[issue19465] selectors: provide a helper to choose a selector using constraints

[STINNER Victor]

STINNER Victor added the comment:

> It still seems to me that this is pretty atypical use of selectors

I already implemented something similar to subprocess.Popen.communicate() when I was working on old Python versions without the timeout parameter of communicate().
http://ufwi.org/projects/edw-svn/repository/revisions/master/entry/trunk/src/nucentral/nucentral/common/process.py#L222

IMO calling select with a few file descriptors (between 1 and 3) and destroying quickly the "selector" is no a rare use case.

If I would port my code to selectors, I don't want to rewrite it to keep the selector alive longer, just because selectors force me to use the super-powerful fast epoll/kqueue selector.

(To be honest, I will probably not notice any performance impact. But I like reducing the number of syscalls, not the opposite :-))

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<http://bugs.python.org/issue19465>
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