from functools import wraps


def cached_class(klass):
    """Decorator to cache class instances by constructor arguments.
    
    We "tuple-ize" the keyword arguments dictionary since
    dicts are mutable; keywords themselves are strings and
    so are always hashable, but if any arguments (keyword
    or positional) are non-hashable, that set of arguments
    is not cached.
    """
    cache = {}
    
    @wraps(klass, assigned=('__name__', '__module__'), updated=())
    class _decorated(klass):
        # The wraps decorator can't do this because __doc__
        # isn't writable once the class is created
        __doc__ = klass.__doc__
        def __new__(cls, *args, **kwds):
            key = (cls,) + args + tuple(kwds.iteritems())
            try:
                inst = cache.get(key, None)
            except TypeError:
                # Can't cache this set of arguments
                inst = key = None
            if inst is None:
                # Technically this is cheating, but it works,
                # and takes care of initializing the instance
                # (so we can override __init__ below safely);
                # calling up to klass.__new__ would be the
                # "official" way to create the instance, but
                # that raises DeprecationWarning if there are
                # args or kwds and klass does not override
                # __new__ (which most classes don't), because
                # object.__new__ takes no parameters (and in
                # Python 3 the warning will become an error)
                inst = klass(*args, **kwds)
                # This makes isinstance and issubclass work
                # properly
                inst.__class__ = cls
                if key is not None:
                    cache[key] = inst
            return inst
        def __init__(self, *args, **kwds):
            # This will be called every time __new__ is
            # called, so we skip initializing here and do
            # it only when the instance is created above
            pass
    
    return _decorated