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[Python-Dev] Re: *very* revised proposal for interfaces
Gerald S. Williamsgsw@agere.com
Wed, 2 Oct 2002 08:59:03 -0400
John Williams wrote:> Here's the new version, in a no-frills stripped-down form, explained by> example....> class I(interface):> prop1 = "Docstring for property prop1."> # It would be good form to put an argument list here,> # but this method will never be called so it's not necessary.> def meth1(): "Docstring for method meth1."This is along the lines of what I was suggesting. Note thatyou can optionally do some argument list checking. You canalso check other properties but that requires you to checkagainst an instantiated object and indicate the presence ofthese properties when instantiating an interface object.> class C(object, I):...> assert I not in C.__bases__ # Interfaces are not really base classes!Isn't this assertion going to fail the way you showed it?Inheritance needn't be used at all for interfaces.> # Example of binding an existing class to an interface. Like creating> # an ordinary implementation, the bases are an interface and a regular> # class, but instead of creating a usable new class, it just associates> # an interface with a class and defines how the interface's methods are> # implmented. The magic class name __binding__ triggers this behavior.> # Instances of this wrapper class are created by calling interface I.I don't think you need any magic class names. If a classwants to present an interface, it can provide a derivedclass and/or surrogate if needed. For example:class MyDerivedInterface(MyClass): def interface_name(self,x): return MyClass.my_name(self,x,1)class MySurrogateInterface(object): def __init__(self,me): self.me = me def __getattr__(self,attr): return self.me.__getattribute__(attr) def interface_name(self,x): return self.me.my_name(x,1)Either one implements the interface.> d = D()> i = I(d) # Note i could really just be d here, not that it matters.But do interfaces really need to have this capabilitybuilt-in? If you want to present an interface that'sconstrained, you can do it using a derived class orsurrogate. I would view "d" as the correct thing touse in this example.-----One thing my simplified example did not show (thoughit was in my local versions) is the ability to haveimplemented_by() register the interface in additionto testing compliance. This requires something like"implementerClass.__implements__ += interfaceClass"(with the appropriate safety checks). This wouldallow a more "contractual" approach to interfaces,since you could check to see if a class has beenregistered as supporting an interface before tryingto use that interface.-----I think we're both saying the same thing--there isa core issue that can be addressed without languageextensions or similar things that complicate theimplementation. But I have a more minimalist view:don't impact anything if your class presents theinterface already. All you need is a simple meansto declare (and verify) that a class implements aninterface.-Jerry
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