Merge ~cjwatson/launchpadlib:fakelaunchpad-methods-returning-None into launchpadlib:main

[Colin Watson]

Support fake methods that return None

`FakeLaunchpad` is useful for writing test suites for packages that use `launchpadlib`. When trying to do so, I noticed a problem: attempting to register a fake method that returns None caused a crash. Real Launchpad webservice methods may return None, so fix this.

I also ran into a couple of issues with this package's `pytest` setup, and fixed those in passing.

[Jürgen Gmach (jugmac00)]

review: Approve

diff --git a/NEWS.rst b/NEWS.rst
index 7d7b2f4..b3475eb 100644
--- a/NEWS.rst
+++ b/NEWS.rst
@@ -5,6 +5,8 @@ NEWS for launchpadlib
 1.11.0 (unreleased)
 ===================
 - Move the ``keyring`` dependency to a new ``keyring`` extra.
+- Support setting fake methods that return None on instances of
+  ``launchpadlib.testing.launchpad.FakeLaunchpad``.
 
 1.10.18 (2022-10-28)
 ====================
diff --git a/src/launchpadlib/testing/launchpad.py b/src/launchpadlib/testing/launchpad.py
index 9a8db06..a079da1 100644
--- a/src/launchpadlib/testing/launchpad.py
+++ b/src/launchpadlib/testing/launchpad.py
@@ -463,7 +463,7 @@ class FakeResource(object):
             isn't valid.
         """
         result = method(*args, **kwargs)
-        if name in self.special_methods:
+        if name in self.special_methods or result is None:
             return result
         else:
             return self._create_resource(self._resource_type, name, result)
diff --git a/src/launchpadlib/testing/tests/test_launchpad.py b/src/launchpadlib/testing/tests/test_launchpad.py
index c118e17..956fc06 100644
--- a/src/launchpadlib/testing/tests/test_launchpad.py
+++ b/src/launchpadlib/testing/tests/test_launchpad.py
@@ -246,6 +246,14 @@ class FakeLaunchpadTest(ResourcedTestCase):
         self.launchpad.me = dict(getBranches=lambda statuses: branches)
         self.assertRaises(IntegrityError, self.launchpad.me.getBranches, [])
 
+    def test_callable_object_return_None(self):
+        """
+        A fake method passes through a return value of None rather than
+        trying to create a L{FakeResource}.
+        """
+        self.launchpad.branches = dict(getByUniqueName=lambda name: None)
+        self.assertIsNone(self.launchpad.branches.getByUniqueName("foo"))
+
     def test_collection_property(self):
         """
         Sample collections can be set on L{FakeLaunchpad} instances.  They are
diff --git a/tox.ini b/tox.ini
index be0759a..d71e61d 100644
--- a/tox.ini
+++ b/tox.ini
@@ -6,7 +6,7 @@ envlist =
 deps =
     .[test,testing]
 commands =
-    coverage run -m pytest src/launchpadlib/tests {posargs}
+    coverage run -m pytest src {posargs}
 
 [testenv:lint]
 # necessary to build the woke linter