sort_by_list.py

class Category(object):
    def __init__(self, slug=None):
        self.slug = slug
    def __repr__(self):
        return self.slug


categories = [
    Category(slug='b'),
    Category(slug='d'),
    Category(slug='c'),
    Category(slug='a'),
]

order = ['a', 'b', 'c']

categories = sorted(categories,
                    key=lambda c: order.index(c.slug) if c.slug in order else len(order))

print(categories)
# [a, b, c, d]