Turn plain classes without bases into new-style classes with language_level=3 (GH-3530)

Cython/Compiler/ExprNodes.py

@@ -9005,9 +9005,11 @@ class Py3ClassNode(ExprNode):
     #  class_def_node  PyClassDefNode  PyClassDefNode defining this class
     #  calculate_metaclass  bool       should call CalculateMetaclass()
     #  allow_py2_metaclass  bool       should look for Py2 metaclass
+    #  force_type           bool       always create a "new style" class, even with no bases
 
     subexprs = []
     type = py_object_type
+    force_type = False
     is_temp = True
 
     def infer_type(self, env):
@@ -9029,6 +9031,8 @@ class Py3ClassNode(ExprNode):
         mkw = class_def_node.mkw.py_result() if class_def_node.mkw else 'NULL'
         if class_def_node.metaclass:
             metaclass = class_def_node.metaclass.py_result()
+        elif self.force_type:
+            metaclass = "((PyObject*)&PyType_Type)"
         else:
             metaclass = "((PyObject*)&__Pyx_DefaultClassType)"
         code.putln(

Cython/Compiler/Nodes.py

@@ -4690,7 +4690,9 @@ class PyClassDefNode(ClassDefNode):
             self.classobj = ExprNodes.Py3ClassNode(
                 pos, name=name, class_def_node=self, doc=doc_node,
                 calculate_metaclass=needs_metaclass_calculation,
-                allow_py2_metaclass=allow_py2_metaclass)
+                allow_py2_metaclass=allow_py2_metaclass,
+                force_type=force_py3_semantics,
+            )
         else:
             # no bases, no metaclass => old style class creation
             self.dict = ExprNodes.DictNode(pos, key_value_pairs=[])

docs/src/userguide/migrating_to_cy30.rst

@@ -29,6 +29,9 @@ Further semantic changes due to the language level include:
 
 * ``/``-division uses the true (float) division operator, unless ``cdivision`` is enabled.
 * ``print`` is a function, not a statement.
+* Python classes that are defined without bases (``class C: ...``) are "new-style"
+  classes also in Py2.x (if you never heard about "old-style classes", you're probably
+  happy without them).
 * Annotations (type hints) are now stored as strings.
   (`PEP 563 <https://github.com/cython/cython/issues/2863>`_)
 * ``StopIteration`` handling in generators has been changed according to

tests/run/cdef_multiple_inheritance_errors.srctree

@@ -48,6 +48,7 @@ cdef class X(Base, Py):
     pass
 
 ######## oldstyle.pyx ########
+# cython: language_level=2
 
 cdef class Base:
     cdef dict __dict__

tests/run/cython3.pyx

@@ -30,6 +30,18 @@ def locals_function(a, b=2):
     return locals()
 
 
+### "new style" classes
+
+class T:
+    """
+    >>> t = T()
+    >>> isinstance(t, T)
+    True
+    >>> isinstance(T, type)  # not a Py2 old style class!
+    True
+    """
+
+
 ### true division
 
 def truediv(x):