[Sqlalchemy-tickets] Issue #3950: 1.1 regression due to TypeDecorator copy() from Column.copy(), interferes with mutable (zzzeek/sqlalchemy)

[Michael B. <iss...@bi...>]

New issue 3950: 1.1 regression due to TypeDecorator copy() from Column.copy(), interferes with mutable
https://bitbucket.org/zzzeek/sqlalchemy/issues/3950/11-regression-due-to-typedecorator-copy

Michael Bayer:

this is a bug that exists since sqlalchemy.ext.mutable started, however it only is apparent when the type object is copied.  The copy is uncommon except in the case of SchemaEventTarget, which TypeDecorator becomes in 1.1.

```
#!python

from sqlalchemy import *
from sqlalchemy.orm import *
from sqlalchemy.ext.declarative import declarative_base
import json
from sqlalchemy.ext import mutable

Base = declarative_base()


class MyType(TypeDecorator):
    impl = Text

    def process_bind_param(self, value, dialect):
        if value is not None:
            value = json.dumps(value)
        return value

    def process_result_value(self, value, dialect):
        if value is not None:
            value = json.loads(value)
        return value

data = Column(mutable.MutableDict.as_mutable(MyType))


class A(Base):
    __tablename__ = 'a'
    id = Column(Integer, primary_key=True)

    # works fine
    # data = data

    # fails in 1.1 because TypeDecorator is a SchemaType,
    # so Column copies the type, so the event we attached
    # to scan mapper columns can't identify that this is the "data"
    # we copied
    data = data.copy()  # as happens in declarative

e = create_engine("sqlite://", echo=True)
Base.metadata.create_all(e)

s = Session(e)

a1 = A(data={"foo": "bar"})
s.add(a1)
s.commit()

a1.data["foo"] = "bat"
s.commit()

assert a1.data["foo"] == "bat"

```