once a..., always a...

once a (something), always a (something)

A person's innate or fundamental nature is not something they are willing or able to change. He says he's trying to leave his life of crime behind, but once a criminal, always a criminal. Even years after leaving the stage, she still can't resist putting a dramatic flair into everything she says—once an actor, always an actor.
See also: always, once

once a..., always a...

in certain respects people do not change, although circumstances change: He can’t resist explaining things in detail. Once a teacher, always a teacher.
See also: always, once