer. Typically layer classes are not instantiated.

## Sub-layers

Layers may subclass other layers:

    class SubLayer(Layer):
        @classmethod
        def setUp(cls):
            # ...

In this case, all tests that belong to the sub-layer also belong to the base layer. For example for this test case:

    class SubTest(unittest.TestCase):
        layer = SubLayer

The `setUp` methods from both `SubLayer` and `Layer` will run before any tests are run. The superclass’s setup will always run before the subclass’s setup. For `teardown`, the reverse: the subclass’s `teardown` runs before the superclass’s.

Warning

One important thing to note: layers that subclass other layers must not call their superclass’s `setUp`, `tearDown`, etc. The test runner will take care of organizing tests so that the superclass’s methods are called in the right order:

    Layer.setUp ->
      SubLayer.setUp ->
        Layer.testSetUp ->
          SubLayer.testSetUp ->
            TestCase.setUp
              TestCase.run
            TestCase.tearDown
          SubLayer.testTearDown <-
        Layer.testTearDown <-
      SubLayer.tearDown <-
    Layer.tearDown <-

If a sublayer calls it superclass’s methods directly, those methods will be called twice.

## Layer method reference

class Layer

  Not an actual class, but reference documentation for the methods layers can implement. There is no layer base class. Layers must be subclasses of `object` or other layers.

  classmethod setUp(cls)

    The layer’s `setUp` method is called before any tests belonging to that layer are executed. If no tests belong to the layer (or one of its sub-layers) then the `setUp` method will not be called.

  classmethod tearDown(cls)

    The layer’s `tearDown` method is called after any tests belonging to the layer are executed, if the layer’s `setUp` method was called and did not raise an exception. It will not be called if the layer has no `setUp` method, or if that method did not run or did raise an exception.

  classmethod testSetUp(cls[, test])

    The layer’s `testSetUp` method is called before each test belonging to the layer (and its sub-layers). If the method is defined to accept an argument, the test case instance is passed to the method. The method may also be defined to take no arguments.

  classmethod testTearDown(cls[, test])

    The layer’s `testTearDown` method is called after each test belonging to the layer (and its sub-layers), if the layer also defines a `setUpTest` method and that method ran successfully (did not raise an exception) for this test case.

## Layers DSL

nose2 includes a DSL for setting up layer-using tests called “such”. Read all about it here: Such: a Functional-Test Friendly DSL.

## Pretty reports

The layers plugin module includes a second plugin that alters test report output to make the layer groupings more clear. When activated with the `--layer-reporter` command-line option (or via a config file), test output that normally looks like this:

    test (test_layers.NoLayer) ... ok
    test (test_layers.Outer) ... ok
    test (test_layers.InnerD) ... ok
    test (test_layers.InnerA) ... ok
    test (test_layers.InnerA_1) ... ok
    test (test_layers.InnerB_1) ... ok
    test (test_layers.InnerC) ... ok
    test2 (test_layers.InnerC) ... ok

    ----------------------------------------------------------------------
    Ran 8 tests in 0.001s

    OK

Will instead look like this:

    test (test_layers.NoLayer) ... ok
    Base
      test (test_layers.Outer) ... ok
      LayerD
        test (test_layers.InnerD) ... ok
      LayerA
        test (test_layers.InnerA) ... ok
      LayerB
        LayerC
          test (test_layers.InnerC) ... ok
          test2 (test_layers.InnerC) ... ok
        LayerB_1
          test (test_layers.InnerB_1) ... ok
        LayerA_1
          test (test_layers.InnerA_1) ... ok

    ----------------------------------------------------------------------
    Ran 8 tests in 0.002s

    OK

The layer reporter plugin can also optionally colorize the keywords (by default, ‘A’, ‘having’, and ‘should’) in output from tests defined with the such DSL.

If you would like to change how the layer is displayed, set the `description` attribute.

    class LayerD(Layer):
        description = '*** This is a very important custom layer description ***'

Now the output will be the following:

    test (test_layers.NoLayer) ... ok
    Base
      test (test_layers.Outer) ... ok
      *** This is a very important custom layer description ***
        test (test_layers.InnerD) ... ok
      LayerA
        test (test_layers.InnerA) ... ok
      LayerB
        LayerC
          test (test_layers.InnerC) ... ok
          test2 (test_layers.InnerC) ... ok
        LayerB_1
          test (test_layers.InnerB_1) ... ok
        LayerA_1
          test (test_layers.InnerA_1) ... ok

    ----------------------------------------------------------------------
    Ran 8 tests in 0.002s

    OK

## Warnings and Caveats

### Test case order and module isolation

Test cases that use layers will not execute in the same order as test cases that do not. In order to execute the layers efficiently, the test runner must reorganize all tests in the loaded test suite to group those having like layers together (and sub-layers under their parents). If you share layers across modules this may result in tests from one module executing interleaved with tests from a different module.

### Mixing layers with `setUpClass` and module fixtures

Don’t cross the streams.

The implementation of class- and module-level fixtures in unittest2 depends on introspecting the class hierarchy inside of the `unittest.TestSuite`. Since the suites that the `layers` plugin uses to organize tests derive from `unittest.BaseTestSuite` (instead of `unittest.TestSuite`), class- and module- level fixtures in TestCase classes that use layers will be ignored.

### Mixing layers and multiprocess testing

In the initial release, test suites using layers are incompatible with the multiprocess plugin. This should be fixed in a future release.

## Plugin reference

### Enable this Plugin

This plugin is built-in, but not loaded by default.

Even if you specify `always-on = True` in the configuration, it will not run unless you also enable it. You can do so by putting the following in a `unittest.cfg` or `nose2.cfg` file

    [unittest]
    plugins = nose2.plugins.layers

The `plugins` parameter may contain a list of plugin names, including `nose2.plugins.layers`

### Configuration [layer-reporter]

always-on
  Default:

    False

  Type:

    boolean

colors
  Default:

    False

  Type:

    boolean

highlight-words
  Default:

    [‘A’, ‘having’, ‘should’]

  Type:

    list

indent
  Default:

  Type:

    str

#### Sample configuration

The default configuration is equivalent to including the following in a `unittest.cfg` file.

    [layer-reporter]
    always-on = False
    colors = False
    highlight-words = A
                      having
                      should
    indent =

### Command-line options

--layer-reporter DEFAULT

  Add layer information to test reports

### Plugin class reference: LayerReporter

class nose2.plugins.layers.LayerReporter(*args, **kwargs)

### Plugin class reference: Layers

class nose2.plugins.layers.Layers(*args, **kwargs)