[Python-ideas] Shorthand syntax for get/set/delattr (was Re: Dict-like object with property access)

[Paul Moore]

On 5 February 2012 20:45, Simon Sapin <simon.sapin at kozea.fr> wrote:
> +1 on extending vars(). I like this idea much more than adding syntax.
>
> In this case, proxy(a).key() would be based on dir(a) (or something similar)
> and have the same (documented) limitations. I think this is acceptable, and
> the proxy object is still useful.

vars() and dir() do very different things:

>>> class A:
...   pass
...
>>> a = A()
>>> a.a = 1
>>> dir(a)
['__class__', '__delattr__', '__dict__', '__doc__', '__eq__',
'__format__', '__ge__', '__getattribute__', '__gt__', '__h
ash__', '__init__', '__le__', '__lt__', '__module__', '__ne__',
'__new__', '__reduce__', '__reduce_ex__', '__repr__', '_
_setattr__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__', 'a']
>>> vars(a)
{'a': 1}
>>>

In my view, following the spec of vars() is far more useful and
matches better the original requirement, which was to simulate
javascript's index/attribute duality. Methods (and even more so
special methods) don't really fit in here.

I'd argue for the definition:

proxy(obj)['a'] <=> obj.a
proxy(obj)['a' = val] <=> obj.a = val
del proxy(obj)['a'] <=> del obj.a
'a' in proxy(obj) <=> hasattr(obj, 'a')
proxy(obj).keys() <=> vars(obj).keys()
len(proxy(obj)) <=> len(vars(obj))

In other words, indexing defers to getattr/setattr/delattr,
containment uses hasattr, but anything else goes via vars. In terms of
ABCs, Sized/Iterable behaviour comes from vars(),
Container/Mapping/MutableMapping behaviour comes from
{has,get,set,del}attr.

It's mildly inconsistent for objects which implement their own
attribute access, but those aren't the key use case, and the behaviour
is well defined even for those.

Paul