test_ok2/doc/en/monkeypatch.rst

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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`` fixture
helps you to safely set/delete an attribute, dictionary item or
environment variable, or to modify ``sys.path`` for importing.
The ``monkeypatch`` fixture provides these helper methods for safely patching and mocking
functionality in tests:
.. code-block:: python
monkeypatch.setattr(obj, name, value, raising=True)
monkeypatch.delattr(obj, name, raising=True)
monkeypatch.setitem(mapping, name, value)
monkeypatch.delitem(obj, name, raising=True)
monkeypatch.setenv(name, value, prepend=False)
monkeypatch.delenv(name, raising=True)
monkeypatch.syspath_prepend(path)
monkeypatch.chdir(path)
All modifications will be undone after the requesting
test function or fixture has finished. The ``raising``
parameter determines if a ``KeyError`` or ``AttributeError``
will be raised if the target of the set/deletion operation does not exist.
Consider the following scenarios:
1. Modifying the behavior of a function or the property of a class for a test e.g.
there is an API call or database connection you will not make for a test but you know
what the expected output should be. Use :py:meth:`monkeypatch.setattr` to patch the
function or property with your desired testing behavior. This can include your own functions.
Use :py:meth:`monkeypatch.delattr` to remove the function or property for the test.
2. Modifying the values of dictionaries e.g. you have a global configuration that
you want to modify for certain test cases. Use :py:meth:`monkeypatch.setitem` to patch the
dictionary for the test. :py:meth:`monkeypatch.delitem` can be used to remove items.
3. Modifying environment variables for a test e.g. to test program behavior if an
environment variable is missing, or to set multiple values to a known variable.
:py:meth:`monkeypatch.setenv` and :py:meth:`monkeypatch.delenv` can be used for
these patches.
4. Use ``monkeypatch.setenv("PATH", value, prepend=os.pathsep)`` to modify ``$PATH``, and
:py:meth:`monkeypatch.chdir` to change the context of the current working directory
during a test.
5. Use py:meth:`monkeypatch.syspath_prepend` to modify ``sys.path`` which will also
call :py:meth:`pkg_resources.fixup_namespace_packages` and :py:meth:`importlib.invalidate_caches`.
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
----------------------------------------
Consider a scenario where you are working with user directories. In the context of
testing, you do not want your test to depend on the running user. ``monkeypatch``
can be used to patch functions dependent on the user to always return a
specific value.
In this example, :py:meth:`monkeypatch.setattr` is used to patch ``Path.home``
so that the known testing path ``Path("/abc")`` is always used when the test is run.
This removes any dependency on the running user for testing purposes.
:py:meth:`monkeypatch.setattr` must be called before the function which will use
the patched function is called.
After the test function finishes the ``Path.home`` modification will be undone.
.. code-block:: python
# contents of test_module.py with source code and the test
from pathlib import Path
def getssh():
"""Simple function to return expanded homedir ssh path."""
return Path.home() / ".ssh"
def test_getssh(monkeypatch):
# mocked return function to replace Path.home
# always return '/abc'
def mockreturn():
return Path("/abc")
# Application of the monkeypatch to replace Path.home
# with the behavior of mockreturn defined above.
monkeypatch.setattr(Path, "home", mockreturn)
# Calling getssh() will use mockreturn in place of Path.home
# for this test with the monkeypatch.
x = getssh()
assert x == Path("/abc/.ssh")
Monkeypatching returned objects: building mock classes
------------------------------------------------------
:py:meth:`monkeypatch.setattr` can be used in conjunction with classes to mock returned
objects from functions instead of values.
Imagine a simple function to take an API url and return the json response.
.. code-block:: python
# contents of app.py, a simple API retrieval example
import requests
def get_json(url):
"""Takes a URL, and returns the JSON."""
r = requests.get(url)
return r.json()
We need to mock ``r``, the returned response object for testing purposes.
The mock of ``r`` needs a ``.json()`` method which returns a dictionary.
This can be done in our test file by defining a class to represent ``r``.
