[Sqlalchemy-tickets] Issue #3102: TypeError: Cannot create a consistent method resolution order (MRO) for bases Comparator, TDComparator (zzzeek/sqlalchemy)

[Martijn v. O. <iss...@bi...>]

New issue 3102: TypeError: Cannot create a consistent method resolution order (MRO) for bases Comparator, TDComparator
https://bitbucket.org/zzzeek/sqlalchemy/issue/3102/typeerror-cannot-create-a-consistent

Martijn van Oosterhout:

I received the above error trying to generate query comparing a variant type. Test program:

=====
import sqlalchemy
from sqlalchemy.types import TypeDecorator
from sqlalchemy import Text, Column
from sqlalchemy.ext.declarative import declarative_base

class TestType(TypeDecorator):
    impl = Text

MyType = TestType().with_variant(Text, 'postgresql')

Base = declarative_base()

class TestTable(Base):
    __tablename__ = 'test'
    column = Column('test', MyType, primary_key=True)

print sqlalchemy.__version__
TestTable.column == None    # Dies
=====

Exception:
=====
0.9.6
Traceback (most recent call last):
  File "test.py", line 18, in <module>
    TestTable.column == None
  File "/tmp/test/local/lib/python2.7/site-packages/sqlalchemy/sql/operators.py", line 296, in __eq__
    return self.operate(eq, other)
  File "/tmp/test/local/lib/python2.7/site-packages/sqlalchemy/orm/attributes.py", line 171, in operate
    return op(self.comparator, *other, **kwargs)
  File "/tmp/test/local/lib/python2.7/site-packages/sqlalchemy/sql/operators.py", line 296, in __eq__
    return self.operate(eq, other)
  File "/tmp/test/local/lib/python2.7/site-packages/sqlalchemy/orm/properties.py", line 251, in operate
    return op(self.__clause_element__(), *other, **kwargs)
  File "/tmp/test/local/lib/python2.7/site-packages/sqlalchemy/sql/annotation.py", line 90, in __eq__
    return self.__element.__class__.__eq__(self, other)
  File "/tmp/test/local/lib/python2.7/site-packages/sqlalchemy/sql/operators.py", line 296, in __eq__
    return self.operate(eq, other)
  File "/tmp/test/local/lib/python2.7/site-packages/sqlalchemy/sql/elements.py", line 664, in operate
    return op(self.comparator, *other, **kwargs)
  File "/tmp/test/local/lib/python2.7/site-packages/sqlalchemy/util/langhelpers.py", line 712, in __get__
    obj.__dict__[self.__name__] = result = self.fget(obj)
  File "/tmp/test/local/lib/python2.7/site-packages/sqlalchemy/sql/elements.py", line 650, in comparator
    return self.type.comparator_factory(self)
  File "/tmp/test/local/lib/python2.7/site-packages/sqlalchemy/sql/type_api.py", line 635, in comparator_factory
    {})
TypeError: Cannot create a consistent method resolution
order (MRO) for bases Comparator, TDComparator
=====

I can fix it in type_api.py by reversing the order of the bases classes, but I'm not sure if that has any other effects.

=====
class TypeDecorator(TypeEngine):
    @property
    def comparator_factory(self):
        return type("TDComparator",
                    (self.impl.comparator_factory, TypeDecorator.Comparator,),  # <- Order reversed
                    {})
=====

Tested on 0.8.4 and 0.9.6.