def lendable_choices():
"""Generate a tuple of lendable choices.
Returns:
A tuple containing value/label pairs representing the
lendable choices for a select widget.
"""
choices = (('', 'Choose lendable ...'),
('all', 'All lendables'))
choices += tuple((lendable.__name__.lower(), lendable.name)
for lendable in Lendable.__subclasses__())
return choices
