test_ok1/doc/monkeypatch.txt

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Monkeypatching/mocking modules and environments
================================================================
.. currentmodule:: _pytest.monkeypatch
Sometimes tests need to invoke functionality which depends
on global settings or which invokes code which cannot be easily
tested such as network access. The ``monkeypatch`` function argument
helps you to safely set/delete an attribute, dictionary item or
environment variable or to modify ``sys.path`` for importing.
See the `monkeypatch blog post`_ for some introduction material
and a discussion of its motivation.
.. _`monkeypatch blog post`: http://tetamap.wordpress.com/2009/03/03/monkeypatching-in-unit-tests-done-right/
Simple example: monkeypatching functions
---------------------------------------------------
If you want to pretend that ``os.expanduser`` returns a certain
directory, you can use the :py:meth:`monkeypatch.setattr` method to
patch this function before calling into a function which uses it::
# content of test_module.py
import os.path
def getssh(): # pseudo application code
return os.path.join(os.path.expanduser("~admin"), '.ssh')
def test_mytest(monkeypatch):
def mockreturn(path):
return '/abc'
monkeypatch.setattr(os.path, 'expanduser', mockreturn)
x = getssh()
assert x == '/abc/.ssh'
Here our test function monkeypatches ``os.path.expanduser`` and
then calls into an function that calls it. After the test function
finishes the ``os.path.expanduser`` modification will be undone.
Method reference of the monkeypatch function argument
-----------------------------------------------------
.. autoclass:: monkeypatch
:members: setattr, delattr, setitem, delitem, setenv, delenv, syspath_prepend, chdir, undo
``monkeypatch.setattr/delattr/delitem/delenv()`` all
by default raise an Exception if the target does not exist.
Pass ``raising=False`` if you want to skip this check.