Welcome to Extend Me’s documentation!

Extend Me - Class based extension/plugin library

This module provides mechanism of extension of your application based on ‘extension via inheritance’. Under this words I mean ability to define new extensions of application objects simply by subclassing of extensible classes of app.

For example we have app with class ‘Worker’ which we would like to make extensible (allowing third party modules to extend or change its behavior). Thinking strait, there are a lot of work to be done, to impelement mechanism of loading, registering, end enabling extension, with lot of glue code, which must define some entry points to connect extension and main app. But why not make it simpler, supposing that any subclass of ‘Worker’ will extend it? And this module provides implementation of this in two ways:

• Explicit (by using metaclass ExtensibleType directly)

  • When using this way You will heve seperatly Base class to be subclassed by extension classes and class getter which will construct class based on all defined extensions using multiple inhertance

• Implicit (by using Extensible class which use metaclass magic implicitly)

  • Extensible class takes care of all metaclass magic related to generation objects of correct class

How it Works

Metaclass (ExtensibleType) tracks all subclasses of class it is applied to, and provides method to build class based on all subclasses of base class, thus using all functionality of all subclasses. Thus generation of correct class is separate process which should be used everywhere where extensible class is requred.

To simplify this class Extensible was implemented. It has redefined method __new__ which automaticaly creates instances of correct class (class that inherited from base class and all its extensions’)

Examples

ExtensibleType

At the begining we should create a metaclass that will automaticaly gether all information about all extensions, and apply this metaclass to class we would like to enable extensions for:

>>> import six  # Used for Python 2/3 compatability
>>> mc = ExtensibleType._("Object")

>>> @six.add_metaclass(mc)
... class Object(object):
...     pass

Not method _ of ExtensibleType. This method is used to create metaclass for specific object. It receives one argument - string that will be used as name of class generated by this metaclass

Next we may define extension for this class. It is very simple. Just subclass previously defined class:

>>> class ObjectExtension(Object):
...     cool_attribute = 1
...     def method1(self):
...         return "Test"

So... at this momet we have base class and extension. And here is that core magic occures. Metaclass that was created at the begining automaticaly collects all subclasses of base class. So it is posible now to create new class that is subclass of all subclasses of base class using multiple inheritance. And metaclass mc will do it for You:

>>> cls = mc.get_class()

And now You can use cls for Your needs, instead of base class. It can do all that base class can, and all that extensions can:

>>> obj = cls()
>>> obj.method1()
'Test'
>>> obj.cool_attribute
1

ExtensibleByHashType

Same as ExtensibleType, but allows to build tree of classes for diferent names (types). Just look examples below.

First, create metaclass that will specify inheritance rules:

>>> import six  # Used for Python 2/3 compatability
>>> mc = ExtensibleByHashType._("Connector", hashattr='name')

Here we see aditional parametr in _ method: hashattr='name' which describes what meta attribute will be used as key(hash).

Next step - we have to create Base class with this metaclass. As example we will look into connection classes of openerp_proxy project:

>>> @six.add_metaclass(mc)
... class ConnectorBase(object):
...     # Base class for all connectors
...
...     def __init__(self, host, port, verbose=False):
...         self.host = host
...         self.port = port
...         self.verbose = verbose
...
...     def _get_service(self, name):
...         raise NotImplementedError
...
...     def get_service(self, name):
...         # Returns service for specified *name*
...         return self._get_service(name)

Base class describes only interface, and may be some part of abstract logic And as next step we will extend it in diferent ways to support different connection types:

>>> class ConnectorXMLRPC(ConnectorBase):
...     # XML-RPC connector
...     class Meta:
...         name = 'xml-rpc' # remember definition of metaclass?
...                          # this attribute is used as hash(key)
...                          # to unique identify each banch of extensions
...                          # of base class
...
...     def __init__(self, *args, **kwargs):
...         super(ConnectorXMLRPC, self).__init__(*args, **kwargs)
...         self.__services = {}
...
...     def get_service_url(self, service_name):
...         return 'http://%s:%s/xmlrpc/%s' % (self.host, self.port, service_name)
...
...     def _get_service(self, name):
...         service = self.__services.get(name, False)
...         if service is False:
...             service = XMLRPCProxy(self.get_service_url(name), verbose=self.verbose)
...             self.__services[name] = service
...         return service
...
...
... # Pay attention on base class.
>>> class ConnectorXMLRPCS(ConnectorXMLRPC):
...     # XML-RPCS Connector
...     class Meta:
...         name = 'xml-rpcs'
...
...     def get_service_url(self, service_name):
...         return 'https://%s:%s/xmlrpc/%s' % (self.host, self.port, service_name)

Code above creates two connectors: one for XML-RPC and one for XML-RPCS. Each of connectors may be extended by simple inheritance. And if required any extension may define new branch(key)(hash) as wee see in example above.

