===============================================
Implementation and Customization of ``py.test``
===============================================

.. contents::
.. sectnum::

.. _`basicpicture`:


Collecting and running tests / implementation remarks 
====================================================== 

In order to customize ``py.test`` it's good to understand 
its basic architure (WARNING: these are not guaranteed 
yet to stay the way they are now!)::

     ___________________
    |                   |
    |    Collector      |
    |___________________|
           / \                
            |                Item.run()
            |               ^
     receive test Items    /
            |             /execute test Item 
            |            /
     ___________________/
    |                   |      
    |     Session       |
    |___________________|
                                          
                        .............................
                        . conftest.py configuration .
                        . cmdline options           .
                        .............................


The *Session* basically receives test *Items* from a *Collector*, 
and executes them via the ``Item.run()`` method.  It monitors 
the outcome of the test and reports about failures and successes. 

.. _`collection process`: 

Collectors and the test collection process 
------------------------------------------

The collecting process is iterative, i.e. the session 
traverses and generates a *collector tree*.  Here is an example of such
a tree, generated with the command ``py.test --collectonly py/xmlobj``:: 

    <Directory 'xmlobj'>
        <Directory 'testing'>
            <Module 'test_html.py' (py.__.xmlobj.testing.test_html)>
                <Function 'test_html_name_stickyness'>
                <Function 'test_stylenames'>
                <Function 'test_class_None'>
                <Function 'test_alternating_style'>
            <Module 'test_xml.py' (py.__.xmlobj.testing.test_xml)>
                <Function 'test_tag_with_text'>
                <Function 'test_class_identity'>
                <Function 'test_tag_with_text_and_attributes'>
                <Function 'test_tag_with_subclassed_attr_simple'>
                <Function 'test_tag_nested'>
                <Function 'test_tag_xmlname'>


By default all directories not starting with a dot are traversed, 
looking for ``test_*.py`` and ``*_test.py`` files.  Those files 
are imported under their `package name`_. 

The Module collector looks for test functions 
and test classes and methods. Test functions and methods
are prefixed ``test`` by default.  Test classes must 
start with a capitalized ``Test`` prefix. 


.. _`collector API`: 

test items are collectors as well
--------------------------------- 

To make the reporting life simple for the session object 
items offer a ``run()`` method as well.  In fact the session
distinguishes "collectors" from "items" solely by interpreting 
their return value.  If it is a list, then we recurse into 
it, otherwise we consider the "test" as passed.  

.. _`package name`: 

constructing the package name for test modules
-------------------------------------------------

Test modules are imported under their fully qualified 
name.  Given a filesystem ``fspath`` it is constructed as follows:

* walk the directories up to the last one that contains 
  an ``__init__.py`` file.  

* perform ``sys.path.insert(0, basedir)``.

* import the root package as ``root``

* determine the fully qualified name for ``fspath`` by either:

  * calling ``root.__pkg__.getimportname(fspath)`` if the
    ``__pkg__`` exists.` or 

  * otherwise use the relative path of the module path to
    the base dir and turn slashes into dots and strike
    the trailing ``.py``. 



Customizing the testing process 
===============================

writing conftest.py files
-----------------------------------

You may put conftest.py files containing project-specific
configuration in your project's root directory, it's usually
best to put it just into the same directory level as your 
topmost ``__init__.py``.  In fact, ``py.test`` performs
an "upwards" search starting from the directory that you specify 
to be tested and will lookup configuration values right-to-left. 
You may have options that reside e.g. in your home directory 
but note that project specific settings will be considered
first.  There is a flag that helps you debugging your
conftest.py configurations::
    
    py.test --traceconfig

adding custom options
+++++++++++++++++++++++

To register a project-specific command line option 
you may have the following code within a ``conftest.py`` file::

    import py
    Option = py.test.config.Option
    option = py.test.config.addoptions("pypy options",
        Option('-V', '--view', action="store_true", dest="view", default=False,
               help="view translation tests' flow graphs with Pygame"),
    )

and you can then access ``option.view`` like this:: 

    if option.view:
        print "view this!"

