Merge pull request #2511 from nicoddemus/addfinalizer-docs

fixture docs: highlight difference between yield and addfinalizer methods

doc/en/fixture.rst

@@ -123,20 +123,13 @@ with a list of available function arguments.
     but is not anymore advertised as the primary means of declaring fixture
     functions.
 
-"Funcargs" a prime example of dependency injection
+Fixtures: a prime example of dependency injection
 ---------------------------------------------------
 
-When injecting fixtures to test functions, pytest-2.0 introduced the
+Fixtures allow test functions to easily receive and work
-term "funcargs" or "funcarg mechanism" which continues to be present
+against specific pre-initialized application objects without having
-also in docs today.  It now refers to the specific case of injecting
+to care about import/setup/cleanup details.
-fixture values as arguments to test functions.  With pytest-2.3 there are
+It's a prime example of `dependency injection`_ where fixture
-more possibilities to use fixtures but "funcargs" remain as the main way
-as they allow to directly state the dependencies of a test function.
-
-As the following examples show in more detail, funcargs allow test
-functions to easily receive and work against specific pre-initialized
-application objects without having to care about import/setup/cleanup
-details.  It's a prime example of `dependency injection`_ where fixture
 functions take the role of the *injector* and test functions are the
 *consumers* of fixture objects.
 
@@ -296,6 +289,9 @@ The ``smtp`` connection will be closed after the test finished execution
 because the ``smtp`` object automatically closes when
 the ``with`` statement ends.
 
+Note that if an exception happens during the *setup* code (before the ``yield`` keyword), the
+*teardown* code (after the ``yield``) will not be called.
+
 
 .. note::
     Prior to version 2.10, in order to use a ``yield`` statement to execute teardown code one
@@ -303,29 +299,51 @@ the ``with`` statement ends.
     fixtures can use ``yield`` directly so the ``yield_fixture`` decorator is no longer needed
     and considered deprecated.
 
-.. note::
-    As historical note, another way to write teardown code is
-    by accepting a ``request`` object into your fixture function and can call its
-    ``request.addfinalizer`` one or multiple times::
 
-        # content of conftest.py
+An alternative option for executing *teardown* code is to
+make use of the ``addfinalizer`` method of the `request-context`_ object to register
+finalization functions.
 
-        import smtplib
+Here's the ``smtp`` fixture changed to use ``addfinalizer`` for cleanup:
-        import pytest
 
-        @pytest.fixture(scope="module")
+.. code-block:: python
-        def smtp(request):
-            smtp = smtplib.SMTP("smtp.gmail.com")
-            def fin():
-                print ("teardown smtp")
-                smtp.close()
-            request.addfinalizer(fin)
-            return smtp  # provide the fixture value
 
-    The ``fin`` function will execute when the last test in the module has finished execution.
+    # content of conftest.py
+    import smtplib
+    import pytest
+
+    @pytest.fixture(scope="module")
+    def smtp(request):
+        smtp = smtplib.SMTP("smtp.gmail.com")
+        def fin():
+            print ("teardown smtp")
+            smtp.close()
+        request.addfinalizer(fin)
+        return smtp  # provide the fixture value
+
+
+Both ``yield`` and ``addfinalizer`` methods work similarly by calling their code after the test
+ends, but ``addfinalizer`` has two key differences over ``yield``:
+
+1. It is possible to register multiple finalizer functions.
+
+2. Finalizers will always be called regardless if the fixture *setup* code raises an exception.
+   This is handy to properly close all resources created by a fixture even if one of them
+   fails to be created/acquired::
+
+        @pytest.fixture
+        def equipments(request):
+            r = []
+            for port in ('C1', 'C3', 'C28'):
+                equip = connect(port)
+                request.addfinalizer(equip.disconnect)
+                r.append(equip)
+            return r
+
+   In the example above, if ``"C28"`` fails with an exception, ``"C1"`` and ``"C3"`` will still
+   be properly closed. Of course, if an exception happens before the finalize function is
+   registered then it will not be executed.
 
-    This method is still fully supported, but ``yield`` is recommended from 2.10 onward because
-    it is considered simpler and better describes the natural code flow.
 
 .. _`request-context`:
 
@@ -782,8 +800,8 @@ Autouse fixtures (xUnit setup on steroids)
 .. regendoc:wipe
 
 Occasionally, you may want to have fixtures get invoked automatically
-without a `usefixtures`_ or `funcargs`_ reference.   As a practical
+without declaring a function argument explicitly or a `usefixtures`_ decorator.
-example, suppose we have a database fixture which has a
+As a practical example, suppose we have a database fixture which has a
 begin/rollback/commit architecture and we want to automatically surround
 each test method by a transaction and a rollback.  Here is a dummy
 self-contained implementation of this idea::