To use this connector mc has method get_class(name[, default=False]) wich will return class generated for hash=*name*:

>>> cls = mc.get_class('xml-rpc')
>>> [b.__name__ for b in cls.__bases__]
['ConnectorXMLRPC', 'ConnectorBase']
>>> cls.__name__
'Connector'

>>> cls = mc.get_class('xml-rpcs')
>>> [b.__name__ for b in cls.__bases__]
['ConnectorXMLRPCS', 'ConnectorBase']
>>> cls.__name__
'Connector'

Example above shows what classes will be generated for specified names. By default, if mc.get_class called with unregistered name (No extension with Meta.name == name defined) it will raise ValueError

If You want to allow creating of classes with not Meta.name defined, just pass default=True to mc.get_class:

>>> cls = mc.get_class('unexisting-protocol', default=True)
>>> [b.__name__ for b in cls.__bases__]
['ConnectorBase']
>>> cls.__name__
'Connector'

Extensible

This class provides one more level of abstraction, allowing to hide all metaclass magic behide the scene. So, using it You don’t need to worry about metaclasses and class creation process. Just inherit extensions form base class, and use in Your program instances of base class. Let’s see it in example:

>>> class MyCoolClass(Extensible):
...     my_attr_1 = 25
...     def my_method1(self, arg1):
...         print('Hello, %s' % arg1)

>>> class MyCoolClassExtension1(MyCoolClass):
...     def my_method1(self, arg1):
...         super(MyCoolClassExtension1, self).my_method1(arg1.upper())
...
...     def my_method2(self, arg1):
...         print("Good by, %s" % arg1)

And now using simply instances of base class You have all abilities that provided by extensions:

>>> my_cool_obj = MyCoolClass()
>>> print(my_cool_obj.my_attr_1)
25
>>> my_cool_obj.my_method1('World')
Hello, WORLD
>>> my_cool_obj.my_method2('World')
Good by, World

ClassReference

class extend_me.ExtensibleType

Bases: type

Metaclass for Extensible objects

To make object (class) extendsible just do following.

Create metaclass for extension point:

>>> import six  # Used for Python 2/3 compatability
>>> # Generate metaclass to be used:
>>> mc = ExtensibleType._("MyClassName")

Define base class for which will be starting point for extensions:

>>> # Define base class
>>> @six.add_metaclass(mc)
... class Object(object):
...   pass

Define just simple extension which introduces new method, simply subclassing base class:

>>> # Define extension
>>> class ObjectExtension(Object):
...     def method1(self):
...         return "Test"

Now to get class with all loaded extension enabled, just do:

>>> # Get class to be used for object creations
>>> cls = mc.get_class()

And now check what we got:

>>> # Now all classes that subclass Object
>>> # will be base classes for one, generated by mc.get_class()
>>> Object in cls.__bases__
True
>>> ObjectExtension in cls.__bases__
True
>>> cls.__name__ == "MyClassName"
True
>>> [b.__name__ for b in cls.__bases__]
['ObjectExtension', 'Object']

>>> # Create class based on all extensions
>>> obj = cls()
>>> obj.method1()
'Test'
>>> obj2 = mc.get_object()
>>> obj2.method1()
'Test'

Next try to use ABC with this class:

>>> import collections
>>> amc = ExtensibleType._("TestABC", with_meta=abc.ABCMeta)
>>> @six.add_metaclass(amc)
... class ABCSequence(collections.Sequence):
...    def __init__(self, seq):
...        self.seq = seq
...    def __getitem__(self, index):
...        return self.seq[index]
...    def __len__(self):
...        return len(self.seq)
>>> seq = amc.get_class()([1, 1, 2])

And now check if Sequence logic forks fine:

>>> seq[1]
1
>>> seq[2]
2
>>> seq[3]
Traceback (most recent call last):
...
IndexError: list index out of range
>>> len(seq)
3
>>> 1 in seq
True
>>> 3 in seq
False
>>> seq.count(2)
1
>>> seq.count(1)
2
>>> [i for i in seq]
[1, 1, 2]

And let’s try to build extension:

>>> class NABCSequence(ABCSequence):
...     def count2(self, *args, **kwargs):
...         return self.count(*args, **kwargs)*2

