Respect PYTEST_DONT_REWRITE for plugins too.

_pytest/assertion/rewrite.py

@@ -179,10 +179,11 @@ class AssertionRewritingHook(object):
         The named module or package as well as any nested modules will
         be rewritten on import.
         """
-        already_imported = set(names).intersection(set(sys.modules))
+        already_imported = (
-        if already_imported:
+            (set(names) & set(sys.modules)) - set(self._rewritten_names))
         for name in already_imported:
-                if name not in self._rewritten_names:
+            if not AssertionRewriter.is_rewrite_disabled(
+                    sys.modules[name].__doc__ or ""):
                 self._warn_already_imported(name)
         self._must_rewrite.update(names)
 
@@ -635,7 +636,8 @@ class AssertionRewriter(ast.NodeVisitor):
                       not isinstance(field, ast.expr)):
                     nodes.append(field)
 
-    def is_rewrite_disabled(self, docstring):
+    @staticmethod
+    def is_rewrite_disabled(docstring):
         return "PYTEST_DONT_REWRITE" in docstring
 
     def variable(self):

_pytest/pytester.py

@@ -347,7 +347,7 @@ class RunResult:
     :stdout: :py:class:`LineMatcher` of stdout, use ``stdout.str()`` to
        reconstruct stdout or the commonly used ``stdout.fnmatch_lines()``
        method
-    :stderrr: :py:class:`LineMatcher` of stderr
+    :stderr: :py:class:`LineMatcher` of stderr
     :duration: duration in seconds
 
     """

changelog/2995.bugfix

@@ -0,0 +1,2 @@
+PYTEST_DONT_REWRITE is now checked for plugins too (rather than only for
+test modules).

testing/test_assertrewrite.py

@@ -128,6 +128,16 @@ class TestAssertionRewrite(object):
             assert len(m.body) == 1
         assert m.body[0].msg is None
 
+    def test_dont_rewrite_plugin(self, testdir):
+        contents = {
+            "conftest.py": "pytest_plugins = 'plugin'; import plugin",
+            "plugin.py": "'PYTEST_DONT_REWRITE'",
+            "test_foo.py": "def test_foo(): pass",
+        }
+        testdir.makepyfile(**contents)
+        result = testdir.runpytest_subprocess()
+        assert "warnings" not in "".join(result.outlines)
+
     def test_name(self):
         def f():
             assert False