You've probably ran into that problem too while writing an application with multi-user access: Python has no apparent way to effectively hide attributes from unauthorized access other than running in an restricted environment (which isn't supported anymore in Python 2.1 and up) or using the standard lib's Bastion object wrapper.

Since using the restricted mode is not an option anymore or can't be used due to being too restrictive (e.g. your code does assignments to __class__ and __dict__ in a few vital places), we've created a new type that hides Python object attributes away in the C implementation. These attributes are not accessible from within Python. Of course, a debugger will give you access and some special extension module that knows about the internal structure of the types could too. But for most applications, this level security suffices.

Instead of creating a special Bastion-like type we decided to write a more generic and extendable version of a Proxy object. The object itself doesn't provide too much functionality, but it effectively hides the wrapped object and provides controlled access to its attributes.

1.1 Object Reference: Weak or Strong

Another nice feature that a proxy object allows you to implement, is that of weak references, i.e. references to objects that don't prevent them to be garbage collected.

Since Python uses a reference count garbage collection scheme, circular references cause memory leakage as soon as the reference to the cycle is dropped (the involved objects are not garbage collected because their reference count never drops to zero).

Even though Python does have weak reference support for a number of basic types nowadays[1], this support only works if the referencing objects know how to handle weak references. mxProxy object by contrast can be used to create weak references to any Python object and store them in any Python object container or attribute.

There are two flavors of Proxy object references: strong references which act just like any other reference you use to the object in Python and weak references. The WeakProxy() constructor must be used for the latter. Details are described in section 1.5Weak References below.

1.2 Access Protocol

To control access to the wrapped objects attributes and methods, the Proxy provides two features:

● Access is only granted if the attributes name appears in a special interface list of accessible names.

● Public access can be further controlled if the object provides the methods .__public_getattr__() and/or .__public_setattr__().

The access scheme first checks the interface list. If given, only attributes with names appearing in it can be set or fetched. All other accesses are cancelled by raising an AccessError exception (which is a subclass of the standard AttributeError).

The presence of the interface list also indicates whether methods that are to be returned to the requesting object should be Proxy-wrapped on-the-fly or not. Wrapped methods only have the callable slot enabled, thus inhibiting access to the enclosed object reference.

In case an attribute it to be fetched, the Proxy checks the availability of a .__public_getattr__() method. If found, this method is called with the name as parameter and whatever this method returns is returned to the requesting object. Otherwise, the Proxy uses the standard getattr()-functionality to fetch the attribute.

Setting attributes is done in a similar way: the Proxy checks the availability of a .__public_setattr__() method. If found, this method is called with parameters name and value. Otherwise, the standard setattr() functionality is used.

1.3 Cleanup Protocol

While Python provides support for garbage collection nowadays, it is still possible to create systems of cyclically referenced objects that the built-in mechanism cannot collect. Since these reference loops cannot automatically be broken by the system, the user has to provide means of breaking the circles at the proper places. One such place is the object destructor of objects that explicitly contain such references.

The Proxy object can be used to provide an indirect pointer into such a circle of objects that reference each other. When the system deletes the Proxy object, its destructor tries to find a special method .__cleanup__() in the wrapped object and calls it before continuing the destruction. This allows the object to break the reference circles, e.g. by performing a self.__dict__.clear() or something similar.

Errors raised in .__cleanup__() calls are ignored. Warnings are printed to stderrin case Python is run in verbose mode (python -v) and tracebacks printed if run in debug mode (python -d).

1.4 Implicit Access

On many occasions object attributes are not explicitly accessed by e.g. using object.attribute, but indirect through built-in functions or the interpreter itself. This poses a problem to the Proxy, since there are different ways to reach an objects implementation. For Python instances all important hooks are reachable via getattr(), e.g. .__len__() and .__add__(). This is different for extension and built-in types: they use a system of slots for providing access to their data.

Proxy objects also allow managing the type slot interface for most applications. This means that you can transparently use the Proxy to wrap built-in types such as lists or tuples and use the Proxy just like you would use the referenced built-in object itself.

Since Proxies are about access management, you can also restrict access to these slots. For simplicity, the names you have to use for different slots are exactly those you would define for Python classes, e.g. the length slot is named '__len__', the sequence and mapping get item slot are grouped under the name '__getitem__'. These names have to be explicitly stated in the interface list you pass to the Proxy constructor when creating the proxy in case you do define an interface list. If you don't specify such a list, no direct interface restriction is applied.

