Generalizing for Public Declarations, Too

Now that we have a Private implementation, it’s straightforward to generalize the code to allow for Public declarations too—they are essentially the inverse of Private declarations, so we need only negate the inner test. The example listed in this section allows a class to use decorators to define a set of either Private or Public instance attributes (attributes stored on an instance or inherited from its classes), with the following semantics:

 

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  • Private declares attributes of a class’s instances that cannot be fetched or assigned, except from within the code of the class’s methods. That is, any name declared Private cannot be accessed from outside the class, while any name not declared Private can be freely fetched or assigned from outside the class.
  • Public declares attributes of a class’s instances that can be fetched or assigned from both outside the class and within the class’s methods. That is, any name declared Public can be freely accessed anywhere, while any name not declared Public cannot be accessed from outside the class.

Private and Public declarations are intended to be mutually exclusive: when using Private, all undeclared names are considered Public, and when using Public, all undeclared names are considered Private. They are essentially inverses, though undeclared names not created by class methods behave slightly differently—they can be assigned and thus created outside the class under Private (all undeclared names are accessible), but not under Public (all undeclared names are inaccessible).

Again, study this code on your own to get a feel for how this works. Notice that this scheme adds an additional fourth level of state retention at the top, beyond that described in the preceding section: the test functions used by the lambdas are saved in an extra enclosing scope. This example is coded to run under either Python 2.6 or 3.0, though it comes with a caveat when run under 3.0 (explained briefly in the file’s docstring and expanded on after the code):

"""
Class decorator with Private and Public attribute declarations.
Controls access to attributes stored on an instance, or inherited
by it from its classes. Private declares attribute names that
cannot be fetched or assigned outside the decorated class, and
Public declares all the names that can. Caveat: this works in
3.0 for normally named attributes only: __X__ operator overloading
methods implicitly run for built-in operations do not trigger
either __getattr__ or __getattribute__ in new-style classes.
Add __X__ methods here to intercept and delegate built-ins.
"""

traceMe = False
def trace(*args):
    if traceMe: print('[' + ' '.join(map(str, args)) + ']')

def accessControl(failIf):
    def onDecorator(aClass):
        class onInstance:
            def __init__(self, *args, **kargs):
                self.__wrapped = aClass(*args, **kargs)
            def __getattr__(self, attr):
                trace('get:', attr)
                if failIf(attr):
                    raise TypeError('private attribute fetch: ' + attr)
                else:
                    return getattr(self.__wrapped, attr)
            def __setattr__(self, attr, value):
                trace('set:', attr, value)
                if attr == '_onInstance__wrapped':
                    self.__dict__[attr] = value
                elif failIf(attr):
                    raise TypeError('private attribute change: ' + attr)
                else:
                    setattr(self.__wrapped, attr, value)
        return onInstance
    return onDecorator

def Private(*attributes):
    return accessControl(failIf=(lambda attr: attr in attributes))

def Public(*attributes):
    return accessControl(failIf=(lambda attr: attr not in attributes))

See the prior example’s self-test code for a usage example. Here’s a quick look at these class decorators in action at the interactive prompt (they work the same in 2.6 and 3.0); as advertised, non-Private or Public names can be fetched and changed from outside the subject class, but Private or non-Public names cannot:

>>> from access import Private, Public

>>> @Private('age')                             # Person = Private('age')(Person)
... class Person:                               # Person = onInstance with state
...     def __init__(self, name, age):
...         self.name = name
...         self.age  = age                     # Inside accesses run normally
...
>>> X = Person('Bob', 40)
>>> X.name                                      # Outside accesses validated
'Bob'
>>> X.name = 'Sue'
>>> X.name
'Sue'
>>> X.age
TypeError: private attribute fetch: age
>>> X.age = 'Tom'
TypeError: private attribute change: age

>>> @Public('name')
... class Person:
...     def __init__(self, name, age):
...         self.name = name
...         self.age  = age
...
>>> X = Person('bob', 40)                       # X is an onInstance
>>> X.name                                      # onInstance embeds Person
'bob'
>>> X.name = 'Sue'
>>> X.name
'Sue'
>>> X.age
TypeError: private attribute fetch: age
>>> X.age = 'Tom'
TypeError: private attribute change: age