# Copyright 2009-2012 Ram Rachum.
# This program is distributed under the MIT license.

'''
This module defines the `Persistent` class.

See its documentation for more information.



Note: This module is still experimental

todo: need to raise an exception if we're getting pickled with
an old protocol?

todo: make it polite to other similar classes. Perhaps make it a metaclass?
'''
from .copy_modes import DontCopyPersistent
from python_toolbox import copy_tools


__all__ = ['Persistent']


class Persistent(object):
    '''
    Object that sometimes shouldn't really be duplicated.

    Say some plain object references a `Persistent` object. Then that plain
    object gets deepcopied with the `DontCopyPersistent` copy mode. The plain
    object will get deepcopied, but the `Persistent` object under it will not!
    The new copy of the plain object will refer to the same old copy of the
    `Persistent` object.
    
    This is useful for objects which are read-only and possibly heavy. You may
    use `Persistent` as a base class for these kinds of objects.
    
    Keep in mind that a `Persistent` is read-only. This means that starting
    from the first time that it is copied or put in a queue, it should not be
    changed.

    There is no mechanism that enforces that the user doesn't change the
    object, so the user must remember not to change it.
    
    Note: This class is still experimental.
    '''    
    
    def __deepcopy__(self, memo):
        '''
        Deepcopy the object. If `DontCopyPersistent` is given, only mock-copy.
        
        When this method receieves an instance of `DontCopyPersistent` as a
        memo dictionary, it will not actually deepcopy the object but only
        return a reference to the original object.
        '''
        if isinstance(memo, DontCopyPersistent):
            memo[id(self)] = self
            return self
        else:
            new_copy = copy_tools.deepcopy_as_simple_object(self, memo)
            return new_copy
    
        
    def __copy__(self):
        return self
    