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• What is Ruby's analog to Python Metaclasses?

  - by Sean Copenhaver

  Python has the idea of metaclasses that, if I understand correctly, allow you to modify an object of a class at the moment of construction. You are not modifying the class, but instead the object that is to be created then initialized. Python (at least as of 3.0 I believe) also has the idea of class decorators. Again if I understand correctly, class decorators allow the modifying of the class definition at the moment it is being declared. Now I believe there is an equivalent feature or features to the class decorator in Ruby, but I'm currently unaware of something equivalent to metaclasses. I'm sure you can easily pump any Ruby object through some functions and do what you will to it, but is there a feature in the language that sets that up like metaclasses do? So again, Does Ruby have something similar to Python's metaclasses? Edit I was off on the metaclasses for Python. A metaclass and a class decorator do very similar things it appears. They both modify the class when it is defined but in different manners. Hopefully a Python guru will come in and explain better on these features in Python. But a class or the parent of a class can implement a __new__(cls[,..]) function that does customize the construction of the object before it is initialized with __init__(self[,..]).

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• When do you use metaclasses?

  - by johannix

  Just started looking into metaclasses and while they seem powerful, I can think of other ways to accomplish the same type of thing. I was wondering when metaclasses have been found to be the right answer and why.

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• Implementing the factory design pattern using metaclasses in python

  - by noam

  I found a lot of links on metaclasses, and most of them mention that they are useful for implementing factory methods. Can you show me an example of using metaclasses to implement the design pattern?

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• Groovy MetaClass - Add category methods to appropriate metaClasses

  - by noah

  I have several categories that I use in my Grails plugin. e.g., class Foo { static foo(ClassA a,Object someArg) { ... } static bar(ClassB b,Object... someArgs) { ... } } I'm looking for the best way to add these methods to the meta-classes so that I don't have to use the category classes and can just invoke them as instance methods. e.g., aInstance.foo(someArg) bInstance.bar(someArgs) Is there a Groovy/Grails class or method that will help me do this or am I stuck iterating through the methods and adding them all myself?

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• methods of metaclasses on class instances.

  - by Stefano Borini

  I was wondering what happens to methods declared on a metaclass. I expected that if you declare a method on a metaclass, it will end up being a classmethod, however, the behavior is different. Example >>> class A(object): ... @classmethod ... def foo(cls): ... print "foo" ... >>> a=A() >>> a.foo() foo >>> A.foo() foo However, if I try to define a metaclass and give it a method foo, it seems to work the same for the class, not for the instance. >>> class Meta(type): ... def foo(self): ... print "foo" ... >>> class A(object): ... __metaclass__=Meta ... def __init__(self): ... print "hello" ... >>> >>> a=A() hello >>> A.foo() foo >>> a.foo() Traceback (most recent call last): File "<stdin>", line 1, in <module> AttributeError: 'A' object has no attribute 'foo' What's going on here exactly ? edit: bumping the question

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• Good real-world uses of metaclasses (e.g. in Python)

  - by Carles Barrobés

  I'm learning about metaclasses in Python. I think it is a very powerful technique, and I'm looking for good uses for them. I'd like some feedback of good useful real-world examples of using metaclasses. I'm not looking for example code on how to write a metaclass (there are plenty examples of useless metaclasses out there), but real examples where you have applied the technique and it was really the appropriate solution. The rule is: no theoretical possibilities, but metaclasses at work in a real application. I'll start with the one example I know: Django models, for declarative programming, where the base class Model uses a metaclass to fill the model objects of useful ORM functionality from the attribute definitions. Looking forward to your contributions.

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• What's the best way to create a static utility class in python? Is using metaclasses code smell?

