mailoutbox fixups (#427)

Make sure `mail.outbox == foo`, `mail.outbox != bar` and `iter(mail.outbox)` raises exceptions.

This commit also improves the readability of the provided assertion message by hiding the internal traceback and showing the documentation link.

pytest_django/plugin.py

@@ -402,9 +402,17 @@ def teardown():
 class _DirectMailboxAccessProtector(list):
 
     def _raise_assertion(*args, **kwargs):
-        raise AssertionError('To access mail.outbox, use the mailoutbox fixture.')
-
-    __len__ = __getitem__ = __nonzero__ = __bool__ = _raise_assertion
+        __tracebackhide__ = True
+        raise AssertionError('''To access mail.outbox, use the mailoutbox fixture.
+See http://pytest-django.readthedocs.io/en/latest/helpers.html#mailoutbox for more information.''')
+
+    __len__ = _raise_assertion
+    __getitem__ = _raise_assertion
+    __nonzero__ = _raise_assertion
+    __bool__ = _raise_assertion
+    __eq__ = _raise_assertion
+    __ne__ = _raise_assertion
+    __iter__ = _raise_assertion
 
 
 @pytest.fixture(autouse=True)

tests/test_environment.py

@@ -19,16 +19,37 @@
 # to do it.
 
 
-def test_direct_mailbox_access_not_allowed():
-    with pytest.raises(AssertionError):
-        len(mail.outbox)
+class Test_direct_mailbox_access_not_allowed():
 
-    with pytest.raises(AssertionError):
-        mail.outbox[0]
+    def test_len(self):
+        with pytest.raises(AssertionError):
+            len(mail.outbox)
 
-    with pytest.raises(AssertionError):
-        if mail.outbox:
-            pass
+    def test_indexing(self):
+        with pytest.raises(AssertionError):
+            mail.outbox[0]
+
+    def test_bool(self):
+        with pytest.raises(AssertionError):
+            if mail.outbox:
+                pass
+
+    def test_equality(self):
+        with pytest.raises(AssertionError):
+            mail.outbox == 'whatever'
+
+    def test_not_equality(self):
+        with pytest.raises(AssertionError):
+            mail.outbox != 'whatever'
+
+    def test_unpacking(self):
+        with pytest.raises(AssertionError):
+            (foo,) = mail.outbox
+
+    def test_iteration(self):
+        with pytest.raises(AssertionError):
+            for x in mail.outbox:
+                pass
 
 
 def test_direct_mailbox_proection_should_not_break_sending_mail():