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https://github.com/neg3ntropy/objproxies

Proxies and wrappers for ordinary Python objects
https://github.com/neg3ntropy/objproxies

design-patterns proxy python python3

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Proxies and wrappers for ordinary Python objects

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README 

        
Simple Proxy Types

==================



The ``objproxies`` module provides some useful base classes for creating

proxies and wrappers for ordinary Python objects. Proxy objects automatically

delegate all attribute access and operations to the proxied object. Wrappers

are similar, but can be subclassed to allow additional attributes and

operations to be added to the wrapped object.



Note that these proxy types are not intended to be tamper-proof; the unproxied

form of an object can be readily accessed using a proxy's ``__subject__``

attribute, and some proxy types even allow this attribute to be set (This can

be handy for algorithms that lazily create circular structures and thus need to

be able to hand out "forward reference" proxies.)



.. contents:: **Table of Contents**



Development status

******************



This is Python 3 port of `ProxyTypes

`_ wrote by Phillip J. Eby as

part of `PEAK `_ for Python 2.



The namespace was changed from ``peak.util.proxies`` to ``objproxies``. Other

than that it should be a compatible replacement.



So far the following was accomplished:



* Streamlined files and setup

* Ported unittests and doctests

* Cleaned up syntax



v1.0 TODO

+++++++++



* Turn the module in a package, separate functionalities in different modules

* Simplify code wherever possible

* Get positive feedback from a couple of users



Contributions and bug reports are welcome.



Testing

+++++++



When nose is available all tests can be run using:



.. code:: bash



    nosetests3 --with-doctest  --doctest-extension=rst .



Otherwise standard python will suffice:



.. code:: bash



    python -m unittest objproxies_tests.py

    python -m doctest README.rst



Proxy Basics

************



Here's a quick demo of the ``ObjectProxy`` type::



    >>> from objproxies import ObjectProxy

    >>> p = ObjectProxy(42)



    >>> p

    42



    >>> isinstance(p, int)

    True



    >>> p.__class__

    



    >>> p*2

    84



    >>> 'X' * p

    'XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX'



    >>> hex(p)

    '0x2a'



    >>> chr(p)

    '*'



    >>> p ^ 1

    43



    >>> p ** 2

    1764



As you can see, a proxy is virtually indistinguishable from the object it

proxies, except via its ``__subject__`` attribute, and its ``type()``::



    >>> p.__subject__

    42



    >>> type(p)

    



You can change the ``__subject__`` of an ``ObjectProxy``, and it will then

refer to something else::



    >>> p.__subject__ = 99

    >>> p

    99

    >>> p-33

    66



    >>> p.__subject__ = "foo"

    >>> p

    'foo'



All operations are delegated to the subject, including ``setattr`` and

``delattr``::



    >>> class Dummy: pass

    >>> d = Dummy()

    >>> p = ObjectProxy(d)



    >>> p.foo = "bar"

    >>> d.foo

    'bar'



    >>> del p.foo

    >>> hasattr(d,'foo')

    False



Callback Proxies

****************



Sometimes, you may want a proxy's subject to be determined dynamically whenever

the proxy is used.  For this purpose, you can use the ``CallbackProxy`` type,

which accepts a callback function and creates a proxy that will invoke the

callback in order to get the target.  Here's a quick example of a counter that

gets incremented each time it's used, from zero to three::



    >>> from objproxies import CallbackProxy



    >>> callback = iter(range(4)).__next__

    >>> counter = CallbackProxy(callback)



    >>> counter

    0

    >>> counter

    1

    >>> str(counter)

    '2'

    >>> hex(counter)

    '0x3'



    >>> counter

    Traceback (most recent call last):

      ...

    StopIteration



As you can see, the callback is automatically invoked on any attempt to use the

proxy.  This is a somewhat silly example; a better one would be something like

a ``thread_id`` proxy that is always equal to the ID # of the thread it's

running in.



A callback proxy's callback can be obtained or changed via the ``get_callback``

and ``set_callback`` functions::



    >>> from objproxies import get_callback, set_callback

    >>> set_callback(counter, lambda: 42)



    >>> counter

    42



    >>> type(get_callback(counter))

    



Lazy Proxies

************



A ``LazyProxy`` is similar to a ``CallbackProxy``, but its callback is called

at most once, and then cached::



    >>> from objproxies import LazyProxy



    >>> def callback():

    ...     print("called")

    ...     return 42



    >>> lazy = LazyProxy(callback)

    >>> lazy

    called

    42

    >>> lazy

    42



You can use the ``get_callback`` and ``set_callback`` functions on lazy

proxies, but it has no effect if the callback was already called::



    >>> set_callback(lazy, lambda: 99)

    >>> lazy

    42



But you can use the ``get_cache`` and ``set_cache`` functions to tamper with

the cached value::



