ext.py

# extension testbed
# autodoc_typehints_docinfo -- typehint をいい感じに表示する拡張
#
# * 型情報は typehint を書いておいてね
# * conf.py で autodoc_typehints = 'docinfo' と設定する

from docutils import nodes
from sphinx import addnodes
from sphinx.util.docfields import DocFieldTransformer
from sphinx.util.inspect import Signature


def process_signature(app, objtype, name, obj, options, args, retann):
    try:
        if callable(obj):
            signature = app.env.temp_data.setdefault('signatures', {}).setdefault(name, {})
            sig = Signature(obj)
            for param in sig.parameters.values():
                if isinstance(param.annotation, str) and param.name in sig.annotations:
                    annotation = sig.annotations[param.name]
                else:
                    annotation = param.annotation

                if annotation is not param.empty:
                    signature[param.name] = sig.format_annotation(param.annotation)
    except TypeError:
        pass


orig_transform_all = DocFieldTransformer.transform_all


def transform_all(self, node):
    signature = node.parent[0]
    fullname = '.'.join([signature['module'], signature['fullname']])
    signatures = self.directive.env.temp_data.get('signatures', {})
    if signatures.get(fullname, {}):
        field_lists = [n for n in node if isinstance(n, nodes.field_list)]
        if field_lists == []:
            insert_field_list(node, signatures[fullname])
        else:
            for field_list in field_lists:
                modify_field_list(field_list, signatures[fullname])
    orig_transform_all(self, node)


def insert_field_list(node, signature):
    field_list = nodes.field_list()
    for name, annotation in signature.items():
        field_name = nodes.field_name("param " + name, "param " + name)
        field_body = nodes.field_body()
        field_list += nodes.field("", field_name, field_body)

        field_name = nodes.field_name("type " + name, "type " + name)
        field_body = nodes.field_body("", nodes.paragraph(annotation, annotation))
        field_list += nodes.field("", field_name, field_body)

    desc = [n for n in node if isinstance(n, addnodes.desc)]
    if desc:
        index = node.index(desc[0])
        node.insert(index - 1, [field_list])
    else:
        node += field_list


def modify_field_list(node, signature):
    for name, annotation in signature.items():
        field_name = nodes.field_name("type " + name, "type " + name)
        field_body = nodes.field_body("", nodes.paragraph(annotation, annotation))
        node += nodes.field("", field_name, field_body)


def config_inited(app, config):
    if config.autodoc_typehints == 'docinfo':
        config.autodoc_typehints = 'none'
        DocFieldTransformer.transform_all = transform_all
        app.connect('autodoc-process-signature', process_signature)


def setup(app):
    app.setup_extension('sphinx.ext.autodoc')
    app.config.values['autodoc_typehints'][2].candidates += ('docinfo',)
    app.connect('config-inited', config_inited)

autodoc_typehints = 'signature' の場合（ext.pyを適用しないデフォルト動作）

この拡張を有効化して autodoc_typehints = 'docinfo' の場合（ext.pyを適用した動作）

autodoc_typehints = 'docinfo' の場合（ext.pyを適用した動作）で、 'sphinx.ext.intersphinx'も適用

設定

extensions = [
    'sphinx.ext.autodoc',
    'ext',
    'sphinx.ext.intersphinx'
]
autodoc_default_options = {
    'members': True,
    'undoc-members': True,
}
# autodoc_typehints = 'signature'
# autodoc_typehints = 'none'
autodoc_typehints = 'docinfo'
intersphinx_mapping = {'python': ('https://docs.python.org/3', None)}

動かしてみた感じ、 :rtype: は未対応でした