[Sqlalchemy-tickets] Issue #2997: Mutable fails to track changes in certain conditions (zzzeek/sqlalchemy)

[Vsevolod S. <iss...@bi...>]

New issue 2997: Mutable fails to track changes in certain conditions
https://bitbucket.org/zzzeek/sqlalchemy/issue/2997/mutable-fails-to-track-changes-in-certain

Vsevolod Solovyov:

Hello, this is a code that illustrates my problem:

```
#!python
>>> product.attrdata
{'key': 1}
>>> product.attrdata = product.attrdata or {}
>>> product.attrdata['key'] = 20
>>> db_session.commit()
>>> db_session.refresh(product)
>>> product.attrdata
{'key': 1}

>>> product.attrdata['key'] = 20
>>> db_session.commit()
>>> db_session.refresh(product)
>>> product.attrdata
{'key': 20}
```

`product.attrdata` is a MutableDict. As a first step, I've found that `Mutable.changed()` fails:

```
#!python
    def changed(self):
        for parent, key in self._parents.items():
            flag_modified(parent, key)
```

Because `self._parents` are empty. That's because in `MutableBase.set()`, when I do `product.attrdata = product.attrdata`:

```
#!python
        def set(target, value, oldvalue, initiator):
            if not isinstance(value, cls):
                value = cls.coerce(key, value)
            if value is not None:
                value._parents[target.obj()] = key
            if isinstance(oldvalue, cls):
                oldvalue._parents.pop(target.obj(), None)
            return value
```

`value is oldvalue` and parent gets popped after it was set. If I switch these two if's, then `Mutable.changed()` will call `flag_modified()`, but it won't flag properly it anyway (I didn't dig much further and don't have a patch to fix a problem).

Is it a bug or I shouldn't do things like `product.attrdata = product.attrdata` in the first place?

SQLAlchemy version 0.9.3