Paul Rudin added the comment:
Although formatannotation is undocumented, its actually documented as a default value for one of the parameters of formatargspec, so it does form part of the module's public interface. However formatargspec is deprecated, so it's still OK to deprecate formatannotation.
formatannotation is also used as a helper within Parameter, which isn't deprecated, but the public interface or docs of Parameter don't mention formatannotation, so using _formatannotation instead is fine.
Since I've looked through this and, as far as I can tell, nobody is working on it, I'll make the changes and make a PR.
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Python tracker
https://bugs.python.org/issue17972
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