.. code-block:: python
# contents of test_app.py, a simple test for our API retrieval
# import requests for the purposes of monkeypatching
import requests
# our app.py that includes the get_json() function
# this is the previous code block example
import app
# custom class to be the mock return value
# will override the requests.Response returned from requests.get
class MockResponse:
# mock json() method always returns a specific testing dictionary
@staticmethod
def json():
return {"mock_key": "mock_response"}
def test_get_json(monkeypatch):
# Any arguments may be passed and mock_get() will always return our
# mocked object, which only has the .json() method.
def mock_get(*args, **kwargs):
return MockResponse()
# apply the monkeypatch for requests.get to mock_get
monkeypatch.setattr(requests, "get", mock_get)
# app.get_json, which contains requests.get, uses the monkeypatch
result = app.get_json("https://fakeurl")
assert result["mock_key"] == "mock_response"
``monkeypatch`` applies the mock for ``requests.get`` with our ``mock_get`` function.
The ``mock_get`` function returns an instance of the ``MockResponse`` class, which
has a ``json()`` method defined to return a known testing dictionary and does not
require any outside API connection.
You can build the ``MockResponse`` class with the appropriate degree of complexity for
the scenario you are testing. For instance, it could include an ``ok`` property that
always returns ``True``, or return different values from the ``json()`` mocked method
based on input strings.
This mock can be shared across tests using a ``fixture``:
.. code-block:: python
# contents of test_app.py, a simple test for our API retrieval
import pytest
import requests
# app.py that includes the get_json() function
import app
# custom class to be the mock return value of requests.get()
class MockResponse:
@staticmethod
def json():
return {"mock_key": "mock_response"}
# monkeypatched requests.get moved to a fixture
@pytest.fixture
def mock_response(monkeypatch):
"""Requests.get() mocked to return {'mock_key':'mock_response'}."""
def mock_get(*args, **kwargs):
return MockResponse()
monkeypatch.setattr(requests, "get", mock_get)
# notice our test uses the custom fixture instead of monkeypatch directly
def test_get_json(mock_response):
result = app.get_json("https://fakeurl")
assert result["mock_key"] == "mock_response"
Furthermore, if the mock was designed to be applied to all tests, the ``fixture`` could
be moved to a ``conftest.py`` file and use the with ``autouse=True`` option.
Global patch 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:
.. code-block:: python
# contents of conftest.py
import pytest
@pytest.fixture(autouse=True)
def no_requests(monkeypatch):
"""Remove requests.sessions.Session.request for all tests."""
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.
.. note::
Be advised that it is not recommended to patch builtin functions such as ``open``,
``compile``, etc., because it might break pytest's internals. If that's
unavoidable, passing ``--tb=native``, ``--assert=plain`` and ``--capture=no`` might
help although there's no guarantee.
.. note::
Mind that patching ``stdlib`` functions and some third-party libraries used by pytest
might break pytest itself, therefore in those cases it is recommended to use
:meth:`MonkeyPatch.context` to limit the patching to the block you want tested:
.. code-block:: python
import functools
def test_partial(monkeypatch):
with monkeypatch.context() as m:
m.setattr(functools, "partial", 3)
assert functools.partial == 3
See issue `#3290 <https://github.com/pytest-dev/pytest/issues/3290>`_ for details.
Monkeypatching environment variables
------------------------------------
If you are working with environment variables you often need to safely change the values
or delete them from the system for testing purposes. ``monkeypatch`` provides a mechanism
to do this using the ``setenv`` and ``delenv`` method. Our example code to test:
.. code-block:: python
# contents of our original code file e.g. code.py
import os
def get_os_user_lower():
"""Simple retrieval function.
