add env example to monkeypatch docs

doc/en/monkeypatch.rst

@@ -16,7 +16,7 @@ and a discussion of its motivation.
 
 
 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
@@ -38,8 +38,8 @@ 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.
 
-example: preventing "requests" from remote operations
-------------------------------------------------------
+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::
@@ -81,6 +81,68 @@ so that any attempts within tests to create http requests will fail.
     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::
+
+    # 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 EnvironmentError("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::
+
+    # 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(EnvironmentError):
+            _ = get_os_user_lower()
+
+This behavior can be be moved into ``fixture`` structures and shared across tests::
+
+    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(EnvironmentError):
+            _ = get_os_user_lower()
+
+
+
+
 .. currentmodule:: _pytest.monkeypatch
 
 API Reference