'A custom list that manages position information for its children.
``OrderingList`` is a ``collection_class`` list implementation that
syncs position in a Python list with a position attribute on the
mapped objects.
This implementation relies on the list starting in the proper order,
so be **sure** to put an ``order_by`` on your relationship.
:param ordering_attr:
Name of the attribute that stores the object\'s order in the
relationship.
:param ordering_func: Optional.  A function that maps the position in
the Python list to a value to store in the
``ordering_attr``.  Values returned are usually (but need not be!)
integers.
An ``ordering_func`` is called with two positional parameters: the
index of the element in the list, and the list itself.
If omitted, Python list indexes are used for the attribute values.
Two basic pre-built numbering functions are provided in this module:
``count_from_0`` and ``count_from_1``.  For more exotic examples
like stepped numbering, alphabetical and Fibonacci numbering, see
the unit tests.
:param reorder_on_append:
Default False.  When appending an object with an existing (non-None)
ordering value, that value will be left untouched unless
``reorder_on_append`` is true.  This is an optimization to avoid a
variety of dangerous unexpected database writes.
SQLAlchemy will add instances to the list via append() when your
object loads.  If for some reason the result set from the database
skips a step in the ordering (say, row \'1\' is missing but you get
\'2\', \'3\', and \'4\'), reorder_on_append=True would immediately
renumber the items to \'1\', \'2\', \'3\'.  If you have multiple sessions
making changes, any of whom happen to load this collection even in
passing, all of the sessions would try to "clean up" the numbering
in their commits, possibly causing all but one to fail with a
concurrent modification error.
Recommend leaving this with the default of False, and just call
``reorder()`` if you\'re doing ``append()`` operations with
previously ordered instances or when doing some housekeeping after
manual sql operations.'

def __init__(self, ordering_attr=None, ordering_func=None, reorder_on_append=False):

    self.ordering_attr = ordering_attr
    if (ordering_func is None):
        ordering_func = count_from_0
    self.ordering_func = ordering_func
    self.reorder_on_append = reorder_on_append