The option will be available if you type ``py.test -h``
Note that you may only register upper case short
options.  ``py.test`` reserves all lower 
case short options for its own cross-project usage. 

customizing the collecting and running process 
-----------------------------------------------

To introduce different test items you can create 
one or more ``conftest.py`` files in your project. 
When the collection process traverses directories 
and modules the default collectors will produce 
custom Collectors and Items if they are found 
in a local ``conftest.py`` file.  

example: perform additional ReST checks 
+++++++++++++++++++++++++++++++++++++++

With your custom collectors or items you can completely 
derive from the standard way of collecting and running
tests in a localized manner.  Let's look at an example. 
If you invoke ``py.test --collectonly py/documentation`` 
then you get:: 

    <DocDirectory 'documentation'>
        <DocDirectory 'example'>
            <DocDirectory 'pytest'>
                <Module 'test_setup_flow_example.py' (test_setup_flow_example)>
                    <Class 'TestStateFullThing'>
                        <Instance '()'>
                            <Function 'test_42'>
                            <Function 'test_23'>
        <ReSTChecker 'TODO.txt'>
            <ReSTSyntaxTest 'TODO.txt'>
            <LinkCheckerMaker 'checklinks'>
        <ReSTChecker 'api.txt'>
            <ReSTSyntaxTest 'api.txt'>
            <LinkCheckerMaker 'checklinks'>
                <CheckLink 'getting-started.html'>
        ... 

In ``py/documentation/conftest.py`` you find the following 
customization:: 

    class DocDirectory(py.test.collect.Directory):

        def run(self):
            results = super(DocDirectory, self).run()
            for x in self.fspath.listdir('*.txt', sort=True):
                    results.append(x.basename)
            return results

        def join(self, name):
            if not name.endswith('.txt'):
                return super(DocDirectory, self).join(name)
            p = self.fspath.join(name)
            if p.check(file=1):
                return ReSTChecker(p, parent=self)

    Directory = DocDirectory

The existence of the 'Directory' name in the 
``pypy/documentation/conftest.py`` module makes the collection 
process defer to our custom "DocDirectory" collector.  We extend 
the set of collected test items by ``ReSTChecker`` instances 
which themselves create ``ReSTSyntaxTest`` and ``LinkCheckerMaker`` 
items.  All of this instances (need to) follow the `collector API`_. 


Customizing the collection process in a module
---------------------------------------------- 

    REPEATED WARNING: details of the collection and running process are 
    still subject to refactorings and thus details will change.  
    If you are customizing py.test at "Item" level then you 
    definitely want to be subscribed to the `py-dev mailing list`_ 
    to follow ongoing development.

If you have a module where you want to take responsibility for
collecting your own test Items and possibly even for executing
a test then you can provide `generative tests`_ that yield 
callables and possibly arguments as a tuple.   This is especially
useful for calling application test machinery with different
parameter sets but counting each of the calls as a separate
tests. 

.. _`generative tests`: test.html#generative-tests

The other extension possibility is about 
specifying a custom test ``Item`` class which 
is responsible for setting up and executing an underlying 
test.  Or you can extend the collection process for a whole 
directory tree by putting Items in a ``conftest.py`` configuration file. 
The collection process dynamically consults the *chain of conftest.py* 
modules to determine collectors and items at ``Directory``, ``Module``, 
``Class``, ``Function`` or ``Generator`` level respectively.  

Customizing execution of Functions 
---------------------------------- 

- ``py.test.collect.Function`` test items control execution 
  of a test function.  ``function.run()`` will get called by the
  session in order to actually run a test.  The method is responsible
  for performing proper setup/teardown ("Test Fixtures") for a 
  Function test. 

- ``Function.execute(target, *args)`` methods are invoked by
  the default ``Function.run()`` to actually execute a python 
  function with the given (usually empty set of) arguments. 

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