[Python-ideas] __vars__ special method

Yahya Abou 'Imran yahya-abou-imran at protonmail.com
Thu Jan 25 06:11:24 EST 2018

> I think you may have misunderstood the purpose of vars(). It isn't to be
> a slightly different version of dir(), instead vars() should return the
> object's namespace. Not a copy of the namespace, but the actual
> namespace used by the object.
> This is how vars() currently works:
> py> class X:
>...     pass
> ...
> py> obj = X()
>py> ns = vars(obj)
>py> ns['spam'] = 999
>py> obj.spam
> If vars() can return a modified copy of the namespace, that will break
> this functionality.

This is not always true, e.g. for classes vars() returns a mappingproxy.

>From Doc:

"Objects such as modules and instances have an updateable __dict__ attribute; however, other objects may have write restrictions on their __dict__ attributes (for example, classes use a types.MappingProxyType to prevent direct dictionary updates)."


But you're right: it's misleading to return a RW mapping wich is a fake namespace...
In the above examples you could just return a mappingproxy.

If you want to support this feature you could use composition:

class C:
    def __init__(self):
        self.publicns = {}  # or: self.proxyattr = MyProxyClass()
    def __vars__(self):
        return self.publicns  # or: return self.proxyattr.__dict__

This namespace will be updateable, and it let you distinguish between the namespace you want to expose to your clients without compromosing the real one.
Of course, the real one could always be accessible via __dict__ (if present).

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