90 lines
3.1 KiB
ReStructuredText
90 lines
3.1 KiB
ReStructuredText
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Monkeypatching/mocking modules and environments
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================================================================
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.. currentmodule:: _pytest.monkeypatch
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Sometimes tests need to invoke functionality which depends
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on global settings or which invokes code which cannot be easily
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tested such as network access. The ``monkeypatch`` fixture
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helps you to safely set/delete an attribute, dictionary item or
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environment variable or to modify ``sys.path`` for importing.
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See the `monkeypatch blog post`_ for some introduction material
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and a discussion of its motivation.
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.. _`monkeypatch blog post`: http://tetamap.wordpress.com/2009/03/03/monkeypatching-in-unit-tests-done-right/
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Simple example: monkeypatching functions
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---------------------------------------------------
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If you want to pretend that ``os.expanduser`` returns a certain
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directory, you can use the :py:meth:`monkeypatch.setattr` method to
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patch this function before calling into a function which uses it::
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# content of test_module.py
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import os.path
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def getssh(): # pseudo application code
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return os.path.join(os.path.expanduser("~admin"), '.ssh')
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def test_mytest(monkeypatch):
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def mockreturn(path):
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return '/abc'
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monkeypatch.setattr(os.path, 'expanduser', mockreturn)
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x = getssh()
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assert x == '/abc/.ssh'
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Here our test function monkeypatches ``os.path.expanduser`` and
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then calls into a function that calls it. After the test function
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finishes the ``os.path.expanduser`` modification will be undone.
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example: preventing "requests" from remote operations
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------------------------------------------------------
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If you want to prevent the "requests" library from performing http
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requests in all your tests, you can do::
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# content of conftest.py
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import pytest
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@pytest.fixture(autouse=True)
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def no_requests(monkeypatch):
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monkeypatch.delattr("requests.sessions.Session.request")
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This autouse fixture will be executed for each test function and it
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will delete the method ``request.session.Session.request``
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so that any attempts within tests to create http requests will fail.
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.. note::
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Be advised that it is not recommended to patch builtin functions such as ``open``,
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``compile``, etc., because it might break pytest's internals. If that's
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unavoidable, passing ``--tb=native``, ``--assert=plain`` and ``--capture=no`` might
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help although there's no guarantee.
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.. note::
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Mind that patching ``stdlib`` functions and some third-party libraries used by pytest
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might break pytest itself, therefore in those cases it is recommended to use
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:meth:`MonkeyPatch.context` to limit the patching to the block you want tested:
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.. code-block:: python
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import functools
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def test_partial(monkeypatch):
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with monkeypatch.context() as m:
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m.setattr(functools, "partial", 3)
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assert functools.partial == 3
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See issue `#3290 <https://github.com/pytest-dev/pytest/issues/3290>`_ for details.
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.. currentmodule:: _pytest.monkeypatch
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API Reference
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-------------
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Consult the docs for the :class:`MonkeyPatch` class.
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