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Mocking system commands
=======================
.. currentmodule:: testpath
Mocking is a technique to replace parts of a system with interfaces that don't
do anything, but which your tests can check whether and how they were called.
The :mod:`unittest.mock` module in Python 3 lets you mock Python functions and
classes. The tools described here let you mock external commands.
Commands are mocked by creating a real file in a temporary directory which is
added to the :envvar:`PATH` environment variable, not by replacing Python
functions. So if you mock ``git``, and your Python code runs a shell script
which calls ``git``, it will be the mocked command that it runs.
By default, mocked commands record each call made to them, so that your test can
check these. Using the :class:`MockCommand` API, you can change what a mocked
command does.
.. note::
Mocking a command affects all running threads or coroutines in a program.
There's no way to mock a command for only the current thread/coroutine,
because it uses environment variables, which are global.
.. autofunction:: assert_calls
.. autoclass:: MockCommand
.. automethod:: fixed_output
.. automethod:: get_calls
.. automethod:: assert_called
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