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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.
example: preventing "requests" from remote operations
------------------------------------------------------
If you want to prevent the "requests" library from performing http
requests in all your tests, you can do::
# content of conftest.py
import pytest
@pytest.fixture(autouse=True)
def no_requests(monkeypatch):
monkeypatch.delattr("requests.sessions.Session.request")
This autouse fixture will be executed for each test function and it
will delete the method ``request.session.Session.request``
so that any attempts within tests to create http requests will fail.
example: setting an attribute on some class
------------------------------------------------------
If you need to patch out ``os.getcwd()`` to return an artificial
value::
def test_some_interaction(monkeypatch):
monkeypatch.setattr("os.getcwd", lambda: "/")
which is equivalent to the long form::
def test_some_interaction(monkeypatch):
import os
monkeypatch.setattr(os, "getcwd", lambda: "/")
Method reference of the monkeypatch function argument
-----------------------------------------------------
.. autoclass:: monkeypatch
:members: setattr, replace, 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.
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