[Python-Dev] PEP 447: add type.locallookup (original) (raw)

Steve Dower Steve.Dower at microsoft.com
Fri Sep 13 06:26:06 CEST 2013

Last I checked, looking up in the instance dict us exactly what it does. Even the example you posted is doing that. And the only difference from getattribute is that it throws instead of following the MRO, which is intended to allow base classes (via super, and another call to this method) to dynamically respond to a getattr without the cooperation of subclasses.

Consider class A, which knows it has a method F, but will not create it until the first getattribute call. Now class B derives from A, and someone calls super(B).F(obj). Currently, super only looks in dict for F, which will fail to invoke A.getattribute. Because super is used to provide MRO traversal, it can't rely on B.getattribute to perform the traversal, so it currently has no choice.

The PEP was originally adding a special class method to provide a getattribute equivalent that would not traverse the MRO, so that super could use it and people can create dynamic classes that can act as base classes. I pointed out that this could be an instance method (thereby avoid automatic-classmethod magic) and implemented on a metaclass for the class behavior. Unless the PEP has changed recently, this is still an instance method that will look up members defined directly on the type and not on base classes.

There may still be valid questions to answer (such as, should overrides if this method on base classes be inherited), but whether it is a type/class method is no longer one of those.

Cheers, Steve

Sent from my Windows Phone

From: Steven D'Aprano<mailto:steve at pearwood.info> Sent: ‎9/‎12/‎2013 21:00 To: python-dev at python.org<mailto:python-dev at python.org> Subject: Re: [Python-Dev] PEP 447: add type.locallookup

On Fri, Sep 13, 2013 at 03:04:49AM +0000, Steve Dower wrote:

What about getlocalattribute or getattributenorecurse? Long, but this isn't going to be used often.

This has nothing to do with locals, nor does it have anything to do with recursion, so both those names are misleading.

Putting "type" or "class" in the name would be misleading. It's an instance method (that is most useful when implemented on a metaclass).

Regardless of whether it is an instance method or not, by default it performs the lookup on the type. Hence the C function _PyType_Lookup and hence my suggestion typelookup.

But I think that typelookup does describe quite well what the method does. It looks up on the type. The PEP is fairly clear on how this is supposed to work, e.g. the default type. method will look up in the class/type dict. PEP 447 includes an example of how you might implement this in Python:

class MetaType(type): def locallookup(cls, name): try: return cls.dict[name] except KeyError: raise AttributeError(name) from None

"local lookup" doesn't even come close to describing what the method does or why you would use it. It suggests something to do with locals, which is not the case. Neither does getattributenorecurse, which suggests looking up an attribute on an object without following the inheritance hierarchy, e.g. looking in the instance dict but not the class dict. So the complete opposite of what it actually does.

-- Steven

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