Proper way to make a TypeDecorator around Enum?

[Nikolaj]

I'm on MySQL and trying to create a type that stores enums as
VARCHAR(255). However, the following code still issues an ENUM() DDL.
What am I doing wrong here?

class StringEnum(types.TypeDecorator):
impl = types.Enum

def __init__(self, *args, **kwargs):
super(StringEnum, self).__init__(*args, native_enum=False,
**kwargs)
self.length = 255

[Michael Bayer]

TypeDecorator doesn't affect the decorated type via subclassing, it affects it via the "impl" attribute, hence "decorates". So you say self.impl = Enum(...). You don't need to use TypeDecorator here if all you want to do is create an Enum with default arguments, just subclass Enum directly.

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[Nikolaj]

It makes sense to subclass the type directly if I'm not messing with
result or bind values. This works:

class StringEnum(types.Enum):

def __init__(self, *args, **kwargs):
super(StringEnum, self).__init__(*args, native_enum=False,
**kwargs)
self.length = 255

I'm still curious why the TypeDecorator I pasted previously doesn't
work though. The superconstructor does the self.impl =
self.__class__.impl(...) anyway so I figured this was the better
approach than calling self.impl = Enum(...).

[Michael Bayer]

On Aug 9, 2010, at 10:32 AM, Nikolaj wrote:

> It makes sense to subclass the type directly if I'm not messing with
> result or bind values. This works:
>
> class StringEnum(types.Enum):
> def __init__(self, *args, **kwargs):
> super(StringEnum, self).__init__(*args, native_enum=False,
> **kwargs)
> self.length = 255
>
> I'm still curious why the TypeDecorator I pasted previously doesn't
> work though. The superconstructor does the self.impl =
> self.__class__.impl(...) anyway so I figured this was the better
> approach than calling self.impl = Enum(...).

its pulling off the "types.Enum" callable from self.__class__ (which actually seems unnecessary here, self.impl should be fine), then calling that with the given arguments. So it should be setting up the native_enum=False part at least. The length attribute though would need to be set on impl. There's no reason we can't make "length" an actual argument of Enum, though, just to help with this kind of thing (I'm assuming that you're planning on adding additional elements to the enum in the future and you don't want to have to alter the column, correct ?).

[Nikolaj]

On Aug 9, 3:38 pm, Michael Bayer <mike...@zzzcomputing.com> wrote:

> its pulling off the "types.Enum" callable from self.__class__ (which actually seems unnecessary here, self.impl should be fine), then calling that with the given arguments.  So it should be setting up the native_enum=False part at least.   The length attribute though would need to be set on impl.        There's no reason we can't make "length" an actual argument of Enum, though, just to help with this kind of thing (I'm assuming that you're planning on adding additional elements to the enum in the future and you don't want to have to alter the column, correct ?).

Correct. About the self.length thing, I simply misread __getattr__ on
TypeDecorator as __setattr__. I've included my test below just for
completness' sake to show that everything works as expected.

from sqlalchemy import create_engine, Column, types
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import scoped_session, sessionmaker

engine = create_engine('sqlite:///:memory:', echo=True)
Base = declarative_base(bind=engine)
Session = scoped_session(sessionmaker(bind=engine))

class StringEnum(types.TypeDecorator):
impl = types.Enum

def __init__(self, *args, **kwargs):
super(StringEnum, self).__init__(*args, native_enum=False,
**kwargs)

self.impl.length = 255

class Item(Base):
__tablename__ = 'items'
name = Column(types.String, primary_key=True)
option = Column(StringEnum(('foo', 'bar')), nullable=True)

Base.metadata.create_all()