class Permanent(Exception):
    """A failure that retrying cannot fix."""


def call_with_retry(fn, attempts=3, sleep=None):
    """Retries transient failures with exponential backoff.

    A `Permanent` failure is raised immediately — retrying it only wastes
    calls. `sleep` is injected so the delays are observable in tests.
    """
    last = None
    for attempt in range(attempts):
        try:
            return fn()
        except Permanent:
            raise
        except Exception as error:
            last = error
            if attempt < attempts - 1 and sleep is not None:
                sleep(2 ** attempt)
    raise last