If a cancel scope becomes cancelled before entering its with block, the Cancelled exception will be raised at the first checkpoint inside the with block. This allows a CancelScope to be created in one task and passed to another, so that the first task can later cancel some work inside the second.

Cancel scopes are not reusable or reentrant; that is, each cancel scope can be used for at most one with block. (You’ll get a RuntimeError if you violate this rule.)