Check the order for an enum's base classes.

Derived from https://docs.python.org/3/library/enum.html#restricted-enum-subclassing

It's not actually documented what a "data-type" is; from what I can glean from the library's source code, it's a class that defines its own `__new__` method. This is annoying, and I don't think it's very valuable to check it.

PiperOrigin-RevId: 398826650

pytype/errors.py

@@ -890,10 +890,11 @@ def invalid_function_call(self, stack, error):
       raise AssertionError(error)
 
   @_error_name("base-class-error")
-  def base_class_error(self, stack, base_var):
+  def base_class_error(self, stack, base_var, details=None):
     base_cls = self._join_printed_types(
         self._print_as_expected_type(t) for t in base_var.data)
-    self.error(stack, "Invalid base class: %s" % base_cls, keyword=base_cls)
+    self.error(stack, "Invalid base class: %s" % base_cls,
+               details=details, keyword=base_cls)
 
   @_error_name("bad-return-type")
   def bad_return_type(self, stack, node, formal, actual, bad):

pytype/overlays/enum_overlay.py

@@ -72,6 +72,18 @@ def make_class(self, node, name_var, bases, class_dict_var, cls_var,
     # errors. EnumMeta turns the class into a full enum, but that's too late for
     # proper error checking.
     # TODO(tsudol): Check enum validity.
+
+    # Enums have a specific ordering for base classes:
+    # https://docs.python.org/3/library/enum.html#restricted-enum-subclassing
+    # Mostly, we just care that the last base is some kind of Enum.
+    if not bases:
+      # This should be impossible.
+      bases = [self.to_variable(node)]
+    elif not any(b.is_enum for b in bases[-1].data):
+      msg = ("The last base class for an enum must be enum.Enum or a subclass "
+             "of enum.Enum")
+      self.vm.errorlog.base_class_error(self.vm.frames, bases[-1], details=msg)
+      return node, self.vm.new_unsolvable(node)
     cls_var = cls_var or self.vm.loaded_overlays["enum"].members["EnumMeta"]
     return self.vm.make_class(node, name_var, bases, class_dict_var, cls_var,
                               new_class_var, class_type=EnumInstance)

pytype/tests/test_enums.py

@@ -1048,5 +1048,12 @@ class NoFn(enum.Enum):
         foo.NoFn.x  # attribute-error
       """, pythonpath=[d.path])
 
+  def test_enum_bases(self):
+    self.CheckWithErrors("""
+      import enum
+      class BadBaseOrder(enum.Enum, int):  # base-class-error
+        A = 1
+    """)
+
 if __name__ == "__main__":
   test_base.main()