Re: [PATCH v3 07/15] devicetree: Add DeviceTree live tree representation

[David Gibson <david@gibson.dropbear.id.au>]

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On Fri, Nov 06, 2026 at 06:43:12PM -0800, Doug Anderson wrote:
> Hi David,
> 
> On Wed, Nov 4, 2026 at 4:44 AM David Gibson <david@gibson.dropbear.id.au> wrote:
> >
> > > +class Property(bytes):
> > > +    """Holds a device tree property name and value.
> > ...
> > > +    """
> > > +    def __new__(cls, name, value=b''):
> > > +        if not isinstance(value, (bytes, bytearray)):
> > > +            raise TypeError(f"Expected bytes or bytearray, got {type(value).__name__}")
> > > +        obj = super().__new__(cls, value)
> > > +        obj.name = name
> > > +        return obj
> >
> > Thanks!  I do wonder if we're doing this if we should update __eq_ and
> > __hash__ methods to take into account the property name.
> 
> I don't think we can include `self.name` in `Property.__eq__` and
> `Property.__hash__` without breaking `Property` comparisons against
> plain `bytes`:
> 
> 1. `fdt.getprop(node, name)` returns a `Property`, and existing callers
>    (including 10 assertions in `tests/pylibfdt_tests.py`, such as
>    `self.assertEqual(prop, b'test_tree1\0')`) compare `Property`
>    directly with `bytes`. If `Property.__eq__` required `other` to be a
>    `Property` with the same `.name`, `prop == b'...'` would evaluate to
>    `False`.
> 
> 2. If `Property.__eq__` only compared `self.name == other.name` when
>    `isinstance(other, Property)` and still allowed `prop == b'...'` for
>    plain `bytes`, then:
>    - `==` would no longer be transitive (`Property('a', b'x') == b'x'`
>      and `Property('b', b'x') == b'x'`, yet
>      `Property('a', b'x') != Property('b', b'x')`).
>    - Python's data model requires `a == b` to imply `hash(a) == hash(b)`,
>      so `hash(Property('a', b'x'))` must equal `hash(b'x')` if
>      `Property('a', b'x') == b'x'` is `True`.
> 
> In `DeviceTree`, properties are stored in `node.props` keyed by
> `prop.name`, so `node1.props == node2.props` already compares both the
> property names and the byte values.
> 
> What do you think?

Good point, never mind.

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson

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