Uninformative message with fast-parser

[medullaskyline]

To reproduce the bug, see https://github.com/medullaskyline/tuplebug

Running the test case:

virtualenv testcase -p `which python3`
source testcase/bin/activate
pip install -r requirements.txt

Annotating a tuple incorrectly will result in an uninformative traceback when running mypy with the --fast-parser flag.

This is the file I'm parsing: https://github.com/medullaskyline/tuplebug/blob/master/testcase.py

$ mypy --fast-parser --silent-imports --py2 --check-untyped-defs --disallow-untyped-defs testcase.py
Traceback (most recent call last):
  File "/srv/zulip-py3-venv/bin/mypy", line 6, in <module>
    main(__file__)
  File "/srv/zulip-py3-venv/lib/python3.5/site-packages/mypy/main.py", line 54, in main
    res = type_check_only(sources, bin_dir, options)
  File "/srv/zulip-py3-venv/lib/python3.5/site-packages/mypy/main.py", line 102, in type_check_only
    python_path=options.python_path)
  File "/srv/zulip-py3-venv/lib/python3.5/site-packages/mypy/build.py", line 209, in build
    dispatch(sources, manager)
  File "/srv/zulip-py3-venv/lib/python3.5/site-packages/mypy/build.py", line 1319, in dispatch
    graph = load_graph(sources, manager)
  File "/srv/zulip-py3-venv/lib/python3.5/site-packages/mypy/build.py", line 1334, in load_graph
    st = State(id=bs.module, path=bs.path, source=bs.text, manager=manager)
  File "/srv/zulip-py3-venv/lib/python3.5/site-packages/mypy/build.py", line 1106, in __init__
    self.parse_file()
  File "/srv/zulip-py3-venv/lib/python3.5/site-packages/mypy/build.py", line 1217, in parse_file
    self.tree = manager.parse_file(self.id, self.xpath, source)
  File "/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/contextlib.py", line 77, in __exit__
    self.gen.throw(type, value, traceback)
  File "/srv/zulip-py3-venv/lib/python3.5/site-packages/mypy/build.py", line 1171, in wrap_context
    yield
  File "/srv/zulip-py3-venv/lib/python3.5/site-packages/mypy/build.py", line 1217, in parse_file
    self.tree = manager.parse_file(self.id, self.xpath, source)
  File "/srv/zulip-py3-venv/lib/python3.5/site-packages/mypy/build.py", line 449, in parse_file
    fast_parser=FAST_PARSER in self.flags)
  File "/srv/zulip-py3-venv/lib/python3.5/site-packages/mypy/parse.py", line 88, in parse
    custom_typing_module=custom_typing_module)
  File "/srv/zulip-py3-venv/lib/python3.5/site-packages/mypy/fastparse.py", line 69, in parse
    ).visit(ast)
  File "/srv/zulip-py3-venv/lib/python3.5/site-packages/typed_ast/ast35.py", line 252, in visit
    return visitor(node)
  File "/srv/zulip-py3-venv/lib/python3.5/site-packages/mypy/fastparse.py", line 221, in visit_Module
    body = self.fix_function_overloads(self.visit_list(mod.body))
  File "/srv/zulip-py3-venv/lib/python3.5/site-packages/mypy/fastparse.py", line 122, in visit_list
    return [self.visit(e) for e in l]
  File "/srv/zulip-py3-venv/lib/python3.5/site-packages/mypy/fastparse.py", line 122, in <listcomp>
    return [self.visit(e) for e in l]
  File "/srv/zulip-py3-venv/lib/python3.5/site-packages/typed_ast/ast35.py", line 252, in visit
    return visitor(node)
  File "/srv/zulip-py3-venv/lib/python3.5/site-packages/mypy/fastparse.py", line 90, in wrapper
    node = f(self, ast)
  File "/srv/zulip-py3-venv/lib/python3.5/site-packages/mypy/fastparse.py", line 242, in visit_FunctionDef
    func_type_ast = ast35.parse(n.type_comment, '<func_type>', 'func_type')
  File "/srv/zulip-py3-venv/lib/python3.5/site-packages/typed_ast/ast35.py", line 42, in parse
    return _ast35._parse(source, filename, mode)
SyntaxError: invalid syntax (<func_type>, line 1)

*** INTERNAL ERROR ***

testcase.py: error: Internal error -- please report a bug at https://github.com/python/mypy/issues

NOTE: you can use "mypy --pdb ..." to drop into the debugger when this happens.

This should be changed to the informative message that results when the --fast-parser flag is omitted

$ mypy --silent-imports --py2 --check-untyped-defs --disallow-untyped-defs testcase.py
testcase.py: note: In function "return_a_tuple":
testcase.py:3: error: Parse error before ,

[ddfisher]

Thanks for the bug report! This was actually recently fixed in #1600.