These type slot names are defined:

Generic:

__call__, __hash__, __str__, __cmp__

Mapping:

__getitem__, __setitem__, __len__

Sequence:

__add__, __repeat__, __getitem__, __getslice__ __setitem__, __getitem__

Number:

__add__, __sub__, __mul__, __div__ __mod__, __divmod__, __pow__, __neg__, __pos__, __abs__, __true__ __invert__, __lshift__, __rshift__, __and__, __str__, __or__, __int__ __long__, __float__

Note: number coercion does not yet work, so most of these are currently useless !

Omissions currently are: __coerce__, __hex__ and __oct__. __repr__ is handled by the Proxy object itself since it would otherwise possibly expose address information about the underlying object.

Proxies also work transparently for Python instances. To achieve this, another proxy-like object which is a real Python instance has to be put in front of the Proxy object. Access to this instance then gets translated into getattr()-calls by the interpreter, these calls are filtered through the Proxy and the wrapped instance object's attribute then gets accessed in the usual way. The result is passed back to the requesting object.

1.5 Weak References

Proxy objects work in two modes: keeping a strong or a weak reference to the object. The Proxy() constructor returns a Proxy object using a strong reference, the WeakProxy() constructor one using a weak reference.

Weak references are called weak because they don't keep the object alive by incrementing the reference count on the referenced object. Since objects get garbage collected when this reference count falls down to 0, a weak reference can become invalid at any time. The mxProxy implementation raises a LostReferenceError in case a weak reference to such an object is used.

This may sound like a pretty flaky feature at first, but the main pro argument for these weak references is that you can build up circular references without having to fear about them not being properly garbage collected. Using strong references (which do increment the reference count and thus keep the object alive as long as the reference is around) would produce cycles in the referencing scheme which the Python garbage collection (GC) mechanism cannot automatically break causing the cycle to become unreachable from the Python namespaces: a severe memory leak.

Weak references in mxProxy work by using a global dictionary of all objects handled through weak reference Proxies. This dictionary is checked prior to every action on a weak Proxy and after its deletion. You can also force a check by calling the checkweakrefs() anytime you like, e.g. at regular intervals.

The dictionary holds a strong reference to the object keeping it alive until the next check is done. During the check all handled objects are inspected to see if their reference count has gone down to 1 (phantom objects: only the dictionary references them). If this is the case, all weak proxies are marked defunct and the object is removed from the dictionary causing it to be garbage collected by the Python GC mechanism. All subsequent actions on the weak references to this object will then cause a LostReferenceError exception to be raised.

2. mx.Proxy.Proxy Object

The Proxy object implements all of the above and provides the following interface.

2.1 Proxy Object Constructors

These constructors are available in the package:

Proxy(object,interface=None,passobj=None)

Returns a new Proxy instance that wraps object.

interface can be given as sequence of strings and/or objects with __name__ attribute or as dictionary with string keys (only the keys are currently used) and tells the Proxy to only allow access to these names. If not given or None, no filtering is done by the Proxy (see above on how access is managed).

passobj can be provided to retrieve the wrapped object from the Proxy at a later point using the .proxy_object()method.

InstanceProxy(object,interface=None,passobj=None)

Same as above, except that a Python instance wraps the Proxy object.

This makes the Proxy transparent for access to wrapped Python instances, meaning that the Proxy will act as if it were the wrapped object itself (with the added features mentioned above).

CachingInstanceProxy(object,interface=None,passobj=None)

Same as InstanceProxy(), except that a read cache is used by the Proxy which caches all queried attributes in the Proxy instance's dictionary.

Note that this may introduce circular references if not used properly. Cached attributes are not looked up in the wrapped instance after the first lookup -- if their value changes, this won't be noticed by objects that access the object through this wrapper.

SelectiveCachingInstanceProxy(object,interface=None,passobj=None)

Same as InstanceProxy(), except that a read cache is used by the Proxy which caches certain queried attributes in the Proxy instance's dictionary depending on their type.

The cached types are defined by the .proxy_cacheable_types attribute. It defaults to only cache Python methods.

MethodCachingProxy(object,interface=None,passobj=None)

Alias for SelectiveCachingInstanceProxy().

ReadonlyInstanceProxy(object,interface=None,passobj=None)

Same as InstanceProxy(), except that write access will result in an AccessError being raised

ProxyFactory(Class,interface=None)

Returns a factory object for producing Class instances that are automatically wrapped using Proxy-instances.

interface is passed to the Proxy constructor, pass-objects are not supported.

