Currently, there is no documented way to uninstall a global opener installed with urllib.request.install_opener(...), however, it seems that either of the following will uninstall the global opener:
urllib.request.install_opener(None): This will set urllib.request._opener to its initial value, None.
The documentation for neither of these indicates that they will uninstall global openers installed with urllib.request.install_opener(...) - so relying on them doing it may not be that wise. However, I think it would be quite useful to have a documented way of uninstalling openers, and I think that urllib.request.install_opener(None) would be best to do that.
Agreed on this point, I did not mean to suggest it should reference it either, I just mentioned it to point out that those methods will uninstall the global opener, even though it is not documented to be the case.