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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 async def get_auth(request):
 """Returns the user_id associated with a particular request. Args: request: aiohttp Request object. Returns: The user_id associated with the request, or None if no user is associated with the request. Raises: RuntimeError: Middleware is not installed """


    auth_val = request.get(AUTH_KEY)
    if auth_val:
        return auth_val

    auth_policy = request.get(POLICY_KEY)
    if auth_policy is None:
        raise RuntimeError('auth_middleware not installed')

    request[AUTH_KEY] = await auth_policy.get(request)
    return request[AUTH_KEY]