A Python interface library that disallows function body content on interfaces and supports adaption.
Project description
pure-interface
A Python interface library that disallows function body content on interfaces and supports adaption.
Jump to the Reference.
Features
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.
Installation
pure_interface depends on the six and typing modules (typing is included in python 3.5 and later).
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): pass
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): pass
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 and abstractproperty decorators work as expected. ABC-style property definitions are also supported (and equivalent):
class IAnimal(Interface): @abstractproperty def height(self): pass @abstractmethod def speak(self, volume): pass
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, and on Python 2.7 abstractclassmethod and abstractstaticmethod are also available. 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
IAnimal() 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): print('hello') 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']
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): print('hello')
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): @abstractmethod def foo(self): pass class MyInterface(ABCInterface, Interface): def bar(self): pass
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 @property def height(self): return self._height def speak(self, volume): print('hello')
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:
readme.py:28: UserWarning: Incomplete Implementation: SilentAnimal does not implement speak class SilentAnimal(IAnimal, object):
Trying to create a SilentAnimal will fail in the standard abc way:
SilentAnimal() 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: print(warning) exit(1)
Note that missing properties are NOT checked for as they may be provided by instance attributes.
Adaption
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): pass class Talker(object): def talk(self): return 'talk' @adapts(Talker) class TalkerToSpeaker(ISpeaker, object): def __init__(self, talker): self._talker = talker def speak(self, volume): return self._talker.talk()
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:
ISpeaker.optional_adapt(optional_talker)
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')
Then:
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 @adapts(int): class IntToB(IB, object): def __init__(self, x): self.foo = self.bar = 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.
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): print('hello') 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): pass 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): pass @dataclass 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_pure_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): pass
In Python 3.5 and later the types can be taken from the argument annotations.:
@adapt_args def my_func(foo: IFoo, bar: IBar=None): pass
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. None values are never adapted, so my_func(foo, None) will work, otherwise AdaptionError 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): pass
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
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): pass
Reference
Classes
- 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.
Functions
- 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_pure_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
Exceptions
- 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.
Project details
Release history Release notifications | RSS feed
Download files
Download the file for your platform. If you're not sure which to choose, learn more about installing packages.
Source Distribution
Built Distribution
Hashes for pure_interface-4.0.2-py2.py3-none-any.whl
Algorithm | Hash digest | |
---|---|---|
SHA256 | c04b5da143398af82fbb88775daf4c01406aeedd8a5a2ecb3690ffde1ecf4c62 |
|
MD5 | 166b76e8a8814d66f1e2e0aeb7fa51cb |
|
BLAKE2b-256 | f36e4778799c5cee56573bc195852b1cf4b7c41c15e0357d620d1285ce73ee81 |