  - by rsimp

  Ok so I need to create a bunch of utility classes in python. Normally I would just use a simple module for this but I need to be able to inherit in order to share common code between them. The common code needs to reference the state of the module using it so simple imports wouldn't work well. I don't like singletons, and classes that use the classmethod decorator do not have proper support for python properties. One pattern I see used a lot is creating an internal python class prefixed with an underscore and creating a single instance which is then explicitly imported or set as the module itself. This is also used by fabric to create a common environment object (fabric.api.env). I've realized another way to accomplish this would be with metaclasses. For example: #util.py class MetaFooBase(type): @property def file_path(cls): raise NotImplementedError def inherited_method(cls): print cls.file_path #foo.py from util import * import env class MetaFoo(MetaFooBase): @property def file_path(cls): return env.base_path + "relative/path" def another_class_method(cls): pass class Foo(object): __metaclass__ = MetaFoo #client.py from foo import Foo file_path = Foo.file_path I like this approach better than the first pattern for a few reasons: First, instantiating Foo would be meaningless as it has no attributes or methods, which insures this class acts like a true single interface utility, unlike the first pattern which relies on the underscore convention to dissuade client code from creating more instances of the internal class. Second, sub-classing MetaFoo in a different module wouldn't be as awkward because I wouldn't be importing a class with an underscore which is inherently going against its private naming convention. Third, this seems to be the closest approximation to a static class that exists in python, as all the meta code applies only to the class and not to its instances. This is shown by the common convention of using cls instead of self in the class methods. As well, the base class inherits from type instead of object which would prevent users from trying to use it as a base for other non-static classes. It's implementation as a static class is also apparent when using it by the naming convention Foo, as opposed to foo, which denotes a static class method is being used. As much as I think this is a good fit, I feel that others might feel its not pythonic because its not a sanctioned use for metaclasses which should be avoided 99% of the time. I also find most python devs tend to shy away from metaclasses which might affect code reuse/maintainability. Is this code considered code smell in the python community? I ask because I'm creating a pypi package, and would like to do everything I can to increase adoption.

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• What is the difference between type and type.__new__ in python?

  - by Jason Baker

  I was writing a metaclass and accidentally did it like this: class MetaCls(type): def __new__(cls, name, bases, dict): return type(name, bases, dict) ...instead of like this: class MetaCls(type): def __new__(cls, name, bases, dict): return type.__new__(cls, name, bases, dict) What exactly is the difference between these two metaclasses? And more specifically, what caused the first one to not work properly (some classes weren't called into by the metaclass)?

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• Python Frequently Asked Questions

  - by Casebash

  After seeing this suggestion for creating tag homepages/FAQs, I thought it'd be best to go on ahead and start collecting links to frequently asked questions to demonstrate how this would work. Object Oriented Metaclasses Missing features Enums Tools Available IDEs?

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• Delphi 2010: whatever happened to TRTTIConstructor?

  - by conciliator

  I've got two questions (of which at least one is regarding RTTI in D2010 and dynamic instancing) I was reading what appears to be the foils for a conference talk by Barry Kelly, and found on p. 13 something that looked really interesting: TRTTIConstructor.Invoke. In an adjacent bullet point, on finds "Dynamically construct instances without needing virtual constructors and metaclasses". This seems like a great feature (and exactly what I need, btw)! However, when I look in the D2010 docs (ms-help://embarcadero.rs2010/vcl/Rtti.html), I can't find it. Did it get revoked? What is the minimal way of creating an instance of a class, provided the class name is stored in a string?

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• Given an instance of a Ruby object, how do I get its metaclass?

  - by Stanislaus Wernstrom

  Normally, I might get the metaclass for a particular instance of a Ruby object with something like this: class C def metaclass class << self; self; end end end # This is this instance's metaclass. C.new.metaclass => #<Class:#<C:0x01234567>> # Successive invocations will have different metaclasses, # since they're different instances. C.new.metaclass => #<Class:#<C:0x01233...>> C.new.metaclass => #<Class:#<C:0x01232...>> C.new.metaclass => #<Class:#<C:0x01231...>> Let's say I just want to know the metaclass of an arbitrary object instance obj of an arbitrary class, and I don't want to define a metaclass (or similar) method on the class of obj. Is there a way to do that?

