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A Python interface library that disallows function body content on interfaces and supports adaption.

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A Python interface library that disallows function body content on interfaces and supports adaption.

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  • Prevents code in method bodies of an interface class
  • Ensures that method overrides have compatible signatures
  • Supports interface adaption.
  • Supports optional structural type checking for Interface.provided_by(a) and Interface.adapt(a)
  • Allows concrete implementations the flexibility to implement abstract properties as instance attributes.
  • Interface.adapt() can return an implementation wrapper that provides only the attributes and methods defined by Interface.
  • Warns if provided_by did a structural type check when inheritance would work.
  • Supports python 2.7 and 3.5+

A note on the name

The phrase pure interface applies only to the first design goal - a class that defines only an interface with no implementation is a pure interface [*]_. In every other respect the zen of 'practicality beats purity' applies.


You can install released versions of pure_interface using pip::

pip install pure-interface

or you can grab the source code from GitHub_.

Defining an Interface

For simplicity in these examples we assume that the entire pure_interface namespace has been imported ::

from pure_interface import *

To define an interface, simply inherit from the class Interface and write a PEP-544_ Protocol-like class leaving all method bodies empty::

class IAnimal(Interface):
    height: float

    def speak(self, volume):

Like Protocols, class annotations are considered part of the interface. In Python versions earlier than 3.6 you can use the following alternate syntax::

class IAnimal(Interface):
    height = None

    def speak(self, volume):

The value assigned to class attributes must be None and the attribute is removed from the class dictionary (since annotations are not in the class dictionary).

Interface is a subtype of abc.ABC and the abstractmethod, abstractclassmethod and abstractstaticmethod decorators work as expected. ABC-style property definitions are also supported (and equivalent)::

class IAnimal(Interface):
    def height(self):

    def speak(self, volume):

Again, the height property is removed from the class dictionary, but, as with the other syntaxes, all concrete subclasses will be required to have a height attribute.

For convenience the abc module abstract decorators are included in the pure_interface namespace. However these decorators are optional as ALL methods and properties on a Interface subclass are abstract. In the examples above, both height and speak are considered abstract and must be overridden by subclasses.

Including abstract decorators in your code can be useful for reminding yourself (and telling your IDE) that you need to override those methods. Another common way of informing an IDE that a method needs to be overridden is for the method to raise NotImplementedError. For this reason methods that just raise NotImplementedError are also considered empty.

Interface classes cannot be instantiated ::

InterfaceError: Interfaces cannot be instantiated.

Including code in a method will result in an InterfaceError being raised when the module is imported. For example::

class BadInterface(Interface):
    def method(self):

InterfaceError: Function "method" is not empty
Did you forget to inherit from object to make the class concrete?

The dir() function will include all interface attributes so that mock.Mock(spec=IAnimal) will work as expected::

>>> dir(IAnimal)
['__abstractmethods__', '__doc__', ..., 'height', 'speak']

The mock_protocol_ package also works well with interfaces.

Concrete Implementations

Simply inheriting from a pure interface and writing a concrete class will result in an InterfaceError exception as pure_interface will assume you are creating a sub-interface. To tell pure_interface that a type should be concrete simply inherit from object as well (or anything else that isn't an Interface). For example::

class Animal(IAnimal, object):
    def __init__(self, height):
        self.height = height

    def speak(self, volume):

Exception: Mixing an Interface class with an abc.ABC interface class that only defines abstract methods and properties that satisfy the empty method criteria will result in a type that is considered a pure interface.::

class ABCInterface(abc.ABC):
    def foo(self):

class MyInterface(ABCInterface, Interface):
    def bar(self):

Concrete implementations may implement interface attributes in any way they like: as instance attributes, properties or custom descriptors, provided that they all exist at the end of __init__(). Here is another valid implementation::

class Animal2(IAnimal, object):
    def __init__(self, height):
        self._height = height

    def height(self):
        return self._height

    def speak(self, volume):

