Currently the headers field of urllib.response.addinfourl, which is also returned by the info() method, is annotated as having type Mapping[str, str]. However, in practice and in the documentation, it has the type email.message.Message. While that is a mapping, it has a lot of extra methods as well.

Note that headers and info() are actually defined on the addinfo base class of addinfourl, but addinfo is never instantiated itself except by the unit test.