Built-in non-data version of property?

Question

class Books():
    def __init__(self):
        self.__dict__['referTable'] = 1

    @property
    def referTable(self):
        return 2

book = Books()
print(book.referTable)
print(book.__dict__['referTable'])

Running:

vic@ubuntu:~/Desktop$ python3 test.py 
2
1

Books.referTable being a data descriptor is not shadowed by book.__dict__['referTable']:

The property() function is implemented as a data descriptor. Accordingly, instances cannot override the behavior of a property.

To shadow it, instead of property built-in descriptor i must use my own descriptor. Is there a built in descriptor like property but which is non-data?

Answer 1

To expand on my comment, why not simply something like this:

>>> class Books():
...     def __init__(self):
...         self.__dict__['referTable'] = 1
...     @property
...     def referTable(self):
...         try:
...             return self.__dict__['referTable']
...         except KeyError:
...             return 2
... 
>>> a = Books()
>>> a.referTable
1
>>> del a.__dict__['referTable']
>>> a.referTable
2

Now, I'd like to note that I don't think this is good design, and you'd be much better off using a private variable rather than accessing __dict__ directly. E.g:

class Books():
    def __init__(self):
        self._referTable = 1

    @property
    def referTable(self):
        return self._referTable if self._referTable else 2

In short, the answer is no, there is no alternative to property() that works in the way you want in the Python standard library.

Answer 2

There is something very similar to a built-in non-data descriptor -- the class attribute:

class Books():

    referTable = 'default'

    def __init__(self, referTable=None):
        if referTable is not None:
            self.referTable = referTable


book = Books()
print(book.referTable)
# default
book.referTable = 'something specific'
print(book.referTable)
# something specific

If you need something more like a property (for example, you want a function to do some heavy-lifting the first time, but then use that first value for all future references), then you will need to build it yourself:

class OneTime(object):

    def __init__(self, method):
        self.name = method.__name__
        self.method = method

    def __get__(self, inst, cls):
        if inst is None:
            return self
        result = self.method(inst)
        inst.__dict__[self.name] = result
        return result

class Books(object):

    @OneTime
    def referTable(self):
        print 'calculating'
        return 1 * 2 * 3 * 4 * 5

b = Books()
print b.__dict__
print b.referTable
print b.__dict__
print b.referTable

With the following results:

{}
calculating
120
{'referTable': 120}
120