[docs] [issue32145] Wrong ExitStack Callback recipe

[Barry A. Warsaw]

Barry A. Warsaw <barry at python.org> added the comment:

I wasn't even aware that pop_all() tries to create the concrete subtype.  I wonder how common subclassing ExitStack is in practice.  (Of course, the answer is that we'll never know.)

For 3.7, we should probably delegate instance creation to a new non-private method, e.g.

    new_stack = self._make_instance()

so *if* you subclass, you can create the new ExitStack subclass instances any way you want.  I would then change the default implementation to make an ExitStack() explicitly, as per the current documentation.

+1 for fixing the recipe and not changing the code for 3.6.  You might want to add a note describing the current behavior for <= 3.6 and indicating that this will change for 3.7+

Finally, while you're at it, is there any reason not to change the recipe to use:

    super().__init__()

instead?

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<https://bugs.python.org/issue32145>
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