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• How to apply stereotypes on UML Relationships' MemberEnds?

  - by Cristi Potlog

  I'm running this code on a UML Class Diagram, and it works just fine, but when trying to apply stereotypes from PropertiesEditor in Visual Studio for relationship ends (FirstRole and SecondRole), the stereotypes combo doesn't load even if in code there seems to be applicable stereotypes valid for association properties. What should I put in metaclasses tag in the UML profile except for IProperty? <metaclassMoniker name="/MyUmlProfile/Microsoft.VisualStudio.Uml.Classes.IProperty"/> This is the code: using Microsoft.VisualStudio.Uml.Classes; foreach( IShape shape in currentDiagram.GetSelectedShapes<IElement>() ) { IElement element = shape.GetElement(); foreach( IStereotype stereotype in element.ApplicableStereotypes ) { if( element is Microsoft.VisualStudio.Uml.Classes.IClass ) { IClass classItem = (IClass)element; if( classItem.SuperClasses.Count() > 0 ) { if( stereotype.Name == "SubclassAttribute" ) { element.ApplyStereotype( stereotype ); } } else if( stereotype.Name == "ClassAttribute" ) { element.ApplyStereotype( stereotype ); } } else if( element is Microsoft.VisualStudio.Uml.Classes.IProperty ) { IProperty property = (IProperty)element; if( property.Association != null ) { if( stereotype.Name == "Set" && property.UpperValue != null && property.UpperValue.ToString() == "*" ) { element.ApplyStereotype( stereotype ); } else if( stereotype.Name == "ManyToOne" && ( property.UpperValue == null || property.UpperValue.ToString() == "1" ) ) { element.ApplyStereotype( stereotype ); } } else if( stereotype.Name == "Property" ) { element.ApplyStereotype( stereotype ); } } } }

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• Subclassed django models with integrated querysets

  - by outofculture

  Like in this question, except I want to be able to have querysets that return a mixed body of objects: >>> Product.objects.all() [<SimpleProduct: ...>, <OtherProduct: ...>, <BlueProduct: ...>, ...] I figured out that I can't just set Product.Meta.abstract to true or otherwise just OR together querysets of differing objects. Fine, but these are all subclasses of a common class, so if I leave their superclass as non-abstract I should be happy, so long as I can get its manager to return objects of the proper class. The query code in django does its thing, and just makes calls to Product(). Sounds easy enough, except it blows up when I override Product.__new__, I'm guessing because of the __metaclass__ in Model... Here's non-django code that behaves pretty much how I want it: class Top(object): _counter = 0 def __init__(self, arg): Top._counter += 1 print "Top#__init__(%s) called %d times" % (arg, Top._counter) class A(Top): def __new__(cls, *args, **kwargs): if cls is A and len(args) > 0: if args[0] is B.fav: return B(*args, **kwargs) elif args[0] is C.fav: return C(*args, **kwargs) else: print "PRETENDING TO BE ABSTRACT" return None # or raise? else: return super(A).__new__(cls, *args, **kwargs) class B(A): fav = 1 class C(A): fav = 2 A(0) # => None A(1) # => <B object> A(2) # => <C object> But that fails if I inherit from django.db.models.Model instead of object: File "/home/martin/beehive/apps/hello_world/models.py", line 50, in <module> A(0) TypeError: unbound method __new__() must be called with A instance as first argument (got ModelBase instance instead) Which is a notably crappy backtrace; I can't step into the frame of my __new__ code in the debugger, either. I have variously tried super(A, cls), Top, super(A, A), and all of the above in combination with passing cls in as the first argument to __new__, all to no avail. Why is this kicking me so hard? Do I have to figure out django's metaclasses to be able to fix this or is there a better way to accomplish my ends?

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