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 [1]. The interface is not a class, it’s an Interface, an instance of zope.interface.interface.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__
'builtins'

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 [2]. (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 implementer decorator on a class:

>>> @zope.interface.implementer(IFoo)
... class Foo:
...
...     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 a class:

>>> list(zope.interface.implementedBy(Foo))
[<InterfaceClass builtins.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 builtins.IFoo>]
>>> list(zope.interface.providedBy(Foo))
[]

We can declare interfaces implemented by other factories (besides classes). We do this using the same implementer decorator.

>>> @zope.interface.implementer(IFoo)
... def yfoo(y):
...     foo = Foo()
...     foo.y = y
...     return foo

>>> list(zope.interface.implementedBy(yfoo))
[<InterfaceClass builtins.IFoo>]

Note that the implementer decorator may modify its 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 builtins.IFoo>]

XXX: Double check and update these version numbers:

In zope.interface 3.5.2 and lower, the implementer 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 builtins.IFoo>]

Note that class decorators using the @implementer(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 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))
[<InterfaceClass builtins.IFooFactory>]
>>> IFooFactory.providedBy(Foo)
True

Declaring class interfaces is common enough that there’s a special decorator for it, provider:

>>> @zope.interface.implementer(IFoo)
... @zope.interface.provider(IFooFactory)
... class Foo2:
...
...     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))
[<InterfaceClass builtins.IFooFactory>]
>>> 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))
[<InterfaceClass builtins.ISpecial>, <InterfaceClass builtins.IFoo>]

We can find out what interfaces are directly provided by an object:

>>> list(zope.interface.directlyProvidedBy(foo))
[<InterfaceClass builtins.ISpecial>]

>>> newfoo = Foo()
>>> list(zope.interface.directlyProvidedBy(newfoo))
[]

Inherited declarations

Normally, declarations are inherited:

>>> @zope.interface.implementer(ISpecial)
... class SpecialFoo(Foo):
...     reason = 'I just am'
...     def brag(self):
...         return "I'm special because %s" % self.reason

>>> list(zope.interface.implementedBy(SpecialFoo))
[<InterfaceClass builtins.ISpecial>, <InterfaceClass builtins.IFoo>]

>>> list(zope.interface.providedBy(SpecialFoo()))
[<InterfaceClass builtins.ISpecial>, <InterfaceClass builtins.IFoo>]

Sometimes, you don’t want to inherit declarations. In that case, you can use implementer_only, instead of implementer:

>>> @zope.interface.implementer_only(ISpecial)
... class Special(Foo):
...     reason = 'I just am'
...     def brag(self):
...         return "I'm special because %s" % self.reason

>>> list(zope.interface.implementedBy(Special))
[<InterfaceClass builtins.ISpecial>]

>>> list(zope.interface.providedBy(Special()))
[<InterfaceClass builtins.ISpecial>]

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))
[<InterfaceClass builtins.IFoo>]

We can use classImplementsOnly to exclude inherited interfaces:

>>> class C(Foo):
...     pass

>>> zope.interface.classImplementsOnly(C, ISpecial)
>>> list(zope.interface.implementedBy(C))
[<InterfaceClass builtins.ISpecial>]

Declaration Objects

When we declare interfaces, we create declaration objects. When we query declarations, declaration objects are returned:

>>> type(zope.interface.implementedBy(Special))
<class 'zope.interface.declarations.Implements'>

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:

>>> @zope.interface.implementer_only(
...     zope.interface.implementedBy(Foo),
...     ISpecial,
... )
... class Special2(Foo):
...     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.implementer(ISpecial), except that the order of interfaces in the resulting declaration is different:

>>> list(zope.interface.implementedBy(Special2))
[<InterfaceClass builtins.IFoo>, <InterfaceClass builtins.ISpecial>]

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__
(<InterfaceClass zope.interface.Interface>,)

>>> class IBaz(IFoo, IBlat):
...     """Baz blah"""
...     def eek(a=1):
...         """eek in baz blah"""
...

>>> IBaz.__bases__
(<InterfaceClass builtins.IFoo>, <InterfaceClass builtins.IBlat>)

>>> 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 [3] 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 can 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 than 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:

>>> @zope.interface.implementer(IBaz)
... class Baz(object):
...     pass

>>> baz_implements = zope.interface.implementedBy(Baz)
>>> baz_implements.__bases__
(<InterfaceClass builtins.IBaz>, <implementedBy ...object>)

>>> 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:

>>> from pprint import pprint
>>> pprint(baz_implements.__sro__)
(<implementedBy builtins.Baz>,
 <InterfaceClass builtins.IBaz>,
 <InterfaceClass builtins.IFoo>,
 <InterfaceClass builtins.IBlat>,
 <InterfaceClass zope.interface.Interface>,
 <implementedBy ...object>)

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')
<InterfaceClass builtins.IBaz>

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:

>>> @zope.interface.implementer(IRange)
... class Range(object):
...     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 its argument:

>>> from zope.interface.exceptions import Invalid
>>> errors = []
>>> try:
...     IRange.validateInvariants(Range(2,1), errors)
... except Invalid as e:
...     str(e)
'[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 semantics are 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, <InterfaceClass builtins.I>)

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:

>>> @zope.interface.implementer(I)
... class C(object):
...     pass

>>> obj = C()
>>> I(obj) is obj
True

If an object implements __conform__, then it will be used:

>>> @zope.interface.implementer(I)
... class C(object):
...     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, <InterfaceClass builtins.I>)

__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:

>>> @zope.interface.implementer(I)
... class C(object):
...     pass

>>> 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)

[1]

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.

[2]

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.

[3]

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 wherever an object that provides IBlat is expected.

The interface implementation doesn’t enforce this, but maybe it should do some checks.