Method Signatures

Method overrides are checked for compatibility with the interface. This means that argument names must match exactly and that no new non-optional arguments are present in the override. This enforces that calling the method with interface parameters will aways work. For example, given the interface method::

def speak(self, volume):

Then these overrides will all fail the checks and raise an InterfaceError::

def speak(self): # too few parameters def speak(self, loudness): # name does not match def speak(self, volume, language): # extra required argument

However new optional parameters are permitted, as are *args and **kwargs::

def speak(self, volume, language='doggy speak') def speak(self, *args, **kwargs)

Implementation Warnings

As with abc.ABC, the abstract method checking for a class is done when an object is instantiated. However it is useful to know about missing methods sooner than that. For this reason pure_interface will issue a warning during module import when methods are missing from a concrete subclass. For example::

class SilentAnimal(IAnimal, object):
    def __init__(self, height):
        self.height = height

will issue this warning:: UserWarning: Incomplete Implementation: SilentAnimal does not implement speak
class SilentAnimal(IAnimal, object):

Trying to create a SilentAnimal will fail in the standard abc way::

InterfaceError: Can't instantiate abstract class SilentAnimal with abstract methods speak

If you have a mixin class that implements part of an interface you can suppress the warnings by adding an class attribute called pi_partial_implementation. The value of the attribute is ignored, and the attribute itself is removed from the class. For example::

class HeightMixin(IAnimal, object):
    pi_partial_implementation = True

    def __init__(self, height):
        self.height = height

will not issue any warnings.

The warning messages are also appended to the module variable missing_method_warnings, irrespective of any warning module filters (but only if is_development=True). This provides an alternative to raising warnings as errors. When all your imports are complete you can check if this list is empty.::

if pure_iterface.missing_method_warnings:
    for warning in pure_iterface.missing_method_warnings:

Note that missing properties are NOT checked for as they may be provided by instance attributes.


Registering Adapters

Adapters for an interface are registered with the adapts decorator or with the register_adapter function. Take for example an interface ISpeaker and a class Talker and an adapter class TalkerToSpeaker::

class ISpeaker(Interface):
    def speak(self, volume):

class Talker(object):
    def talk(self):
        return 'talk'

class TalkerToSpeaker(ISpeaker, object):
    def __init__(self, talker):
        self._talker = talker

    def speak(self, volume):

The adapts decorator call above is equivalent to::

register_adapter(TalkerToSpeaker, Talker, ISpeaker)

The ISpeaker parameter passed to register_adapter is the first interface in the MRO of the class being decorated (TalkerToSpeaker). If there are no interface types in the MRO of the decorated class an InterfaceError exception is raised.

Adapter factory functions can be decorated too, in which case the interface being adapted to needs to be specified::

@adapts(Talker, ISpeaker)
def talker_to_speaker(talker):
    return TalkerToSpeaker(talker)

The decorated adapter (whether class for function) must be callable with a single parameter - the object to adapt.

Adapting Objects

The Interface.adapt method will adapt an object to the given interface such that Interface.provided_by is True or raise AdaptionError if no adapter could be found. For example::

speaker = ISpeaker.adapt(talker)
isinstance(speaker, ISpeaker)  --> True

If you want to get None rather than an exception then use::

speaker = ISpeaker.adapt_or_none(talker)

You can filter a list of objects returning those objects that provide an interface using filter_adapt(objects)::

list(ISpeaker.filter_adapt([None, Talker(), a_speaker, 'text']) --> [TalkerToSpeaker, a_speaker]

To adapt an object only if it is not None then use::


This is equivalent to::

