Source code for nxpy.core.memo

# nxpy.core package ----------------------------------------------------------

# Copyright Nicola Musatti 2008 - 2012
# Use, modification, and distribution are subject to the Boost Software
# License, Version 1.0. (See accompanying file LICENSE.txt or copy at
# http://www.boost.org/LICENSE_1_0.txt)

# See http://sourceforge.net/nxpy for library home page. ---------------------

r"""
Memoize class instances according to a given key.
 
By default the key only assumes the 'True' value, thus implementing a singleton. 
"""

class Memo(object):
    r"""
    Base class for classes that require memoization.
    
    Care should be taken to avoid calling __init__() again for entities already constructed.
    
    """
    _dict = {}
    def __new__(cls, *args, **kwargs):
        k = cls._key(*args, **kwargs)
        try:
            return Memo._dict[(cls, k)]
        except KeyError:
            i = super(Memo, cls).__new__(cls)
            Memo._dict[(cls, k)] = i
            return i

    @staticmethod
    def _key(*args, **kwargs):
        r"""
        Returns the key by which instances are memoized.

        Subclasses should override it as a function of their construction arguments.
        
        """
        return True