    >>> from objproxies import get_cache, set_cache

    >>> get_cache(lazy)

    42

    >>> set_cache(lazy, 99)

    >>> lazy

    99



Wrappers

********



The ``ObjectWrapper``, ``CallbackWrapper`` and ``LazyWrapper`` classes are

similar to their proxy counterparts, except that they are intended to be

subclassed in order to add custom extra attributes or methods.  Any attribute

that exists in a subclass of these classes will be read or written from the

wrapper instance, instead of the wrapped object.  For example::



    >>> from objproxies import ObjectWrapper

    >>> class NameWrapper(ObjectWrapper):

    ...     name = None

    ...     def __init__(self, ob, name):

    ...         ObjectWrapper.__init__(self, ob)

    ...         self.name = name

    ...     def __str__(self):

    ...         return self.name



    >>> w = NameWrapper(42, "The Ultimate Answer")

    >>> w

    42



    >>> print(w)

    The Ultimate Answer



    >>> w * 2

    84



    >>> w.name

    'The Ultimate Answer'



Notice that any attributes you add must be defined *in the class*.  You can't

add arbitrary attributes at runtime, because they'll be set on the wrapped

object instead of the wrapper::



    >>> w.foo = 'bar'

    Traceback (most recent call last):

      ...

    AttributeError: 'int' object has no attribute 'foo'



Note that this means that all instance attributes must be implemented as either

slots, properties, or have a default value defined in the class body (like the

``name = None`` shown in the example above.



The ``CallbackWrapper`` and ``LazyWrapper`` base classes are basically the same

as ``ObjectWrapper``, except that they use a callback or cached lazy callback

instead of expecting an object as their subject.



``LazyWrapper`` objects are particularly useful when working with expensive

resources, like connections or web browsers, to avoid their creation unless

absolutely needed. 



However resources usually must be released after use by calling a "``close``"

method of some sort. In this case the lazy creation could be triggered just

when the object is not needed anymore, by the call to ``close`` itself. For

this reason when extending ``LazyWrapper`` these methods can be overridden with

a ``@lazymethod`` replacement::



    >>> from objproxies import LazyWrapper, lazymethod



    >>> class LazyCloseable(LazyWrapper):

    ...     @lazymethod

    ...     def tell(self):

    ...         return 0

    ...     @lazymethod

    ...     def close(self):

    ...         print("bye")

    ...     @lazymethod

    ...     def __bool__(self):

    ...         return False



    >>> import tempfile



    >>> def openf():

    ...     print("called")

    ...     return tempfile.TemporaryFile('w')



    >>> lazyfile = LazyCloseable(openf)

    >>> lazyfile.tell()

    0

    >>> lazyfile.close()

    bye

    >>> bool(lazyfile)

    False



    >>> lazyfile = LazyCloseable(openf)

    >>> lazyfile.write('wake up')

    called

    7

    >>> lazyfile.tell()

    7

    >>> lazyfile.close()  # close for real

    >>> bool(lazyfile)

    True



Advanced: custom subclasses and mixins

**************************************



In addition to all the concrete classes described above, there are also two

abstract base classes: ``AbstractProxy`` and ``AbstractWrapper``.  If you want

to create a mixin type that can be used with any of the concrete types, you

should subclass the abstract version and set ``__slots__`` to an empty list::



    >>> from objproxies import AbstractWrapper



    >>> class NamedMixin(AbstractWrapper):

    ...     __slots__ = []

    ...     name = None

    ...     def __init__(self, ob, name):

    ...         super(NamedMixin, self).__init__(ob)

    ...         self.name = name

    ...     def __str__(self):

    ...         return self.name



Then, when you mix it in with the respective base class, you can add back in

any necessary slots, or leave off ``__slots__`` to give the subclass instances

a dictionary of their own::



    >>> from objproxies import CallbackWrapper, LazyWrapper



    >>> class NamedObject(NamedMixin, ObjectWrapper): pass

    >>> class NamedCallback(NamedMixin, CallbackWrapper): pass

    >>> class NamedLazy(NamedMixin, LazyWrapper): pass



    >>> print(NamedObject(42, "The Answer"))

    The Answer



    >>> n = NamedCallback(callback, "Test")

    >>> n

    called

    42

    >>> n

    called

    42



    >>> n = NamedLazy(callback, "Once")

    >>> n

    called

    42

    >>> n

    42



Both the ``AbstractProxy`` and ``AbstractWrapper`` base classes work by

assuming that ``self.__subject__`` will be the wrapped or proxed object.  If

you don't want to use any of the standard three ways of defining

``__subject__`` (i.e., as an object, callback, or lazy callback), you will need

to subclass ``AbstractProxy`` or ``AbstractWrapper`` and provide your own way

of defining ``__subject__``.