Package: objproxies
Theobjproxiesmodule 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.)Table of ContentsDevelopment statusv1.0 TODOTestingProxy BasicsCallback ProxiesLazy ProxiesWrappersAdvanced: custom subclasses and mixinsDevelopment statusThis is Python 3 port ofProxyTypeswrote by Phillip J. Eby as
part ofPEAKfor Python 2.The namespace was changed frompeak.util.proxiestoobjproxies. Other
than that it should be a compatible replacement.So far the following was accomplished:Streamlined files and setupPorted unittests and doctestsCleaned up syntaxv1.0 TODOTurn the module in a package, separate functionalities in different modulesSimplify code wherever possibleGet positive feedback from a couple of usersContributions and bug reports are welcome.TestingWhen nose is available all tests can be run using:nosetests3--with-doctest--doctest-extension=rst.Otherwise standard python will suffice:python-munittestobjproxies_tests.pypython-mdoctestREADME.rstProxy BasicsHere’s a quick demo of theObjectProxytype:>>> from objproxies import ObjectProxy
>>> p = ObjectProxy(42)

>>> p
42

>>> isinstance(p, int)
True

>>> p.__class__
<class 'int'>

>>> p*2
84

>>> 'X' * p
'XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX'

>>> hex(p)
'0x2a'

>>> chr(p)
'*'

>>> p ^ 1
43

>>> p ** 2
1764As you can see, a proxy is virtually indistinguishable from the object it
proxies, except via its__subject__attribute, and itstype():>>> p.__subject__
42

>>> type(p)
<class 'objproxies.ObjectProxy'>You can change the__subject__of anObjectProxy, 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, includingsetattranddelattr:>>> class Dummy: pass
>>> d = Dummy()
>>> p = ObjectProxy(d)

>>> p.foo = "bar"
>>> d.foo
'bar'

>>> del p.foo
>>> hasattr(d,'foo')
FalseCallback ProxiesSometimes, you may want a proxy’s subject to be determined dynamically whenever
the proxy is used.  For this purpose, you can use theCallbackProxytype,
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):
  ...
StopIterationAs 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
athread_idproxy 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 theget_callbackandset_callbackfunctions:>>> from objproxies import get_callback, set_callback
>>> set_callback(counter, lambda: 42)

>>> counter
42

>>> type(get_callback(counter))
<class 'function'>Lazy ProxiesALazyProxyis similar to aCallbackProxy, 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
42You can use theget_callbackandset_callbackfunctions on lazy
proxies, but it has no effect if the callback was already called:>>> set_callback(lazy, lambda: 99)
>>> lazy
42But you can use theget_cacheandset_cachefunctions to tamper with
the cached value:>>> from objproxies import get_cache, set_cache
>>> get_cache(lazy)
42
>>> set_cache(lazy, 99)
>>> lazy
99WrappersTheObjectWrapper,CallbackWrapperandLazyWrapperclasses 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 definedin 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 thename = Noneshown in the example above.TheCallbackWrapperandLazyWrapperbase classes are basically the same
asObjectWrapper, except that they use a callback or cached lazy callback
instead of expecting an object as their subject.LazyWrapperobjects 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 tocloseitself. For
this reason when extendingLazyWrapperthese methods can be overridden with
a@lazymethodreplacement:>>> 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)
TrueAdvanced: custom subclasses and mixinsIn addition to all the concrete classes described above, there are also two
abstract base classes:AbstractProxyandAbstractWrapper.  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.nameThen, 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
42Both theAbstractProxyandAbstractWrapperbase classes work by
assuming thatself.__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 subclassAbstractProxyorAbstractWrapperand provide your own way
of defining__subject__.