Hybrid decorators' "mutator decorators" actually mutate state, preventing specialising in a subclass

[Markus Meskanen]

As it stands, hybrid_method and hybrid_property both mutate their state with the modifier decorators (setter, deleter, expression, comparator) in the following manner:

class hybrid_property:
    ...

    def setter(self, fset):
        self.fset = fset
        return self

But this prevents me from specialising the property in a subclass:

class Foo(Model):
    first_name = Column(String)
    last_name = Column(String)

    @hybrid_property
    def name(self):
        return self.first_name + ' ' + self.last_name

    @name.setter
    def name(self, value):
        self.first_name, self.last_name = value.split(' ', maxsplit=1)


class Bar(Foo):

    @Foo.name.setter
    def name(self, value):
        if run_name_check(value) > 0:  # Error code received
            raise InvalidName
        super().name.fset(value)

Since that would result into modifying the original Foo.name:

>>> f = Foo(first_name='Markus', last_name='Meskanen')
>>> f.name = 'Markus^% Meskanen'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
__main__.InvalidName

The correct way to implement these mutator decorators is to have them return a copy instance instead:

class hybrid_property:
    ...
    def setter(self, fset):
        return type(self)(self.fget, fset, self.fdel, self.expr)

I would PR such fix myself, but I'm in a hurry and couldn't quicky figure out how the expressions and comparators work exactly, and wasn't sure what is the intended behavior after this suggested fix would be applied.

[Markus Meskanen]

For what it's worth, this is how Python's built-in property works too; it always returns a new instance:

>>> class Foo:
...
...   @property
...   def x(self):
...     pass
...
...   _x = x  # Copy a reference of the current property
...
...   @x.setter
...   def x(self, value):
...     pass
...
...   print(_x is x)
...   print(_x, x)
... 
False
<property object at 0x7fa243d599a8> <property object at 0x7fa243d59958>

[Mike Bayer]

I'm putting this as "enhancement" since I hadn't realized this use case originally, else I would have made it copy from the start. This would be bundled with #3911 and noted in changes to hybrids in the migration document. targeting for 1.2 but this may move out unless I can get contributions including tests.

[Markus Meskanen]

Yeah I guess it's a bit of both, but fair enough.

I've never contributed to SQLAlchemy before but would love to help, any ETA when 1.2 will be out? Got a bit busy schedule and will have to investigate the project further before contributing too much.

[Mike Bayer]

1.2 is a "big" release and I haven't really started organizing all the patches, it's at best for spring 2017 and probably later. for now I'd consider vendoring your own hybrid decorator as this is a simple extension.

[Mike Bayer]

Allow reuse of hybrid_property across subclasses

The :class:sqlalchemy.ext.hybrid.hybrid_property class now supports calling mutators like @setter, @expression etc. multiple times across subclasses, and now provides a @getter mutator, so that a particular hybrid can be repurposed across subclasses or other classes. This now matches the behavior of @property in standard Python.

Co-authored-by: Mike Bayer [e-mail] Fixes: #3911 Fixes: #3912 Change-Id: Iff033d8ccaae20ded9289cbfa789c376759381f5

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