>>> # Get object with all extensions applied
>>> seq2 = amc.get_object([1, 1, 2])
>>> seq2.count2(1)
4
>>> seq2.count2(2)
2
>>> seq2.count(1)
2

classmethod _(mcs, cls_name='Object', with_meta=None)

Method to generate real metaclass to be used:

mc = ExtensibleType._("MyClass")  # note this line
@six.add_metaclass(mc)
class MyClassBase(object):
    pass

Parameters:

• cls_name (str) – name of generated class
• with_meta (class) – Mix aditional metaclass in. (default: None)

Returns:

specific metaclass to track new inheritance tree

classmethod get_class(mcs)

Generates new class to gether logic of all available extensions

mc = ExtensibleType._("MyClass")
@six.add_metaclass(mc)
class MyClassBase(object):
    pass
MyClass = mc.get_class()   # get class with all extensions enabled

classmethod get_object(mcs, *args, **kwargs)

Creates new object with all extensions applied

all args and keyword arguments will be forwarded to generated class constructor

Same as .get_class()(*args, **kwargs)

class extend_me.ExtensibleByHashType

Bases: extend_me.ExtensibleType

Metaclass for extensible object that allows to build extension trees. This may be useful in situations where there is some set of similar objects which in some cases my behave diferently, which looks like all of them have same base class and some set of subclasses for each situation. For example we have some set of services which have same protocol, but we would like to code some aditional logic for each service. In this case we could build next architecture

At first define metaclass which will control building extensions

>>> mc = ExtensibleByHashType._("MyCoolClass", hashattr='name')

Then define base class which represents common inteface for each service

>>> @six.add_metaclass(mc)
... class ServiceBase(object):
...     def do_reduce(self, f, *args):
...         # Think that here is call to remote server by API for example
...         # simple reduce is taken only to test that inheritance works fine
...         it = iter(args)
...         res = next(it)
...         for x in it:
...             res = f(res, x)
...         return res

Try define service:

>>> class ServiceAddition(ServiceBase):
...     class Meta:
...         name = 'Addition'
...
...     def add(self, a, b):
...         return self.do_reduce(lambda x,y: x+y, a, b)

Define second service

>>> class ServiceMul(ServiceBase):
...     class Meta:
...         name = 'Mul'
...
...     def mul(self, a, b):
...         return self.do_reduce(lambda x,y: x*y, a, b)

And let’s check what we have now

>>> adder = mc.get_class('Addition')()
>>> adder.do_reduce(lambda x,y: x+y, 2, 5)
7
>>> adder.add(2, 5)
7
>>> adder.mul(2, 5)
Traceback (most recent call last):
...
AttributeError: 'MyCoolClass' object has no attribute 'mul'
>>> adder.do_reduce(lambda x, y: x * y, 2, 5)
10
>>> multiplier = mc.get_class('Mul')()
>>> multiplier.do_reduce(lambda x, y: x * y, 2, 5)
10
>>> multiplier.do_reduce(lambda x,y: x+y, 2, 5)
7
>>> multiplier.mul(2, 5)
10
>>> multiplier.add(2, 5)
Traceback (most recent call last):
...
AttributeError: 'MyCoolClass' object has no attribute 'add'

Let’s try to get unregistered service

>>> srv = mc.get_class('X')()
Traceback (most recent call last):
...
ValueError: There is no class registered for key 'X'
>>> srv = mc.get_class('X', default=True)()
>>> srv.do_reduce(lambda x,y: x+y, 2, 5)
7

And now let’s check what base classes were used to build each instance of service

>>> [b.__name__ for b in adder.__class__.__bases__]
['ServiceAddition', 'ServiceBase']
>>> [b.__name__ for b in multiplier.__class__.__bases__]
['ServiceMul', 'ServiceBase']
>>> [b.__name__ for b in srv.__class__.__bases__]
['ServiceBase']

Check if get_registered_names works fine:

>>> sorted(mc.get_registered_names())
['Addition', 'Mul']

And the simple example of integration with ABC (or other metaclassess):

>>> mc = ExtensibleByHashType._("TestABC", with_meta=abc.ABCMeta, hashattr='name')
>>> @six.add_metaclass(mc)
... class TestBase(collections.Sequence):
...     def __init__(self, seq):
...         self.seq = seq
...     def __getitem__(self, index):
...         return self.seq[index]
...     def __len__(self):
...         return len(self.seq)
...     def count_x2(self, *args, **kwargs):
...        return self.seq.count(*args, **kwargs) * 2
...
>>> class TestX3(TestBase):
...     class Meta:
...         name = 'X3'
...
...     def count_x3(self, *args, **kwargs):
...         return self.seq.count(*args, **kwargs) * 3
...
>>> Test = mc.get_class(None, default=True)
>>> test = Test([1, 1, 2])
>>> test.count(1)
2
>>> test.count_x2(1)
4
>>> test.count_x3(1)
Traceback (most recent call last):
...
AttributeError: 'TestABC' object has no attribute 'count_x3'
>>>
>>> Test3 = mc.get_class('X3')
>>> test3 = Test3([1, 1, 2])
>>> test3.count_x3(1)
6
>>> test3.count_x2(1)
4

classmethod _(mcs, cls_name='Object', with_meta=None, hashattr='_name')

