Harden tests, fix doc/msg

src/_pytest/assertion/rewrite.py

@@ -880,12 +880,13 @@ class AssertionRewriter(ast.NodeVisitor):
 
     def warn_about_none_ast(self, node, module_path, lineno):
         """
-        Returns an ast issuing a warning if the value of node is `None`
-        This is used to warn the user when asserting a function that asserts internally.
-        See issue #3191 for more details
+        Returns an AST issuing a warning if the value of node is `None`.
+        This is used to warn the user when asserting a function that asserts
+        internally already.
+        See issue #3191 for more details.
         """
 
-        # using parse because it's different between py2 py3
+        # Using parse because it is different between py2 and py3.
         AST_NONE = ast.parse("None").body[0].value
         val_is_none = ast.Compare(node, [ast.Is()], [AST_NONE])
         send_warning = ast.parse(
@@ -893,7 +894,7 @@ class AssertionRewriter(ast.NodeVisitor):
 from _pytest.warning_types import PytestWarning
 from warnings import warn_explicit
 warn_explicit(
-    PytestWarning('assertion the value None, Please use "assert is None"'),
+    PytestWarning('asserting the value None, please use "assert is None"'),
     category=None,
     filename={filename!r},
     lineno={lineno},

testing/test_warnings.py

@@ -627,8 +627,8 @@ def test_removed_in_pytest4_warning_as_error(testdir, change_default):
 
 class TestAssertionWarnings:
     @staticmethod
-    def assert_result_warns(result):
-        result.stdout.fnmatch_lines(["*PytestWarning*"])
+    def assert_result_warns(result, msg):
+        result.stdout.fnmatch_lines(["*PytestWarning: %s*" % msg])
 
     def test_tuple_warning(self, testdir):
         testdir.makepyfile(
@@ -638,7 +638,9 @@ class TestAssertionWarnings:
             """
         )
         result = testdir.runpytest()
-        self.assert_result_warns(result)
+        self.assert_result_warns(
+            result, "assertion is always true, perhaps remove parentheses?"
+        )
 
     @staticmethod
     def create_file(testdir, return_none):
@@ -660,10 +662,11 @@ class TestAssertionWarnings:
     def test_none_function_warns(self, testdir):
         self.create_file(testdir, True)
         result = testdir.runpytest()
-        self.assert_result_warns(result)
+        self.assert_result_warns(
+            result, 'asserting the value None, please use "assert is None"'
+        )
 
     def test_assert_is_none_no_warn(self, testdir):
-        """Tests a more simple case of `test_none_function_warns` where `assert None` is explicitly called"""
         testdir.makepyfile(
             """
             def foo():