Types and Class Membership Tests in VS .NET

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Types and Class Membership Tests
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When you create an instance of a class, the type of that instance is the class itselfTo test for membership in a class, use the built-in function isinstance(obj,cname)This
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Types and Class Membership Tests
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function returns True if an object, obj, belongs to the class cname or any class derived from cname Here s an example:
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class A(object): pass class B(A): pass class C(object): pass a = A() b = B() c = C() type(a) isinstance(a,A) isinstance(b,A) isinstance(b,C) # Instance of 'A' # Instance of 'B' # Instance of 'C' # # # # Returns Returns Returns Returns the class object A True True, B derives from A False, C not derived from A
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Similarly, the built-in function issubclass(A,B) returns True if the class A is a subclass of class B Here s an example:
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issubclass(B,A) issubclass(C,A) # Returns True # Returns False
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A subtle problem with type-checking of objects is that programmers often bypass inheritance and simply create objects that mimic the behavior of another object As an example, consider these two classes:
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class Foo(object): def spam(self,a,b): pass class FooProxy(object): def _ _init_ _(self,f): selff = f def spam(self,a,b): return selffspam(a,b)
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In this example, FooProxy is functionally identical to Foo It implements the same methods, and it even uses Foo underneath the coversYet, in the type system, FooProxy is different than Foo For example:
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f = Foo() g = FooProxy(f) isinstance(g, Foo) # Create a Foo # Create a FooProxy # Returns False
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If a program has been written to explicitly check for a Foo using isinstance(), then it certainly won t work with a FooProxy object However, this degree of strictness is often not exactly what you want Instead, it might make more sense to assert that an object can simply be used as Foo because it has the same interfaceTo do this, it is possible to define an object that redefines the behavior of isinstance() and issubclass() for the purpose of grouping objects together and type-checking Here is an example:
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class IClass(object): def _ _init_ _(self): selfimplementors = set() def register(self,C): selfimplementorsadd(C) def _ _instancecheck_ _(self,x): return self_ _subclasscheck_ _(type(x))
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7 Classes and Object-Oriented Programming
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def _ _subclasscheck_ _(self,sub): return any(c in selfimplementors for c in submro()) # Now, use the above object IFoo = IClass() IFooregister(Foo) IFooregister(FooProxy)
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In this example, the class IClass creates an object that merely groups a collection of other classes together in a setThe register() method adds a new class to the setThe special method _ _instancecheck_ _() is called if anyone performs the operation isinstance(x, IClass)The special method _ _subclasscheck_ _() is called if the operation issubclass(C,IClass) is called By using the IFoo object and registered implementers, one can now perform type checks such as the following:
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f = Foo() # Create g = FooProxy(f) # Create isinstance(f, IFoo) # isinstance(g, IFoo) # issubclass(FooProxy, IFoo) # a Foo a FooProxy Returns True Returns True Returns True
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In this example, it s important to emphasize that no strong type-checking is occurring The IFoo object has overloaded the instance checking operations in a way that allows a you to assert that a class belongs to a group It doesn t assert any information on the actual programming interface, and no other verification actually occurs In fact, you can simply register any collection of objects you want to group together without regard to how those classes are related to each otherTypically, the grouping of classes is based on some criteria such as all classes implementing the same programming interface However, no such meaning should be inferred when overloading _ _instancecheck_ _() or _ _subclasscheck_ _()The actual interpretation is left up to the application Python provides a more formal mechanism for grouping objects, defining interfaces, and type-checkingThis is done by defining an abstract base class, which is defined in the next section
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Abstract Base Classes
In the last section, it was shown that the isinstance() and issubclass() operations can be overloadedThis can be used to create objects that group similar classes together and to perform various forms of type-checking Abstract base classes build upon this concept and provide a means for organizing objects into a hierarchy, making assertions about required methods, and so forth To define an abstract base class, you use the abc moduleThis module defines a metaclass (ABCMeta) and a set of decorators (@abstractmethod and @abstractproperty) that are used as follows:
from abc import ABCMeta, abstractmethod, abstractproperty class Foo: # In Python 3, you use the syntax _ _metaclass_ _ = ABCMeta # class Foo(metaclass=ABCMeta) @abstractmethod def spam(self,a,b): pass @abstractproperty