ProxyProperty: Properties with proxy & instance varibale emulation

discriptor_magic.py

# -*- coding: utf-8 -*-

__author__ = "Arun KR (kra3) <the1.arun@gmail.com>"
__license__ = "Simplified BSD"

'''
Can be found at: https://gist.github.com/2409397
Public Clone URL: git://gist.github.com/2409397.git
Private Clone URL: git@gist.github.com:2409397.git
'''

class ProxyProperty(object):
    '''
    How to use:
    1) For class variable: set <is_class_var> to True, also pass in a <name>
    2) For instance variable: just create an object with a <name>;
    3) Optionally provide (in case #1 & #2), gfn & sfn: those should be closures or functions, which take a single value and transform it into another value.
    PS: You should catch errors in gfn/sfn, if you need different behaviour than class is provided.
       Default behaviour if gfn raised error will be: default value will be send back, which is None.
       Default behaviour if sfn raised error will be: value will not be stored.
    PS: a instance/class variable using provided 'name' will be created in the original class, automatically.
       it will not be accessible as <name> in the object. If you really need it, use _ProxyProperty__<name>

    What if second option is not there?
    -> You can't emulate instance variable behaviour.
    -> provided the purpose of doing all this is to transform data when getting & saving using closures.

    Behaviour when applied to a class variable:
    -> As instance variable if by default, as class variable if argument <is_class_var> is True.

    Now: Here are some examples:

    >>> class B(object):
    ...     aa = ProxyProperty('aa', True) # creating class variable
    ...     bb = ProxyProperty('bb') # this is instance variable
    ...     def __init__(self):
    ...         self._bb = ''
    >>> b=B()
    >>> b.aa
    >>> B.aa
    >>> b.bb
    >>> B.bb
    >>> #b.aa = 1  # this will raise Exception
    >>> b.bb = 2
    >>> b.bb
    2
    >>> B.aa = 1
    >>> B.aa
    1
    >>> b.aa
    1

    '''

    def __init__(self, name, is_class_var=False, gfn=lambda s:s, sfn=lambda s:s):
        self.is_class_var = is_class_var
        # prepend name with _ProxyProperty__, so that there will be no clash in actual object about storing variable that we create (automatically) in originating class
        self.name = "_ProxyProperty__%s" % name
        self.gfn = gfn
        self.sfn = sfn

    def __get__(self, instance, owner):
        try:
            if not self.is_class_var:
                value = self.gfn(getattr(instance, self.name))
            else:
                value = self.gfn(getattr(owner, self.name))
        except:
            return None # default value
        else:
            return value

    def __set__(self, instance, value):
        try:
            value = self.sfn(value)
        except:
            # sfn caused error, so we will not save value
            pass
        else:
            if not self.is_class_var:
                setattr(instance, self.name, value)
            else:
                setattr(owner, self.name, value)


if __name__ == '__main__':
    ''' Run as "python discriptor_magic.py -v" to see verbose doctest result'''
    import doctest
    doctest.testmod()