Documentation: expansion of Monkeypatch to include mocked classes and dictionaries

changelog/5315.doc.rst

@@ -0,0 +1 @@
+Expand docs on mocking classes and dictionaries with ``monkeypatch``.

doc/en/monkeypatch.rst

@@ -8,46 +8,215 @@ 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.
+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 :py:meth:`monkeypatch.syspath_prepend` to modify the system ``$PATH`` safely, and
+:py:meth:`monkeypatch.chdir` to change the context of the current working directory
+during a test.
+
 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::
+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")
 
-    # content of test_module.py
-    import os.path
-    def getssh(): # pseudo application code
-        return os.path.join(os.path.expanduser("~admin"), '.ssh')
+        # Application of the monkeypatch to replace Path.home
+        # with the behavior of mockreturn defined above.
+        monkeypatch.setattr(Path, "home", mockreturn)
 
-    def test_mytest(monkeypatch):
-        def mockreturn(path):
-            return '/abc'
-        monkeypatch.setattr(os.path, 'expanduser', mockreturn)
+        # Calling getssh() will use mockreturn in place of Path.home
+        # for this test with the monkeypatch.
         x = getssh()
-        assert x == '/abc/.ssh'
+        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.
 
-Here our test function monkeypatches ``os.path.expanduser`` and
-then calls into a function that calls it.  After the test function
-finishes the ``os.path.expanduser`` modification will be undone.
 
 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::
+requests in all your tests, you can do:
 
-    # content of conftest.py
+.. 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
@@ -85,7 +254,7 @@ 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
+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
@@ -131,6 +300,7 @@ 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
 
 
@@ -144,7 +314,7 @@ This behavior can be moved into ``fixture`` structures and shared across tests:
         monkeypatch.delenv("USER", raising=False)
 
 
-    # Notice the tests reference the fixtures for mocks
+    # notice the tests reference the fixtures for mocks
     def test_upper_to_lower(mock_env_user):
         assert get_os_user_lower() == "testinguser"
 
@@ -154,6 +324,112 @@ This behavior can be moved into ``fixture`` structures and shared across tests:
             _ = 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