InstanceProxyFactory(Class,interface=None)

Returns a factory object for producing Class instances that are automatically wrapped using InstanceProxy-instances.

WeakProxy(object,interface=None,passobj=None)

Returns a new weak referencing Proxy instance that points to object.

passobj can be provided to retrieve the wrapped object from the Proxy at a later point using the .proxy_object()method.

For details on weak references and how they work, see section 1.5Weak References.

2.2 Proxy Object Instance Methods

A Proxyinstance proxy defines these methods in addition to the ones available through restricted access to the wrapped object:

.proxy_defunct()

Return 1 iff the referenced object has already been garbage collected.

.proxy_getattr(name)

Same as getattr(proxy,name).

.proxy_object(passobj)

Returns the wrapped object provided the given passobj is identical to the one used for creating the Proxy.

Simple equality is not enough -- it has to be the same object.

.proxy_setattr(name,value)

Same as setattr(proxy,name,value).

Note that all attribute names starting with 'proxy_' are interpreted as being Proxy attributes and are not passed to the wrapped object. Access to these attributes is not subject to the restrictions explained above.

2.3 Proxy Object Instance Variables

Proxy instances do not provide any instance variables themselves. They do provide restricted access to the variables of the wrapped object though.

Note that all attribute names starting with 'proxy_' are interpreted as being Proxy attributes and are not passed to the wrapped object. Also, access to these attributes is not subject to the restrictions explained above.

3. mx.Proxy Functions

These functions are available:

checkweakrefs()

Calling this function causes the global dictionary used for managing weak references to be checked for phantom objects. If such objects are found, they are garbage collected during the call and weak referencing Proxies pointing to them are defunct.

Weak references cause the objects to stay alive until either a proxy is used on them (causing an exception), a proxy referencing them is deleted or this function is called. To ensure the timely garbage collection of the objects, call this function at regular intervals.

finalizeweakrefs()

For internal use only:
Forces finalization of the weak reference implementation. Subsequent usage of weak references will cause errors to be raised.

Calling this function after finalization is not an error.

initweakrefs()

For internal use only:
Initializes or reinitializes the weak reference implementation. This first forces a finalization of the previous state if the implementation has already been used and then starts again with a clean weak reference dictionary.

4. mx.Proxy Constants

These constants are available:

AccessError

Exception object used for access specific errors that the module raises. It is a subclass of AttributeError.

5. Examples of Use

Here is a very simple one:

from mx import Proxy

class DataRecord:

a = 2

b = 3

# Make read-only:

def __public_setattr__(self,what,to):

raise Proxy.AccessError('read-only')

# Cleanup protocol

def __cleanup__(self):

print 'cleaning up',self

o = DataRecord()

# Wrap the instance:

p = Proxy.InstanceProxy(o,('a',))

# Remove o from the accessible system:

del o

print 'Read p.a through Proxy:',p.a

# This will cause an exception, because the object is read-only

p.a = 3

# This will cause an exception, because no access is given to .b

print p.b

# Clear all traces of the provious exceptions (they might contain

# references to p) by issuing another one. Note that not doing

# so will cause the following 'del p' to not destroy the final

# reference to p... (don't ask why).

1/0

# Deleting the Proxy will also delete the wrapped object, if there

# is no other reference to it in the system. It will invoke

# the __cleanup__ method in that case.

del p

# If you want to have the wrapping done automatically, you can use

# the InstanceProxyFactory:

DataRecord = Proxy.InstanceProxyFactory(DataRecord,('a',))

# This gives the same behaviour...

p = DataRecord()

print p.a

p.a = 3

print p.b

More examples will eventually appear in the Examples subdirectory of the package.

6. Package Structure

[Proxy]

Doc/

Examples/

[mxProxy]

Proxy.py

Entries enclosed in brackets are packages (i.e. they are directories that include a __init__.py file). Ones without brackets are just simple subdirectories that are not accessible via import.

The package imports all symbols from the Proxy sub module which in turn imports the extension module, so you only need to 'from mx import Proxy' to start working.

7. Support

eGenix.com is providing commercial support for this package. If you are interested in receiving information about this service please see the eGenix.com Support Conditions.

8. Copyright & License

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By downloading, copying, installing or otherwise using the software, you agree to be bound by the terms and conditions of the following eGenix.com Public License Agreement.

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[1] It didn't support weak references at the time mxProxy was written. The weak reference implementation in Python was in part inspired by mxProxy.

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