Write a retry decorator

Medium~20 min

Write a decorator factory retry(times, exceptions):

The decorated function is called up to times times in total.
If it raises an exception that is an instance of one of exceptions (a tuple of exception classes), try again; any other exception propagates immediately.
If every attempt fails, re-raise the last exception.
Preserve the wrapped function's __name__ (use functools.wraps).

The harness decorates a flaky function that fails a given number of times and reports what happened.

Function

Implement retry. Input: times: int, exceptions: tuple[type[BaseException], ...]. Returns: a decorator.

Examples

>>> retry(3, 2, 'transient')

{'name': 'flaky', 'calls': 3, 'result': 42}

Fails twice, succeeds on the third call.

>>> retry(3, 5, 'transient')

{'name': 'flaky', 'calls': 3, 'raised': 'TransientError', 'message': 'attempt 3'}

Provided environment

This code runs before yours (fake clients, clocks and the test harness). You can use its names; don't edit them.

class TransientError(Exception):
    pass

class FatalError(Exception):
    pass

def __harness__(retry, times, failures, kind):
    calls = {"n": 0}
    error = {"transient": TransientError, "fatal": FatalError}[kind]

    def flaky(x):
        """Fails `failures` times, then returns x * 2."""
        calls["n"] += 1
        if calls["n"] <= failures:
            raise error(f"attempt {calls['n']}")
        return x * 2

    wrapped = retry(times, (TransientError,))(flaky)
    try:
        result = wrapped(21)
        outcome = {"result": result}
    except Exception as exc:
        outcome = {"raised": type(exc).__name__, "message": str(exc)}
    outcome["calls"] = calls["n"]
    outcome["name"] = getattr(wrapped, "__name__", None)
    return outcome

Details

Level: API integration against fakes, data processing, error handling, decorators and asyncio. Several cooperating parts.
Roles: Python Developer, Backend Engineer, Site Reliability Engineer
Topics: Decorators, Retries and backoff, Exceptions
Tests: 2 public (run with Run), 3 hidden (graded on Submit)
Grading: Exact match of the return value