[issue19172] selectors: add keys() method

[Antoine Pitrou]

Antoine Pitrou added the comment:

Indeed it's easy enough to call list() or set() on the result if you need it.

On the other hand, the problem with returning a dict view is that it makes the return type dependent on an implementation detail. Returning a simple iterator would be great, except that you can't as easily test it for emptiness.

So perhaps two methods are needed:
- a keys() method returning an iterator
- a has_keys() method returning a boolean

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue19172>
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