iterator.py

# This library is free software.
#
# It was originally distributed under the terms of the GNU Lesser
# General Public License Version 2.
#
# python-messaging opts to apply the terms of the ordinary GNU
# General Public License v2, as permitted by section 3 of the LGPL
# v2.1. This re-licensing allows the entirety of python-messaging to
# be distributed according to the terms of GPL-2.
#
# See the COPYING file included in this archive
#
# The docstrings in this module contain epytext markup; API documentation
# may be created by processing this file with epydoc: http://epydoc.sf.net
"""Iterator with "value preview" capability."""


class PreviewIterator:
    """An ``iter`` wrapper class providing a "previewable" iterator.

    This "preview" functionality allows the iterator to return successive
    values from its ``iterable`` object, without actually moving forward
    itself. This is very usefuly if the next item(s) in an iterator must
    be used for something, after which the iterator should "undo" those
    read operations, so that they can be read again by another function.

    From the user point of view, this class supersedes the builtin iter()
    function: like iter(), it is called as PreviewIter(iterable).
    """
    def __init__(self, data):
        self._it = iter(data)
        self._cached_values = []
        self._preview_pos = 0

    #pylint: disable=non-iterator-returned
    def __iter__(self):
        return self

    def __next__(self):
        self.reset_preview()
        if len(self._cached_values) > 0:
            return self._cached_values.pop(0)
        return next(self._it)

    def preview(self):
        """
        Return the next item in the ``iteratable`` object

        But it does not modify the actual iterator (i.e. do not
        intefere with :func:`next`.

        Successive calls to :func:`preview` will return successive values from
        the ``iterable`` object, exactly in the same way :func:`next` does.

        However, :func:`preview` will always return the next item from
        ``iterable`` after the item returned by the previous :func:`preview`
        or :func:`next` call, whichever was called the most recently.
        To force the "preview() iterator" to synchronize with the "next()
        iterator" (without calling :func:`next`), use :func:`reset_preview`.
        """
        if self._preview_pos < len(self._cached_values):
            value = self._cached_values[self._preview_pos]
        else:
            value = next(self._it)
            self._cached_values.append(value)

        self._preview_pos += 1
        return value

    def reset_preview(self):
        self._preview_pos = 0