
.. currentmodule:: testfixtures.popen

Testing use of the subprocess package
=====================================

When using the :mod:`subprocess` package there are two approaches to testing:

* Have your tests exercise the real processes being instantiated and used.

* Mock out use of the :mod:`subprocess` package and provide expected output
  while recording interactions with the package to make sure they are as
  expected.

While the first of these should be preferred, it means that you need to have all
the external software available everywhere you wish to run tests. Your tests
will also need to make sure any dependencies of that software on
an external environment are met. If that external software takes a long time to
run, your tests will also take a long time to run.

These challenges can often make the second approach more practical and can
be the more pragmatic approach when coupled with a mock that accurately
simulates the behaviour of a subprocess. :class:`~testfixtures.popen.MockPopen`
is an attempt to provide just such a mock.

.. note:: To use :class:`~testfixtures.popen.MockPopen`, you must have the
          :mod:`mock` package installed.


Example usage
-------------

As an example, suppose you have code such as the following that you need to
test:

.. literalinclude:: ../testfixtures/tests/test_popen_docs.py
   :lines: 4-12

Tests that exercises this code using :class:`~testfixtures.popen.MockPopen`
could be written as follows:

.. literalinclude:: ../testfixtures/tests/test_popen_docs.py
   :lines: 16-52


Passing input to processes
--------------------------

If your testing requires passing input to the subprocess, you can do so by
checking for the input passed to :meth:`~subprocess.Popen.communicate` method
when you check the calls on the mock as shown in this example:

.. literalinclude:: ../testfixtures/tests/test_popen_docs.py
   :pyobject: TestMyFunc.test_communicate_with_input
   :dedent: 4

.. note:: Accessing ``.stdin`` isn't current supported by this mock.


Reading from ``stdout`` and ``stderr``
--------------------------------------

The ``.stdout`` and ``.stderr`` attributes of the mock returned by
:class:`~testfixtures.popen.MockPopen` will be file-like objects as with
the real :class:`~subprocess.Popen` and can be read as shown in this example:

.. literalinclude:: ../testfixtures/tests/test_popen_docs.py
   :pyobject: TestMyFunc.test_read_from_stdout_and_stderr
   :dedent: 4

.. warning::

    While these streams behave a lot like the streams of a real
    :class:`~subprocess.Popen` object, they do not exhibit the deadlocking
    behaviour that can occur when the two streams are read as in the example
    above. Be very careful when reading ``.stdout`` and ``.stderr`` and
    consider using :class:`~subprocess.Popen.communicate` instead.

Specifying the return code
--------------------------

Often code will need to behave differently depending on the return code of the
launched process. Specifying a simulated response code, along with testing for
the correct usage of :meth:`~subprocess.Popen.wait`, can be seen in the
following example:

.. literalinclude:: ../testfixtures/tests/test_popen_docs.py
   :pyobject: TestMyFunc.test_wait_and_return_code
   :dedent: 4

Checking for signal sending
---------------------------

Calls to ``.send_signal()``, ``.terminate()`` and  ``.kill()`` are all recorded
by the mock returned by :class:`~testfixtures.popen.MockPopen`
but otherwise do nothing as shown in the following example, which doesn't
make sense for a real test of sub-process usage but does show how the mock
behaves:

.. literalinclude:: ../testfixtures/tests/test_popen_docs.py
   :pyobject: TestMyFunc.test_send_signal
   :dedent: 4

Polling a process
-----------------

The :meth:`~subprocess.Popen.poll` method is often used as part of a loop
in order to do other work while waiting for a sub-process to complete.
The mock returned by :class:`~testfixtures.popen.MockPopen` supports this
by allowing the ``.poll()`` method to be called a number of times before
the ``returncode`` is set using the ``poll_count`` parameter as shown in
the following example:

.. literalinclude:: ../testfixtures/tests/test_popen_docs.py
   :pyobject: TestMyFunc.test_poll_until_result
   :dedent: 4

Using default behaviour
-----------------------

If you're testing something that needs to make many calls to many different
commands that all behave the same, it can be tedious to specify the behaviour
of each with :class:`~MockPopen.set_command`. For this case, :class:`~MockPopen`
has the :class:`~MockPopen.set_default` method which can be used to set the
behaviour of any command that has not been specified with
:class:`~MockPopen.set_command` as shown in the
following example:

.. literalinclude:: ../testfixtures/tests/test_popen_docs.py
   :pyobject: TestMyFunc.test_default_behaviour
   :dedent: 4


