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Frequently Asked Questions
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On naming, nosetests, licensing and magic
Why the ``py`` naming? Why not ``pytest``?
This mostly has historic reasons - the aim is
to get away from the somewhat questionable 'py' name
at some point. These days (2010) the 'py' library
almost completely comprises APIs that are used
by the ``py.test`` tool. There also are some
other uses, e.g. of the ``py.path.local()`` and
other path implementations. So it requires some
work to factor them out and do the shift.
Why the ``py.test`` naming?
because of TAB-completion under Bash/Shells. If you hit
``py.<TAB>`` you'll get a list of available development
tools that all share the ``py.`` prefix. Another motivation
was to unify the package ("py.test") and tool filename.
What's py.test's relation to ``nosetests``?
py.test and nose_ share basic philosophy when it comes
to running Python tests. In fact,
with py.test-1.1.0 it is ever easier to run many test suites
that currently work with ``nosetests``. nose_ was created
as a clone of ``py.test`` when py.test was in the ``0.8`` release
cycle so some of the newer features_ introduced with py.test-1.0
and py.test-1.1 have no counterpart in nose_.
.. _nose:
.. _features: test/features.html
.. _apipkg:
What's this "magic" with py.test?
issues where people have used the term "magic" in the past:
* `py/`_ uses the apipkg_ mechanism for lazy-importing
and full control on what API you get when importing "import py".
* when an ``assert`` statement fails, py.test re-interprets the expression
to show intermediate values if a test fails. If your expression
has side effects the intermediate values may not be the same, obfuscating
the initial error (this is also explained at the command line if it happens).
``py.test --no-assert`` turns off assert re-intepretation.
Sidenote: it is good practise to avoid asserts with side effects.
.. _`py namespaces`: index.html
.. _`py/`:
Where does my ``py.test`` come/import from?
You can issue::
py.test --version
which tells you both version and import location of the tool.
function arguments, parametrized tests and setup
.. _funcargs: test/funcargs.html
Is using funcarg- versus xUnit-based setup a style question?
It depends. For simple applications or for people experienced
with nose_ or unittest-style test setup using `xUnit style setup`_
make some sense. For larger test suites, parametrized testing
or setup of complex test resources using funcargs_ is recommended.
Moreover, funcargs are ideal for writing advanced test support
code (like e.g. the monkeypatch_, the tmpdir_ or capture_ funcargs)
because the support code can register setup/teardown functions
in a managed class/module/function scope.
.. _monkeypatch: test/plugin/monkeypatch.html
.. _tmpdir: test/plugin/tmpdir.html
.. _capture: test/plugin/capture.html
.. _`xUnit style setup`: test/xunit_setup.html
.. _`pytest_nose`: test/plugin/nose.html
.. _`why pytest_pyfuncarg__ methods?`:
Why the ``pytest_funcarg__*`` name for funcarg factories?
When experimenting with funcargs an explicit registration mechanism
was considered. But lacking a good use case for this indirection and
flexibility we decided to go for `Convention over Configuration`_ and
allow to directly specify the factory. Besides removing the need
for an indirection it allows to "grep" for ``pytest_funcarg__MYARG``
and will safely find all factory functions for the ``MYARG`` function
argument. It helps to alleviate the de-coupling of function
argument usage and creation.
.. _`Convention over Configuration`:
Can I yield multiple values from a factory function?
There are two conceptual reasons why yielding from a factory function
is not possible:
* Calling factories for obtaining test function arguments
is part of setting up and running a test. At that
point it is not possible to add new test calls to
the test collection anymore.
* If multiple factories yielded values there would
be no natural place to determine the combination
policy - in real-world examples some combinations
often should not run.
Use the `pytest_generate_tests`_ hook to solve both issues
and implement the `parametrization scheme of your choice`_.
.. _`pytest_generate_tests`: test/funcargs.html#parametrizing-tests
.. _`parametrization scheme of your choice`:
py.test interaction with other packages
Issues with py.test, multiprocess and setuptools?
On windows the multiprocess package will instantiate sub processes
by pickling and thus implicitely re-import a lot of local modules.
Unfortuantely, setuptools-0.6.11 does not ``if __name__=='__main__'``
protect its generated command line script. This leads to infinite
recursion when running a test that instantiates Processes.
There are these workarounds:
* `install Distribute`_ as a drop-in replacement for setuptools
and install py.test
* `directly use a checkout`_ which avoids all setuptools/Distribute
If those options are not available to you, you may also manually
fix the script that is created by setuptools by inserting an
``if __name__ == '__main__'``. Or you can create a ""
script with this content and invoke that with the python version::
import py
if __name__ == '__main__':
.. _`directly use a checkout`: install.html#directly-use-a-checkout
.. _`install distribute`: