========== 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) 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) >>> 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 "", 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)) [] 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)) [] >>> 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)) [] Note that the implementer decorator may modify it's argument. Callers should not assume that a new object is created. Also note that, at least for now, implementer can't be used with classes:: >>> zope.interface.implementer(IFoo)(Foo) ... # doctest: +NORMALIZE_WHITESPACE Traceback (most recent call last): ... TypeError: Can't use implementer with classes. Use one of the class-declaration functions instead. 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): ... """Create foos""" ... ... def __call__(x=None): ... """Create a foo ... ... The argument provides the initial value for x ... ... """ It's the class that provides this interface, so we declare the interface on the class:: >>> zope.interface.directlyProvides(Foo, IFooFactory) And then, we'll see that Foo provides some interfaces:: >>> list(zope.interface.providedBy(Foo)) [] >>> IFooFactory.providedBy(Foo) True Declaring class interfaces is common enough that there's a special declaration function for it, `classProvides`, that allows the declaration from within a class statement:: >>> class Foo2: ... zope.interface.implements(IFoo) ... zope.interface.classProvides(IFooFactory) ... ... 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 >>> list(zope.interface.providedBy(Foo2)) [] >>> IFooFactory.providedBy(Foo2) True There's a similar function, `moduleProvides`, that supports interface declarations from within module definitions. For example, see the use of `moduleProvides` call in `zope.interface.__init__`, which declares that the package `zope.interface` provides `IInterfaceDeclaration`. Sometimes, we want to declare interfaces on instances, even though those instances get interfaces from their classes. Suppose we create a new interface, `ISpecial`:: >>> class ISpecial(zope.interface.Interface): ... reason = zope.interface.Attribute("Reason why we're special") ... def brag(): ... "Brag about being special" We can make an existing foo instance special by providing `reason` and `brag` attributes:: >>> foo.reason = 'I just am' >>> def brag(): ... return "I'm special!" >>> foo.brag = brag >>> foo.reason 'I just am' >>> foo.brag() "I'm special!" and by declaring the interface:: >>> zope.interface.directlyProvides(foo, ISpecial) then the new interface is included in the provided interfaces:: >>> ISpecial.providedBy(foo) True >>> list(zope.interface.providedBy(foo)) [, ] We can find out what interfaces are directly provided by an object:: >>> list(zope.interface.directlyProvidedBy(foo)) [] >>> newfoo = Foo() >>> list(zope.interface.directlyProvidedBy(newfoo)) [] Inherited declarations ---------------------- Normally, declarations are inherited:: >>> class SpecialFoo(Foo): ... zope.interface.implements(ISpecial) ... reason = 'I just am' ... def brag(self): ... return "I'm special because %s" % self.reason >>> list(zope.interface.implementedBy(SpecialFoo)) [, ] >>> list(zope.interface.providedBy(SpecialFoo())) [, ] Sometimes, you don't want to inherit declarations. In that case, you can use `implementsOnly`, instead of `implements`:: >>> class Special(Foo): ... zope.interface.implementsOnly(ISpecial) ... reason = 'I just am' ... def brag(self): ... return "I'm special because %s" % self.reason >>> list(zope.interface.implementedBy(Special)) [] >>> list(zope.interface.providedBy(Special())) [] External declarations --------------------- Normally, we make implementation declarations as part of a class definition. Sometimes, we may want to make declarations from outside the class definition. For example, we might want to declare interfaces for classes that we didn't write. The function `classImplements` can be used for this purpose:: >>> class C: ... pass >>> zope.interface.classImplements(C, IFoo) >>> list(zope.interface.implementedBy(C)) [] We can use `classImplementsOnly` to exclude inherited interfaces:: >>> class C(Foo): ... pass >>> zope.interface.classImplementsOnly(C, ISpecial) >>> list(zope.interface.implementedBy(C)) [] Declaration Objects ------------------- When we declare interfaces, we create *declaration* objects. When we query declarations, declaration objects are returned:: >>> type(zope.interface.implementedBy(Special)) Declaration objects and interface objects are similar in many ways. In fact, they share a common base class. The important thing to realize about them is that they can be used where interfaces are expected in declarations. Here's