Fix 'NoneType' object is not iterable on Generics

[icgood]

Fix the following exception during doc builds

Exception occurred:
  File "/path/to/.venv/lib/python3.7/site-packages/sphinx_autodoc_typehints.py", line 119, in format_annotation
    extra = '\\[{}]'.format(', '.join(format_annotation(param) for param in params))
TypeError: 'NoneType' object is not iterable

# Sphinx version: 1.8.2
# Python version: 3.7.1 (CPython)
# Docutils version: 0.14
# Jinja2 version: 2.10
# Last messages:
#   19 added, 0 changed, 0 removed
#
#   reading sources...
#
# Loaded extensions:
#   sphinx.ext.mathjax (1.8.2) from /path/to/.venv/lib/python3.7/site-packages/sphinx/ext/mathjax.py
#   alabaster (0.7.12) from /path/to/.venv/lib/python3.7/site-packages/alabaster/__init__.py
#   sphinx.ext.autodoc (1.8.2) from /path/to/.venv/lib/python3.7/site-packages/sphinx/ext/autodoc/__init__.py
#   sphinx.ext.intersphinx (1.8.2) from /path/to/.venv/lib/python3.7/site-packages/sphinx/ext/intersphinx.py
#   sphinx.ext.napoleon (1.8.2) from /path/to/.venv/lib/python3.7/site-packages/sphinx/ext/napoleon/__init__.py
#   sphinx_autodoc_typehints (unknown version) from /path/to/.venv/lib/python3.7/site-packages/sphinx_autodoc_typehints.py
Traceback (most recent call last):
  File "/path/to/.venv/lib/python3.7/site-packages/sphinx/cmd/build.py", line 304, in build_main
    app.build(args.force_all, filenames)
  File "/path/to/.venv/lib/python3.7/site-packages/sphinx/application.py", line 341, in build
    self.builder.build_update()
  File "/path/to/.venv/lib/python3.7/site-packages/sphinx/builders/__init__.py", line 347, in build_update
    len(to_build))
  File "/path/to/.venv/lib/python3.7/site-packages/sphinx/builders/__init__.py", line 360, in build
    updated_docnames = set(self.read())
  File "/path/to/.venv/lib/python3.7/site-packages/sphinx/builders/__init__.py", line 468, in read
    self._read_serial(docnames)
  File "/path/to/.venv/lib/python3.7/site-packages/sphinx/builders/__init__.py", line 490, in _read_serial
    self.read_doc(docname)
  File "/path/to/.venv/lib/python3.7/site-packages/sphinx/builders/__init__.py", line 534, in read_doc
    doctree = read_doc(self.app, self.env, self.env.doc2path(docname))
  File "/path/to/.venv/lib/python3.7/site-packages/sphinx/io.py", line 318, in read_doc
    pub.publish()
  File "/path/to/.venv/lib/python3.7/site-packages/docutils/core.py", line 217, in publish
    self.settings)
  File "/path/to/.venv/lib/python3.7/site-packages/docutils/readers/__init__.py", line 72, in read
    self.parse()
  File "/path/to/.venv/lib/python3.7/site-packages/docutils/readers/__init__.py", line 78, in parse
    self.parser.parse(self.input, document)
  File "/path/to/.venv/lib/python3.7/site-packages/sphinx/parsers.py", line 88, in parse
    self.statemachine.run(inputstring, document, inliner=self.inliner)
  File "/path/to/.venv/lib/python3.7/site-packages/docutils/parsers/rst/states.py", line 171, in run
    input_source=document['source'])
  File "/path/to/.venv/lib/python3.7/site-packages/docutils/statemachine.py", line 239, in run
    context, state, transitions)
  File "/path/to/.venv/lib/python3.7/site-packages/docutils/statemachine.py", line 460, in check_line
    return method(match, context, next_state)
  File "/path/to/.venv/lib/python3.7/site-packages/docutils/parsers/rst/states.py", line 2753, in underline
    self.section(title, source, style, lineno - 1, messages)
  File "/path/to/.venv/lib/python3.7/site-packages/docutils/parsers/rst/states.py", line 327, in section
    self.new_subsection(title, lineno, messages)
  File "/path/to/.venv/lib/python3.7/site-packages/docutils/parsers/rst/states.py", line 395, in new_subsection
    node=section_node, match_titles=True)
  File "/path/to/.venv/lib/python3.7/site-packages/docutils/parsers/rst/states.py", line 282, in nested_parse
    node=node, match_titles=match_titles)
  File "/path/to/.venv/lib/python3.7/site-packages/docutils/parsers/rst/states.py", line 196, in run
