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

Migrated issue, originally created by Markus Meskanen (@Mahi)

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.

[sqlalchemy-bot]

Michael Bayer (@zzzeek) wrote:

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

→ caeb274e287f

[sqlalchemy-bot]

Changes by Michael Bayer (@zzzeek):

• changed status to closed