__vars__ special method
On top of this old proposition: https://bugs.python.org/issue13290 We could have a __vars__ method that would be called by vars() if defined. The use cases: 1. let vars() work with instances without __dict__; 2. hide some attributes from the public API. Example for 1: class C: __slots__ = 'eggs', 'spam' def __vars__(self): d = {} for attr in self.__slots__: if hasattr(self, attr): d[attr] = getattr(self, attr) return d Exemple for 2: class C: def __vars__(self): return {attr: value for attr, value in self.__dict__.items() if not attr.startswith('_')}
On Mon, Jan 22, 2018 at 09:20:16AM -0500, Yahya Abou 'Imran via Python-ideas wrote:
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 999 If vars() can return a modified copy of the namespace, that will break this functionality. -- Steve
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)." https://docs.python.org/3.6/library/functions.html#vars 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).
On Mon, Jan 22, 2018 at 09:20:16AM -0500, Yahya Abou 'Imran via Python-ideas wrote:
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 999 If vars() can return a modified copy of the namespace, that will break this functionality. -- Steve
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)." https://docs.python.org/3.6/library/functions.html#vars 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).
participants (2)
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Steven D'Aprano -
Yahya Abou 'Imran