Parsing error when using dataclasses in Py36 without default values

[theendsofinvention]

Subject: sphinx build fails when using dataclasses in Py36 without default values

Problem

Using Python 3.6.5 on Win7 inside a Pipenv venv.

When building the documentation, Sphinx is unable to parse a .py file containing a simple dataclass (see .py file below).

    raise PycodeError('parsing %r failed: %r' % (self.srcname, exc))
sphinx.errors.PycodeError: parsing ...\\agent_loop_args.py' failed: AssertionError()

Procedure to reproduce the problem

make html

(nothing fancy going on)

Error logs / results

https://pastebin.com/qQi0NLW4

Expected results

I expected Sphinx to document the dataclass, showing the attributes' expected types.

Reproducible project / your project

import threading
import dataclasses


@dataclasses.dataclass
class AgentLoopArgs:
    """
    Defines the parameters necessary for an Agent to run.
    """
    on_start: callable
    on_stop: callable
    target: callable
    target_args: tuple
    target_kwargs: dict
    delay: int
    name: str
    stop_event: threading.Event

Environment info

• OS: Win7
• Python version: 3.6.5
• Sphinx version: 1.7.4

Extensions:

#   alabaster (0.7.10) 
#   sphinx.ext.autodoc (1.7.4) 
#   sphinx.ext.intersphinx (1.7.4) 
#   sphinx.ext.todo (1.7.4)
#   sphinx.ext.coverage (1.7.4)
#   sphinx.ext.mathjax (1.7.4)
#   sphinx.ext.ifconfig (1.7.4)
#   sphinx.ext.viewcode (1.7.4)
#   sphinx.ext.githubpages (1.7.4)

This is my first time reporting an issue here, please let me know if I've broken any guideline and I'll do my best to fix it!

EDIT: I forgot to emphasize that if I'm specifying default values like so:

import threading
import dataclasses


@dataclasses.dataclass
class AgentLoopArgs:
    """
    Defines the parameters necessary for an Agent to run.
    """
    on_start: callable = None
    on_stop: callable = None
    ...

then Sphinx has no issue whatsoever building the documentation.

[tk0miya]

PEP-557 says dataclasses is a new feature of 3.7.
https://www.python.org/dev/peps/pep-0557/

But it seems you're using py3.6

Python version: 3.6.5

[theendsofinvention]

@tk0miya indeed ! I'm using https://github.com/ericvsmith/dataclasses to emulate the behaviour.

The PEP states that the syntax is:

@dataclass
class C:
    a: int       # 'a' has no default value
    b: int = 0   # assign a default value for 'b'

, which is the exact same syntax I'm using.

I know this isn't official Python (yet), but since it's coming pretty soon, I figured I'd raise the issue =)

[tk0miya]

On my local, with python-3.6.4 and dataclasses-0.5, I can't reproduce the error.

BTW, the error was raised on parssing AnnAssign (assignment with variable annotation). So I guess this is not related with dataclasses.
Could you find the reason of this error by cutting down your code?

...
  File "...sphinx\pycode\parser.py", line 343, in visit_AnnAssign
    self.visit_Assign(node)  # type: ignore
  File "...sphinx\pycode\parser.py", line 312, in visit_Assign
    parser.parse()
  File "...sphinx\pycode\parser.py", line 229, in parse
    assert self.current
AssertionError

[tk0miya]

Ah, sorry. I noticed both a and b are AnnAssign statements. AnnAssign statement can omit rvalue.

@dataclass
class C:
    a: int       # 'a' has no default value
    b: int = 0   # assign a default value for 'b'

Anyway, I can't reproduce the error yet. So we have to reproduce it to fix.

[theendsofinvention]

Thanks for looking into it. I'm away for work but I'll look into it this week end and try to narrow it down!

[tk0miya]

We will ship next stable release soon. I'd like to fix this until then if possible. So please let me know if any updates.

[theendsofinvention]

I'm sorry I didn't come back to you earlier, something popped up on another front. Thank you for the reminder !

I am at a complete loss, the bug has disappeared without any reason.

My environment is locked, and conf.py hasn't changed. I just went and checked out every commit in my history from the day I reported the issue, and on every one of them sphinx builds successfully.

I have not the slightest idea about what could have gone wrong at the time ... I will however report back if it ever happens again.

I'm sorry for wasting your time on this without an actual outcome.

[tk0miya]

I noticed the error has been happened silently. We can see it with -vvv option.

[autodoc] module analyzer failed: parsing '/Users/tkomiya/work/tmp/doc/zzz/foo.py' failed: AssertionError()

[theendsofinvention]

Oh, ok.

I wonder why it decided to pop for me (I'm not using -vvv). It was crashing my build entirely.

Please let me know if I can do anything to help.

Thanks a bunch for what you're doing guys, it's much, much appreciated! I'm looking forward using the upcoming stable release!

[tk0miya]

Please try #4996 and confirm it works fine if you have time.
I'd like to merge it to next stable release :-)

[tk0miya]

Fixed by #4996.
Thank you for reporting.