Method to generate real metaclass to be used

mc = ExtensibleByHashType._("MyClass", hashattr='name')  # note this line
@six.add_metaclass(mc)
class MyClassBase(object):
    pass

Parameters:

• cls_name (str) – name of generated class
• with_meta (class) – Mix aditional metaclass in. (default: None)
• hashattr – name of class Meta attribute to be used as hash. default=’_name’

Returns:

specific metaclass to track new inheritance tree

classmethod get_class(mcs, name, default=False)

Generates new class to gether logic of all available extensions

mc = ExtensibleByHashType._("MyClass", hashattr='name')  # note this line
@six.add_metaclass(mc)
class MyClassBase(object):
    pass

class MyClassX1(MyClassBase):
    class Meta:
        name = 'X1'

MyClass = mc.get_class(None, default=True)  # get default class
MyX1 = mc.get_class('X1')                   # get specific class

Parameters:

• name – key to get class for
• default (bool) – if set to True will generate default class for if there no special class defined for such key

Returns:

generated class for requested type

classmethod get_object(mcs, *args, **kwargs)

Not implemented!!!

classmethod get_registered_names(mcs)

Return’s list of names (keys) registered in this tree. For each name specific classes exists

class extend_me.Extensible

Bases: object

All direct subclasses of this class will be extensible through inhertance

So to create new class that should be extensible just inhert it from this class:

>>> class MyClass(Extensible): pass

After this all subclasses of MyClass will extend it. For example, create simple extension which will add attribute and method to class:

>>> class MyClassExtension(MyClass):
...     my_attr = 42
...     def my_method(self):
...         print ("Hello, %s" % self.my_attr)

After this we expect that attributes and methods defined in extensions will be available in MyClass object:

>>> my_obj = MyClass()
>>> my_obj.my_attr
42
>>> my_obj.my_method()
Hello, 42

So as we see, we could extend any objects derived from Extensible class using simple inheritence with out any long code. This is useful for plugin or extension systems, making extension work when it just imported. But as disadvantage of this approach, when we ll get error when we try to access attributes of extensions from base class:

>>> MyClass.my_attr
Traceback (most recent call last):
...
AttributeError: type object 'MyClass' has no attribute 'my_attr'

>>> MyClass.my_method()
Traceback (most recent call last):
...
AttributeError: type object 'MyClass' has no attribute 'my_method'

This happens because Extensible class implements some black magic using inheritance and metaclasses. Extensible class have overridden __new__ method in the way when it creates instance not of base class but of automatically generated class which inherits from all its extensions.

>>> [b.__name__ for b in my_obj.__class__.__bases__]
['MyClassExtension', 'MyClass']

Thus any behavior of objects created by MyClass may be changed by extensions. And as one more thing, extensions that inherits from extensions will be extensions of base class, not extension of extension. So we could do things like:

>>> class MyNewExtension(MyClassExtension):
...     my_attr = 25
...     def my_method_2(self):
...         print ("Method 2")
>>> my_obj_2 = MyClass()  # Not after creating new extension we
>>>                       # create new object
>>> my_obj_2.my_attr
25
>>> my_obj_2.my_method()
Hello, 25
>>> my_obj_2.my_method_2()
Method 2

and objects created before definition of new extension will not be changed, they will not get new methods or attributes from new extensions

>>> my_obj.my_attr
42
>>> my_obj.my_method()
Hello, 42
>>> my_obj.my_method_2()
Traceback (most recent call last):
...
AttributeError: 'MyClass' object has no attribute 'my_method_2'

Test that initialization of new objects performed only once (such bug was present in versions <= 1.1.0)

>>> class ZBase(Extensible):
...     def __init__(self):
...         if not hasattr(self, 'counter'):
...             self.counter = 1
...         else:
...             self.counter += 1
>>> z0 = ZBase()
>>> z0.counter
1
>>> class Z1(ZBase):
...     pass
>>> z1 = ZBase()
>>> z1.counter
1

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