Prevent a recursion error to happen when inferring the declared metac…

…lass of a class

Close #749

ChangeLog

@@ -10,6 +10,10 @@ Release Date: TBA
 
   Close PyCQA/pylint#3417
 
+* Prevent a recursion error to happen when inferring the declared metaclass of a class
+
+  Close #749
+
 * Raise ``AttributeInferenceError`` when ``getattr()`` receives an empty name
 
   Close PyCQA/pylint#2991

astroid/scoped_nodes.py

@@ -2456,7 +2456,8 @@ def _metaclass_lookup_attribute(self, name, context):
         """Search the given name in the implicit and the explicit metaclass."""
         attrs = set()
         implicit_meta = self.implicit_metaclass()
-        metaclass = self.metaclass()
+        context = contextmod.copy_context(context)
+        metaclass = self.metaclass(context=context)
         for cls in {implicit_meta, metaclass}:
             if cls and cls != self and isinstance(cls, ClassDef):
                 cls_attributes = self._get_attribute_from_metaclass(cls, name, context)

tests/testdata/python3/data/metaclass_recursion/monkeypatch.py

@@ -0,0 +1,17 @@
+# https://github.com/PyCQA/astroid/issues/749
+# Not an actual module but allows us to reproduce the issue
+from tests.testdata.python3.data.metaclass_recursion import parent
+
+class MonkeyPatchClass(parent.OriginalClass):
+    _original_class = parent.OriginalClass
+
+    @classmethod
+    def patch(cls):
+        if parent.OriginalClass != MonkeyPatchClass:
+            cls._original_class = parent.OriginalClass
+            parent.OriginalClass = MonkeyPatchClass
+
+    @classmethod
+    def unpatch(cls):
+        if parent.OriginalClass == MonkeyPatchClass:
+            parent.OriginalClass = cls._original_class

tests/testdata/python3/data/metaclass_recursion/parent.py

@@ -0,0 +1,3 @@
+# https://github.com/PyCQA/astroid/issues/749
+class OriginalClass:
+    pass

tests/unittest_inference.py

@@ -5755,5 +5755,14 @@ def test(args):
     assert inferred is util.Uninferable
 
 
+def test_recursion_error_metaclass_monkeypatching():
+    module = resources.build_file(
+        "data/metaclass_recursion/monkeypatch.py", "data.metaclass_recursion"
+    )
+    cls = next(module.igetattr("MonkeyPatchClass"))
+    assert isinstance(cls, nodes.ClassDef)
+    assert cls.declared_metaclass() is None
+
+
 if __name__ == "__main__":
     unittest.main()