ISpeaker.adapt(optional_talker) if optional_talker is not None else None

By default the adaption functions will return an object which provides only the functions and properties specified by the interface. For example given the following implementation of the ISpeaker interface above::

class TopicSpeaker(ISpeaker): def init(self, topic): self.topic = topic

  def speak(self, volume):
      return 'lets talk about {} very {}'.format(self.topic, volume)

topic_speaker = TopicSpeaker('python')


speaker = ISpeaker.adapt(topic_speaker) speaker is topic_speaker --> False speaker.topic --> AttributeError("ISpeaker interface has no attribute topic")

This is controlled by the optional interface_only parameter to adapt which defaults to True. Pass interface_only=False if you want the actual adapted object rather than a wrapper::

speaker = ISpeaker.adapt(topic_speaker, interface_only=False) speaker is topic_speaker --> True speaker.topic --> 'Python'

Accessing the topic attribute on an ISpeaker may work for all current implementations of ISpeaker, but this code will likely break at some inconvenient time in the future.

Adapters from sub-interfaces may be used to perform adaption if necessary. For example::

class IA(Interface):
   foo = None

class IB(IA):
    bar = None

class IntToB(IB, object):
    def __init__(self, x): = = x

Then IA.adapt(4) will use the IntToB adapter to adapt 4 to IA (unless there is already an adapter from int to IA)

Structural Type Checking

Structural_ type checking checks if an object has the attributes and methods defined by the interface.

.. _Structural:

As interfaces are inherited, you can usually use isinstance(obj, MyInterface) to check if an interface is provided. An alternative to isinstance() is the Interface.provided_by(obj) classmethod which will fall back to structural type checking if the instance is not an actual subclass. This can be controlled by the allow_implicit parameter which defaults to True. The structural type-checking does not check function signatures.::

class Parrot(object):
    def __init__(self):
        self.height = 43

    def speak(self, volume):

p = Parrot()
isinstance(p, IAnimal) --> False
IAnimal.provided_by(p) --> True
IAnimal.provided_by(p, allow_implicit=False) --> False

The structural type checking makes working with data transfer objects (DTO's) much easier.::

class IMyDataType(Interface):
    thing: str

class DTO(object):

d = DTO()
d.thing = 'hello'
IMyDataType.provided_by(d) --> True
e = DTO()
e.something_else = True
IMyDataType.provided_by(e) --> False

Adaption also supports structural typing by passing allow_implicit=True (but this is not the default)::

speaker = ISpeaker.adapt(Parrot(), allow_implicit=True)
ISpeaker.provided_by(speaker)  --> True

When using provided_by() or adapt() with allow_implicit=True, a warning may be issued informing you that the structurally typed object should inherit the interface. The warning is only issued if the interface is implemented by the class (and not by instance attributes as in the DTO case above) and the warning is only issued once for each class, interface pair. For example::

s = ISpeaker.adapt(Parrot())
UserWarning: Class Parrot implements ISpeaker.
Consider inheriting ISpeaker or using ISpeaker.register(Parrot)

Dataclass Support

dataclasses_ were added in Python 3.7. When used in this and later versions of Python, pure_interface provides a dataclass decorator. This decorator can be used to create a dataclass that implements an interface. For example::

class IAnimal2(Interface):
    height: float
    species: str

    def speak(self):

class Animal(Concrete, IAnimal2):
    def speak(self):
        print('Hello, I am a {} metre tall {}', self.height, self.species)

a = Animal(height=4.5, species='Giraffe')

The builtin Python dataclass decorator cannot be used because it will not create attributes for the height and species annotations on the interface base class IAnimal2. As per the built-in dataclass decorator, only interface attributes defined using annotation syntax are supported (and not the alternatives syntaxes provided by pure_interface).

