[Python-Dev] multi-with statement (original) (raw)

R. David Murray rdmurray at bitdance.com
Sun May 3 00:33:15 CEST 2009

oN Sat, 2 May 2009 at 22:12, Georg Brandl wrote:

I see; you want to construct your context manager programmatically and pass it to "with" without knowing what is in there.

While this would be possible, we have to be aware that with this we would effectively change the context manager protocol, rather like the iterator protocol's getitem alternate realization. This muddies the definition of a context manager. (The interesting thing is that you could already implement that version without any new syntactic support, by giving tuples an enter/exit method pair.)

With the syntax in the patch, I will still have to implement a custom nesting context manager to do this, which sort of defeats the purpose. Not really. Having an unknown number of stacked context managers is not the purpose -- for that, I'd still say a custom nesting context manager is better, because it is also more explicit when created not at the "with" site. (You could even write it as a tuple subclass, if you like the tuple interface.)

As I understand it, the primary problem the patch Georg is talking about solves is the fact that currently if you pass multiple contexts to contextlib.nested, and one of the later items in the argument list throws an error, the context(s) from the earlier context manager(s) does not get cleaned up properly. This patch solves that problem very neatly.

I'm +1 on the patch, including preferring the syntax over the alternative.

Georg, maybe you should post the link to the python-ideas discussion?

--David

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