Returns lowercase USER or raises EnvironmentError."""
username = os.getenv("USER")
if username is None:
raise OSError("USER environment is not set.")
return username.lower()
There are two potential paths. First, the ``USER`` environment variable is set to a
value. Second, the ``USER`` environment variable does not exist. Using ``monkeypatch``
both paths can be safely tested without impacting the running environment:
.. code-block:: python
# contents of our test file e.g. test_code.py
import pytest
def test_upper_to_lower(monkeypatch):
"""Set the USER env var to assert the behavior."""
monkeypatch.setenv("USER", "TestingUser")
assert get_os_user_lower() == "testinguser"
def test_raise_exception(monkeypatch):
"""Remove the USER env var and assert EnvironmentError is raised."""
monkeypatch.delenv("USER", raising=False)
with pytest.raises(OSError):
_ = get_os_user_lower()
This behavior can be moved into ``fixture`` structures and shared across tests:
.. code-block:: python
# contents of our test file e.g. test_code.py
import pytest
@pytest.fixture
def mock_env_user(monkeypatch):
monkeypatch.setenv("USER", "TestingUser")
@pytest.fixture
def mock_env_missing(monkeypatch):
monkeypatch.delenv("USER", raising=False)
# notice the tests reference the fixtures for mocks
def test_upper_to_lower(mock_env_user):
assert get_os_user_lower() == "testinguser"
def test_raise_exception(mock_env_missing):
with pytest.raises(OSError):
_ = get_os_user_lower()
Monkeypatching dictionaries
---------------------------
:py:meth:`monkeypatch.setitem` can be used to safely set the values of dictionaries
to specific values during tests. Take this simplified connection string example:
.. code-block:: python
# contents of app.py to generate a simple connection string
DEFAULT_CONFIG = {"user": "user1", "database": "db1"}
def create_connection_string(config=None):
"""Creates a connection string from input or defaults."""
config = config or DEFAULT_CONFIG
return f"User Id={config['user']}; Location={config['database']};"
For testing purposes we can patch the ``DEFAULT_CONFIG`` dictionary to specific values.
.. code-block:: python
# contents of test_app.py
# app.py with the connection string function (prior code block)
import app
def test_connection(monkeypatch):
# Patch the values of DEFAULT_CONFIG to specific
# testing values only for this test.
monkeypatch.setitem(app.DEFAULT_CONFIG, "user", "test_user")
monkeypatch.setitem(app.DEFAULT_CONFIG, "database", "test_db")
# expected result based on the mocks
expected = "User Id=test_user; Location=test_db;"
# the test uses the monkeypatched dictionary settings
result = app.create_connection_string()
assert result == expected
You can use the :py:meth:`monkeypatch.delitem` to remove values.
.. code-block:: python
# contents of test_app.py
import pytest
# app.py with the connection string function
import app
def test_missing_user(monkeypatch):
# patch the DEFAULT_CONFIG t be missing the 'user' key
monkeypatch.delitem(app.DEFAULT_CONFIG, "user", raising=False)
# Key error expected because a config is not passed, and the
# default is now missing the 'user' entry.
with pytest.raises(KeyError):
_ = app.create_connection_string()
The modularity of fixtures gives you the flexibility to define
separate fixtures for each potential mock and reference them in the needed tests.
.. code-block:: python
# contents of test_app.py
import pytest
# app.py with the connection string function
import app
# all of the mocks are moved into separated fixtures
@pytest.fixture
def mock_test_user(monkeypatch):
"""Set the DEFAULT_CONFIG user to test_user."""
monkeypatch.setitem(app.DEFAULT_CONFIG, "user", "test_user")
@pytest.fixture
def mock_test_database(monkeypatch):
"""Set the DEFAULT_CONFIG database to test_db."""
monkeypatch.setitem(app.DEFAULT_CONFIG, "database", "test_db")
@pytest.fixture
def mock_missing_default_user(monkeypatch):
"""Remove the user key from DEFAULT_CONFIG"""
monkeypatch.delitem(app.DEFAULT_CONFIG, "user", raising=False)
# tests reference only the fixture mocks that are needed
def test_connection(mock_test_user, mock_test_database):
expected = "User Id=test_user; Location=test_db;"
result = app.create_connection_string()
assert result == expected
def test_missing_user(mock_missing_default_user):
with pytest.raises(KeyError):
_ = app.create_connection_string()
.. currentmodule:: _pytest.monkeypatch
API Reference
-------------
Consult the docs for the :class:`MonkeyPatch` class.