Interface Type Information

The pure_interface module provides these functions for returning information about interface types.

type_is_interface(cls) Return True if cls is a pure interface, False otherwise or if cls is not a class.

get_type_interfaces(cls) Returns all interfaces in the cls mro including cls itself if it is an interface

get_interface_names(cls) Returns a frozenset of names (methods and attributes) defined by the interface. if interface is not a Interface subtype then an empty set is returned.

get_interface_method_names(interface) Returns a frozenset of names of methods defined by the interface. if interface is not a Interface subtype then an empty set is returned

get_interface_attribute_names(interface) Returns a frozenset of names of attributes defined by the interface. if interface is not a Interface subtype then an empty set is returned

Automatic Adaption

The function decorator adapt_args adapts arguments to a decorated function to the types given. For example::

@adapt_args(foo=IFoo, bar=IBar)
def my_func(foo, bar=None):

In Python 3.5 and later the types can be taken from the argument annotations.::

def my_func(foo: IFoo, bar: IBar=None):

This would adapt the foo parameter to IFoo (with IFoo.optional_adapt(foo)) and bar to IBar (using IBar.optional_adapt(bar)) before passing them to my_func. Nonevalues are never adapted, somy_func(foo, None)will work, otherwiseAdaptionError`` is raised if the parameter is not adaptable. All arguments must be specified as keyword arguments::

@adapt_args(IFoo, IBar)   # NOT ALLOWED
def other_func(foo, bar):

Development Flag

Much of the empty function and other checking is awesome whilst writing your code but ultimately slows down production code. For this reason the pure_interface module has an is_development switch.::

is_development = not hasattr(sys, 'frozen')

is_development defaults to True if running from source and default to False if bundled into an executable by py2exe_, cx_Freeze_ or similar tools.

If you manually change this flag it must be set before modules using the Interface type are imported or else the change will not have any effect.

If is_development if False then:

* Signatures of overriding methods are not checked
* No warnings are issued by the adaption functions
* No incomplete implementation warnings are issued
* The default value of ``interface_only`` is set to ``False``, so that interface wrappers are not created.

PyContracts Integration

You can use pure_interface with PyContracts_

.. _PyContracts:

Simply import the pure_contracts module and use the ContractInterface class defined there as you would the Interface class described above. For example::

from pure_contracts import ContractInterface
from contracts import contract

class ISpeaker(ContractInterface):
    @contract(volume=int, returns=unicode)
    def speak(self, volume):



InterfaceType(abc.ABCMeta) Metaclass for checking interface and implementation classes. Adding InterfaceType as a meta-class to a class will not make that class an interface, you need to inherit from Interface class to define an interface.

In addition to the ``register`` method provided by ``ABCMeta``, the following functions are defined on
``InterfaceType`` and can be accessed directly when the ``Interface`` methods are overridden
for other purposes.

**adapt** *(cls, obj, allow_implicit=False, interface_only=None)*
    See ``Interface.adapt`` for a description.

**adapt_or_none** *(cls, obj, allow_implicit=False, interface_only=None)*
    See ``Interface.adapt_or_none`` for a description

**optional_adapt** *(cls, obj, allow_implicit=False, interface_only=None)*
    See ``Interface.optional_adapt`` for a description

**can_adapt** *(cls, obj, allow_implicit=False)*
    See ``Interface.can_adapt`` for a description

**filter_adapt** *(cls, objects, allow_implicit=False, interface_only=None)*
    See ``Interface.filter_adapt`` for a description

**interface_only** *(cls, implementation)*
    See ``Interface.interface_only`` for a description

**provided_by** *(cls, obj, allow_implicit=True)*
    See ``Interface.provided_by`` for a description

Classes created with a metaclass of ``InterfaceType`` will have the following property:

**_pi** Information about the class that is used by this meta-class.  This attribute is reserved for use by
        ``pure_interface`` and must not be overridden.

Interface Base class for defining interfaces. The following methods are provided:

**adapt** *(obj, allow_implicit=False, interface_only=None)*
    Adapts ``obj`` to this interface. If ``allow_implicit`` is ``True`` permit structural adaptions.
    If ``interface_only`` is ``None`` the it is set to the value of ``is_development``.
    If ``interface_only`` resolves to ``True`` a wrapper object that provides
    the properties and methods defined by the interface and nothing else is returned.
    Raises ``AdaptionError`` if no adaption is possible or a registered adapter returns an object not providing
    this interface.