a silly example:: >>> class Special2(Foo): ... zope.interface.implementsOnly( ... zope.interface.implementedBy(Foo), ... ISpecial, ... ) ... reason = 'I just am' ... def brag(self): ... return "I'm special because %s" % self.reason The declaration here is almost the same as ``zope.interface.implements(ISpecial)``, except that the order of interfaces in the resulting declaration is different:: >>> list(zope.interface.implementedBy(Special2)) [, ] Interface Inheritance ===================== Interfaces can extend other interfaces. They do this simply by listing the other interfaces as base interfaces:: >>> class IBlat(zope.interface.Interface): ... """Blat blah blah""" ... ... y = zope.interface.Attribute("y blah blah") ... def eek(): ... """eek blah blah""" >>> IBlat.__bases__ (,) >>> class IBaz(IFoo, IBlat): ... """Baz blah""" ... def eek(a=1): ... """eek in baz blah""" ... >>> IBaz.__bases__ (, ) >>> names = list(IBaz) >>> names.sort() >>> names ['bar', 'eek', 'x', 'y'] Note that `IBaz` overrides eek:: >>> IBlat['eek'].__doc__ 'eek blah blah' >>> IBaz['eek'].__doc__ 'eek in baz blah' We were careful to override eek in a compatible way. When extending an interface, the extending interface should be compatible [#compat]_ with the extended interfaces. We can ask whether one interface extends another:: >>> IBaz.extends(IFoo) True >>> IBlat.extends(IFoo) False Note that interfaces don't extend themselves:: >>> IBaz.extends(IBaz) False Sometimes we wish they did, but we can, instead use `isOrExtends`:: >>> IBaz.isOrExtends(IBaz) True >>> IBaz.isOrExtends(IFoo) True >>> IFoo.isOrExtends(IBaz) False When we iterate over an interface, we get all of the names it defines, including names defined by base interfaces. Sometimes, we want *just* the names defined by the interface directly. We bane use the `names` method for that:: >>> list(IBaz.names()) ['eek'] Inheritance of attribute specifications --------------------------------------- An interface may override attribute definitions from base interfaces. If two base interfaces define the same attribute, the attribute is inherited from the most specific interface. For example, with:: >>> class IBase(zope.interface.Interface): ... ... def foo(): ... "base foo doc" >>> class IBase1(IBase): ... pass >>> class IBase2(IBase): ... ... def foo(): ... "base2 foo doc" >>> class ISub(IBase1, IBase2): ... pass ISub's definition of foo is the one from IBase2, since IBase2 is more specific that IBase:: >>> ISub['foo'].__doc__ 'base2 foo doc' Note that this differs from a depth-first search. Sometimes, it's useful to ask whether an interface defines an attribute directly. You can use the direct method to get a directly defined definitions:: >>> IBase.direct('foo').__doc__ 'base foo doc' >>> ISub.direct('foo') Specifications -------------- Interfaces and declarations are both special cases of specifications. What we described above for interface inheritance applies to both declarations and specifications. Declarations actually extend the interfaces that they declare:: >>> class Baz: ... zope.interface.implements(IBaz) >>> baz_implements = zope.interface.implementedBy(Baz) >>> baz_implements.__bases__ (,) >>> baz_implements.extends(IFoo) True >>> baz_implements.isOrExtends(IFoo) True >>> baz_implements.isOrExtends(baz_implements) True Specifications (interfaces and declarations) provide an `__sro__` that lists the specification and all of it's ancestors:: >>> baz_implements.__sro__ (, , , , ) Tagged Values ============= Interfaces and attribute descriptions support an extension mechanism, borrowed from UML, called "tagged values" that lets us store extra data:: >>> IFoo.setTaggedValue('date-modified', '2004-04-01') >>> IFoo.setTaggedValue('author', 'Jim Fulton') >>> IFoo.getTaggedValue('date-modified') '2004-04-01' >>> IFoo.queryTaggedValue('date-modified') '2004-04-01' >>> IFoo.queryTaggedValue('datemodified') >>> tags = list(IFoo.getTaggedValueTags()) >>> tags.sort() >>> tags ['author', 'date-modified'] Function attributes are converted to tagged values when method attribute definitions are created:: >>> class IBazFactory(zope.interface.Interface): ... def __call__(): ... "create one" ... __call__.return_type = IBaz >>> IBazFactory['__call__'].getTaggedValue('return_type') Tagged values can also be defined from within an interface definition:: >>> class IWithTaggedValues(zope.interface.Interface): ... zope.interface.taggedValue('squish', 'squash') >>> IWithTaggedValues.getTaggedValue('squish') 'squash' Invariants ========== Interfaces can express conditions that must hold for objects that provide them. These conditions are expressed using one or more invariants. Invariants are callable objects that will be called with an object that provides