源码
def backref(name, **kwargs):
"""Create a back reference with explicit keyword arguments, which are the same arguments one can send to :func:`relationship`.
Used with the ``backref`` keyword argument to :func:`relationship` in place of a string argument, e.g.::
'items':relationship(
SomeItem, backref=backref('parent', lazy='subquery'))
.. seealso::
:ref:`relationships_backref`
"""
return (name, kwargs)