Python StringView implementation. Supports slicing, iteration and creating sub-views of existing StringViews. No copying, only reference semantics.

stringview.py

class StringView:
    """
    StringView implementation using minimal copying with maximum use of
    reference semantics. Creating a sub-view of an existing StringView using
    either object slicing or constructing one from another will reüse the same
    source string object, using a reference rather than a copy.
    The contents() method can similarly be used to get an iterator (Generator)
    to access the view contents sequentially without putting it all in memory
    at once.
    A brand new string object is only created if the StringView is cast to str.
    """
    def __init__(self, source: str, start=0, stop=None):
        if isinstance(source, StringView):
            self.__source = source.__source
            self.__start = source.__start + start
            self.__stop = source.__start + (min(stop, len(source)) if stop is not None else 0)
        else:
            self.__source = source
            self.__start = start
            self.__stop = stop if stop is not None else len(source)
        assert self.__start <= self.__stop
    def __str__(self):
        return self.__source[self.__start:self.__stop]
    def __repr__(self):
        return 'StringView({}, {}, {})'.format(
            repr(self.__source),
            self.__start,
            self.__stop
        )
    # these next two methods are provided so we can produce StringViews from StringViews
    def __len__(self):
        return self.__stop - self.__start
    def __getitem__(self, key):
        if isinstance(key, slice):
            if key.step is not None:
                raise TypeError('StringView does not support step when slicing')
            return StringView(self, key.start, key.stop)
        """
        not only is there no point returning a StringView of length 1, it's also
        slightly less memory-intensive to just return a str of length 1...
        """
        return self.__source[key]
    def contents(self):
        """
        Returns Generator for efficient no-copy iteration over string contents
        """
        return (self.__source[i] for i in range(self.__start, self.__stop))