    results = StateMachineWS.run(self, input_lines, input_offset)
  File "/path/to/.venv/lib/python3.7/site-packages/docutils/statemachine.py", line 239, in run
    context, state, transitions)
  File "/path/to/.venv/lib/python3.7/site-packages/docutils/statemachine.py", line 460, in check_line
    return method(match, context, next_state)
  File "/path/to/.venv/lib/python3.7/site-packages/docutils/parsers/rst/states.py", line 2326, in explicit_markup
    nodelist, blank_finish = self.explicit_construct(match)
  File "/path/to/.venv/lib/python3.7/site-packages/docutils/parsers/rst/states.py", line 2338, in explicit_construct
    return method(self, expmatch)
  File "/path/to/.venv/lib/python3.7/site-packages/docutils/parsers/rst/states.py", line 2081, in directive
    directive_class, match, type_name, option_presets)
  File "/path/to/.venv/lib/python3.7/site-packages/docutils/parsers/rst/states.py", line 2130, in run_directive
    result = directive_instance.run()
  File "/path/to/.venv/lib/python3.7/site-packages/sphinx/ext/autodoc/directive.py", line 131, in run
    documenter.generate(more_content=self.content)
  File "/path/to/.venv/lib/python3.7/site-packages/sphinx/ext/autodoc/__init__.py", line 819, in generate
    self.document_members(all_members)
  File "/path/to/.venv/lib/python3.7/site-packages/sphinx/ext/autodoc/__init__.py", line 740, in document_members
    check_module=members_check_module and not isattr)
  File "/path/to/.venv/lib/python3.7/site-packages/sphinx/ext/autodoc/__init__.py", line 1247, in generate
    all_members=all_members)
  File "/path/to/.venv/lib/python3.7/site-packages/sphinx/ext/autodoc/__init__.py", line 819, in generate
    self.document_members(all_members)
  File "/path/to/.venv/lib/python3.7/site-packages/sphinx/ext/autodoc/__init__.py", line 1235, in document_members
    ModuleLevelDocumenter.document_members(self, all_members)
  File "/path/to/.venv/lib/python3.7/site-packages/sphinx/ext/autodoc/__init__.py", line 740, in document_members
    check_module=members_check_module and not isattr)
  File "/path/to/.venv/lib/python3.7/site-packages/sphinx/ext/autodoc/__init__.py", line 816, in generate
    self.add_content(more_content)
  File "/path/to/.venv/lib/python3.7/site-packages/sphinx/ext/autodoc/__init__.py", line 554, in add_content
    for i, line in enumerate(self.process_doc(docstrings)):
  File "/path/to/.venv/lib/python3.7/site-packages/sphinx/ext/autodoc/__init__.py", line 514, in process_doc
    self.options, docstringlines)
  File "/path/to/.venv/lib/python3.7/site-packages/sphinx/application.py", line 510, in emit
    return self.events.emit(event, self, *args)
  File "/path/to/.venv/lib/python3.7/site-packages/sphinx/events.py", line 80, in emit
    results.append(callback(*args))
  File "/path/to/.venv/lib/python3.7/site-packages/sphinx_autodoc_typehints.py", line 190, in process_docstring
    formatted_annotation = format_annotation(annotation)
  File "/path/to/.venv/lib/python3.7/site-packages/sphinx_autodoc_typehints.py", line 103, in format_annotation
    extra = '\\[{}]'.format(', '.join(format_annotation(param) for param in params))
  File "/path/to/.venv/lib/python3.7/site-packages/sphinx_autodoc_typehints.py", line 103, in <genexpr>
    extra = '\\[{}]'.format(', '.join(format_annotation(param) for param in params))
  File "/path/to/.venv/lib/python3.7/site-packages/sphinx_autodoc_typehints.py", line 119, in format_annotation
    extra = '\\[{}]'.format(', '.join(format_annotation(param) for param in params))
TypeError: 'NoneType' object is not iterable

[agronholm]

Please add at least one test for this.

[icgood]

@agronholm done, and I believe I fully understand the problem now. My project makes extensive use of Protocol which can use type parameters (like Generic) but can also be standalone (unlike Generic). Because Protocol is in the typing_extensions package I guarded the import.

[agronholm]

Thanks!