an interface. An invariant raises an `Invalid` exception if the condition doesn't hold. Here's an example:: >>> class RangeError(zope.interface.Invalid): ... """A range has invalid limits""" ... def __repr__(self): ... return "RangeError(%r)" % self.args >>> def range_invariant(ob): ... if ob.max < ob.min: ... raise RangeError(ob) Given this invariant, we can use it in an interface definition:: >>> class IRange(zope.interface.Interface): ... min = zope.interface.Attribute("Lower bound") ... max = zope.interface.Attribute("Upper bound") ... ... zope.interface.invariant(range_invariant) Interfaces have a method for checking their invariants:: >>> class Range(object): ... zope.interface.implements(IRange) ... ... def __init__(self, min, max): ... self.min, self.max = min, max ... ... def __repr__(self): ... return "Range(%s, %s)" % (self.min, self.max) >>> IRange.validateInvariants(Range(1,2)) >>> IRange.validateInvariants(Range(1,1)) >>> IRange.validateInvariants(Range(2,1)) Traceback (most recent call last): ... RangeError: Range(2, 1) If you have multiple invariants, you may not want to stop checking after the first error. If you pass a list to `validateInvariants`, then a single `Invalid` exception will be raised with the list of exceptions as it's argument:: >>> errors = [] >>> IRange.validateInvariants(Range(2,1), errors) Traceback (most recent call last): ... Invalid: [RangeError(Range(2, 1))] And the list will be filled with the individual exceptions:: >>> errors [RangeError(Range(2, 1))] >>> del errors[:] Adaptation ========== Interfaces can be called to perform adaptation. The sematics based on those of the PEP 246 adapt function. If an object cannot be adapted, then a TypeError is raised:: >>> class I(zope.interface.Interface): ... pass >>> I(0) Traceback (most recent call last): ... TypeError: ('Could not adapt', 0, ) unless an alternate value is provided as a second positional argument:: >>> I(0, 'bob') 'bob' If an object already implements the interface, then it will be returned:: >>> class C(object): ... zope.interface.implements(I) >>> obj = C() >>> I(obj) is obj True If an object implements __conform__, then it will be used:: >>> class C(object): ... zope.interface.implements(I) ... def __conform__(self, proto): ... return 0 >>> I(C()) 0 Adapter hooks (see __adapt__) will also be used, if present:: >>> from zope.interface.interface import adapter_hooks >>> def adapt_0_to_42(iface, obj): ... if obj == 0: ... return 42 >>> adapter_hooks.append(adapt_0_to_42) >>> I(0) 42 >>> adapter_hooks.remove(adapt_0_to_42) >>> I(0) Traceback (most recent call last): ... TypeError: ('Could not adapt', 0, ) __adapt__ --------- >>> class I(zope.interface.Interface): ... pass Interfaces implement the PEP 246 __adapt__ method. This method is normally not called directly. It is called by the PEP 246 adapt framework and by the interface __call__ operator. The adapt method is responsible for adapting an object to the reciever. The default version returns None:: >>> I.__adapt__(0) unless the object given provides the interface:: >>> class C(object): ... zope.interface.implements(I) >>> obj = C() >>> I.__adapt__(obj) is obj True Adapter hooks can be provided (or removed) to provide custom adaptation. We'll install a silly hook that adapts 0 to 42. We install a hook by simply adding it to the adapter_hooks list:: >>> from zope.interface.interface import adapter_hooks >>> def adapt_0_to_42(iface, obj): ... if obj == 0: ... return 42 >>> adapter_hooks.append(adapt_0_to_42) >>> I.__adapt__(0) 42 Hooks must either return an adapter, or None if no adapter can be found. Hooks can be uninstalled by removing them from the list:: >>> adapter_hooks.remove(adapt_0_to_42) >>> I.__adapt__(0) .. [#create] The main reason we subclass `Interface` is to cause the Python class statement to create an interface, rather than a class. It's possible to create interfaces by calling a special interface class directly. Doing this, it's possible (and, on rare occasions, useful) to create interfaces that don't descend from `Interface`. Using this technique is beyond the scope of this document. .. [#factory] Classes are factories. They can be called to create their instances. We expect that we will eventually extend the concept of implementation to other kinds of factories, so that we can declare the interfaces provided by the objects created. .. [#compat] The goal is substitutability. An object that provides an extending interface should be substitutable for an object that provides the extended interface. In our example, an object that provides IBaz should be usable whereever an object that provides IBlat is expected. The interface implementation doesn't enforce this. but maybe it should do some checks.