**adapt_or_none** *(obj, allow_implicit=False, interface_only=None)*
    As per **adapt()** except returns ``None`` instead of raising a ``AdaptionError``

**optional_adapt** *(obj, allow_implicit=False, interface_only=None)*
    Adapts obj to this interface if it is not ``None`` returning ``None`` otherwise.
    Short-cut for ``adapt(obj) if obj is not None else None``

**can_adapt** *(obj, allow_implicit=False)*
    Returns ``True`` if ``adapt(obj, allow_implicit)`` will succeed.  Short-cut for
    ``adapt_or_none(obj) is not None``

**filter_adapt** *(objects, allow_implicit=False, interface_only=None)*
    Generates adaptions of each item in *objects* that provide this interface.
    *allow_implicit* and *interface_only* are as for **adapt**.
    Objects that cannot be adapted to this interface are silently skipped.

**interface_only** *(implementation)*
    Returns a wrapper around *implementation* that provides the properties and methods defined by
    the interface and nothing else.

**provided_by** *(obj, allow_implicit=True)*
    Returns ``True`` if *obj* provides this interface. If ``allow_implicit`` is ``True`` the also
    return ``True`` for objects that provide the interface structure but do not inherit from it.
    Raises ``InterfaceError`` if the class is a concrete type.


adapts (from_type, to_interface=None) Class or function decorator for declaring an adapter from from_type to to_interface. The class or function being decorated must take a single argument (an instance of from_type) and provide (or return and object providing) to_interface. The adapter may return an object that provides the interface structurally only, however adapt must be called with allow_implicit=True for this to work. If decorating a class, to_interface may be None to use the first interface in the class's MRO.

register_adapter (adapter, from_type, to_interface) Registers an adapter to convert instances of from_type to objects that provide to_interface for the to_interface.adapt() method. adapter must be a callable that takes a single argument (an instance of from_type) and returns and object providing to_interface.

type_is_interface (cls) Return True if cls is a pure interface and False otherwise

get_type_interfaces (cls) Returns all interfaces in the cls mro including cls itself if it is an interface

get_interface_names (cls) Returns a frozenset of names (methods and attributes) defined by the interface. if interface is not a Interface subtype then an empty set is returned.

get_interface_method_names (cls) Returns a frozenset of names of methods defined by the interface. If cls is not a Interface subtype then an empty set is returned.

get_interface_attribute_names (cls) Returns a frozenset of names of class attributes and annotations defined by the interface If cls is not a Interface subtype then an empty set is returned.

dataclass (_cls=None, init=True, repr=True, eq=True, order=False, unsafe_hash=False, frozen=False) This function is a re-implementation of the standard Python dataclasses.dataclass decorator. In addition to the fields on the decorated class, all annotations on interface base classes are added as fields. See the Python dataclasses_ documentation for more details.

3.7+ Only


PureInterfaceError Base exception class for all exceptions raised by pure_interface.

InterfaceError Exception raised for problems with interfaces

AdaptionError Exception raised for problems with adapters or adapting.

Module Attributes

is_development Set to True to enable all checks and warnings. If set to False then:

* Signatures of overriding methods are not checked
* No warnings are issued by the adaption functions
* No incomplete implementation warnings are issued
* The default value of ``interface_only`` is set to ``False``, so that interface wrappers are not created.

missing_method_warnings The list of warning messages for concrete classes with missing interface (abstract) method overrides. Note that missing properties are NOT checked for as they may be provided by instance attributes.

.. _typing: .. _PEP-544: .. _GitHub: .. _mypy: .. _py2exe: .. _cx_Freeze: .. _dataclasses: .. _mock_protocol: .. [*] We don't talk about the methods on the base Interface class. In earlier versions they were all on the meta class but then practicality got in the way.

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