Get rid of the circular dependency between root and client in twisted.names

[exarkun]

twisted.names.client uses twisted.names.root to construct the object returned by createResolver (sometimes). twisted.names.root uses twisted.names.client to bootstrap itself.

This circularity could be removed by giving twisted.names.root.Resolver a resolver argument so it doesn't need to import twisted.names.client to create one.

[borko]

I guess it's obsolate. root imports only dns, common, error from twisted.names. Cannot see client. So I think it should be closed.

[exarkun]

See http://twistedmatrix.com/trac/browser/trunk/twisted/names/root.py?rev=36682#L115

[borko]

pls give an idea of easiest unit testing this one.

[exarkun]

As long as all the code is exercised by existing unit tests, this kind of change doesn't require new unit tests (perhaps partly because it is too difficult to test, and we lack the tools to do so, but also because the unit tests exist to demonstrate the code works, not that it has a particular shape - any shape that makes the unit tests pass is fine, as far as the tests go; and then as developers we can apply additional constraints like "shorter is better than longer" or "easier to read is better than harder to read").

You can use  coverage.py to verify that the changed code really is exercised by the unit tests (or discover that it is not).