python twisted zope.interface

========== Interfaces ========== Interfaces are objects that specify (document) the external behavior of objects that "provide" them. An interface specifies behavior through: - Informal documentation in a doc string - Attribute definitions - Invariants, which are conditions that must hold for objects that provide the interface Attribute definitions specify specific attributes. They define the attribute name and provide documentation and constraints of attribute values. Attribute definitions can take a number of forms, as we'll see below. Defining interfaces =================== Interfaces are defined using Python class statements:: >>> import zope.interface >>> class IFoo(zope.interface.Interface): ... """Foo blah blah""" ... ... x = zope.interface.Attribute("""X blah blah""") ... ... def bar(q, r=None): ... """bar blah blah""" In the example above, we've created an interface, `IFoo`. We subclassed `zope.interface.Interface`, which is an ancestor interface for all interfaces, much as `object` is an ancestor of all new-style classes [#create]_. The interface is not a class, it's an Interface, an instance of `InterfaceClass`:: >>> type(IFoo) <class 'zope.interface.interface.InterfaceClass'> We can ask for the interface's documentation:: >>> IFoo.__doc__ 'Foo blah blah' and its name:: >>> IFoo.__name__ 'IFoo' and even its module:: >>> IFoo.__module__ '__main__' The interface defined two attributes: `x` This is the simplest form of attribute definition. It has a name and a doc string. It doesn't formally specify anything else. `bar` This is a method. A method is defined via a function definition. A method is simply an attribute constrained to be a callable with a particular signature, as provided by the function definition. Note that `bar` doesn't take a `self` argument. Interfaces document how an object is *used*. When calling instance methods, you don't pass a `self` argument, so a `self` argument isn't included in the interface signature. The `self` argument in instance methods is really an implementation detail of Python instances. Other objects, besides instances can provide interfaces and their methods might not be instance methods. For example, modules can provide interfaces and their methods are usually just functions. Even instances can have methods that are not instance methods. You can access the attributes defined by an interface using mapping syntax:: >>> x = IFoo['x'] >>> type(x) <class 'zope.interface.interface.Attribute'> >>> x.__name__ 'x' >>> x.__doc__ 'X blah blah' >>> IFoo.get('x').__name__ 'x' >>> IFoo.get('y') You can use `in` to determine if an interface defines a name:: >>> 'x' in IFoo True You can iterate over interfaces to get the names they define:: >>> names = list(IFoo) >>> names.sort() >>> names ['bar', 'x'] Remember that interfaces aren't classes. You can't access attribute definitions as attributes of interfaces:: >>> IFoo.x Traceback (most recent call last): File "<stdin>", line 1, in ? AttributeError: 'InterfaceClass' object has no attribute 'x' Methods provide access to the method signature:: >>> bar = IFoo['bar'] >>> bar.getSignatureString() '(q, r=None)' TODO Methods really should have a better API. This is something that needs to be improved. Declaring interfaces ==================== Having defined interfaces, we can *declare* that objects provide them. Before we describe the details, lets define some terms: *provide* We say that objects *provide* interfaces. If an object provides an interface, then the interface specifies the behavior of the object. In other words, interfaces specify the behavior of the objects that provide them. *implement* We normally say that classes *implement* interfaces. If a class implements an interface, then the instances of the class provide the interface. Objects provide interfaces that their classes implement [#factory]_. (Objects can provide interfaces directly, in addition to what their classes implement.) It is important to note that classes don't usually provide the interfaces that they implement. We can generalize this to factories. For any callable object we can declare that it produces objects that provide some interfaces by saying that the factory implements the interfaces. Now that we've defined these terms, we can talk about the API for declaring interfaces. Declaring implemented interfaces -------------------------------- The most common way to declare interfaces is using the implements function in a class statement:: >>> class Foo: ... zope.interface.implements(IFoo) ... ... def __init__(self, x=None): ... self.x = x ... ... def bar(self, q, r=None): ... return q, r, self.x ... ... def __repr__(self): ... return "Foo(%s)" % self.x In this example, we declared that `Foo` implements `IFoo`. This means that instances of `Foo` provide `IFoo`. Having made this declaration, there are several ways we can introspect the declarations. First, we can ask an interface whether it is implemented by a class:: >>> IFoo.implementedBy(Foo) True And we can ask whether an interface is provided by an object:: >>> foo = Foo() >>> IFoo.providedBy(foo) True Of course, `Foo` doesn't provide `IFoo`, it implements it:: >>> IFoo.providedBy(Foo) False We can also ask what interfaces are implemented by an object:: >>> list(zope.interface.implementedBy(Foo)) [<InterfaceClass __main__.IFoo>] It's an error to ask for interfaces implemented by a non-callable object:: >>> IFoo.implementedBy(foo) Traceback (most recent call last): ... TypeError: ('ImplementedBy called for non-factory', Foo(None)) >>> list(zope.interface.implementedBy(foo)) Traceback (most recent call last): ... TypeError: ('ImplementedBy called for non-factory', Foo(None)) Similarly, we can ask what interfaces are provided by an object:: >>> list(zope.interface.providedBy(foo)) [<InterfaceClass __main__.IFoo>] >>> list(zope.interface.providedBy(Foo)) [] We can declare interfaces implemented by other factories (besides classes). We do this using a Python-2.4-style decorator named `implementer`. In versions of Python before 2.4, this looks like:: >>> def yfoo(y): ... foo = Foo() ... foo.y = y ... return foo >>> yfoo = zope.interface.implementer(IFoo)(yfoo) >>> list(zope.interface.implementedBy(yfoo)) [<InterfaceClass __main__.IFoo>] Note that the implementer decorator may modify it's argument. Callers should not assume that a new object is created. Using implementer also works on callable objects. This is used by zope.formlib, as an example. >>> class yfactory: ... def __call__(self, y): ... foo = Foo() ... foo.y = y ... return foo >>> yfoo = yfactory() >>> yfoo = zope.interface.implementer(IFoo)(yfoo) >>> list(zope.interface.implementedBy(yfoo)) [<InterfaceClass __main__.IFoo>] XXX: Double check and update these version numbers: In zope.interface 3.5.2 and lower, the implementor decorator can not be used for classes, but in 3.6.0 and higher it can: >>> Foo = zope.interface.implementer(IFoo)(Foo) >>> list(zope.interface.providedBy(Foo())) [<InterfaceClass __main__.IFoo>] Note that class decorators using the @implementor(IFoo) syntax are only supported in Python 2.6 and later. Declaring provided interfaces ----------------------------- We can declare interfaces directly provided by objects. Suppose that we want to document what the `__init__` method of the `Foo` class does. It's not *really* part of `IFoo`. You wouldn't normally call the `__init__` method on Foo instances. Rather, the `__init__` method is part of the `Foo`'s `__call__` method:: >>> class IFooFactory(zope.interface.Interface): ..