Nick Coghlan <ncoghlan at gmail.com> added the comment:
I'm not clear on what you mean about allowing arbitrary names for the instance creation function - at that point we're back to subclasses not being able to use the default `pop_all()` implementation at all, and needing to duplicate the state transfer logic in the subclass (which we don't provide a public API to do, since the `_exit_callbacks` queue is deliberately private).
However, I'm thinking we could potentially change `pop_all` *itself* to accept a target object:
def pop_all(self, *, target=None):
"""Preserve the context stack by transferring it to a new instance
If a *target* stack is given, it must be either an `ExitStack`
instance, or else provide a callback registration method akin to `ExitStack.callback`.
"""
if target is None:
target = type(self)()
exit_callbacks = self._exit_callbacks
self._exit_callbacks = deque()
if isinstance(target, ExitStack):
target._exit_callbacks = exit_callbacks
else:
for cb in exit_callbacks:
target.callback(cb)
return target
The recipe fix would then be to make `Callback.cancel()` look like:
def cancel(self):
# We deliberately *don't* clean up the cancelled callback
self.pop_all(target=ExitStack())
(Tangent: https://bugs.python.org/issue32445 suggests adding a small optimisation to `ExitStack.callback` to skip adding the wrapper when `args` and `kwds` are both empty)
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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue32145>
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