Look into using mypy's daemon

[brettcannon]

https://mypy.readthedocs.io/en/latest/mypy_daemon.html

[erichiller]

Looks like as of June 2018 work has begun for Windows daemon support.
python/mypy#5019

[emmatyping]

FYI, mypy master now has a working implementation of the mypy daemon for Windows if you want to play with it.

[erichiller]

How would we be able to use dmypy within vscode? Do you mean just for testing via the command line? Or is there a proper way to integrate this with vscode currently? If not are there plans to use the daemon or give the option?

[emmatyping]

Eric Hiller (@erichiller) Until there is more official support, as a work around you can use something like the following:

{
    "python.linting.mypyEnabled": true,
    "python.linting.mypyPath": "C:\\Python37\\Scripts\\dmypy.exe",
    "python.linting.mypyArgs": [
        "run",
        "--",
        "--ignore-missing-imports",
        "--follow-imports=skip"
    ]
}

Though this is rather hackish. (It won't kill the daemon when VS Code closes, which likely is not the behavior you want). Also this is rather abusing the whole "set an executable and args" settings system...

Do note you must change the argument for following imports to either skip or error for now.

[brettcannon]

Eric Hiller (@erichiller) we would like to add support, but we don't have any ETA on when this will get prioritized high enough for us to get around to it.

ethanhs thanks for the work-around!

[DonJayamanne]

Ivan Levkivskyi (@ilevkivskyi)

Thanks for connecting with us at PyCon, looking forward to landing this in the extension.

Lets get the ball rolling.
A few questions (there'll be a few more, but lets start with a few simple ones):

• How do we start multiple daemons for different repositories? The docs don't cover that.
  • Assume user opens two instances of VS Code, both instances of VSC will now run dmypy start.
  • My assumption is, this will result in two instances of the daemon.
• Will shutting down the parent process kill the daemon automatically or is it a detached sub process?
  • I.e. if we were to kill vscode (non-graceful shutdown), will this result in the daemon evetually killing itself, or will it stay alive and never die?
• Silyy question:
  • Assume we have two instances of vscode, one using Python 2.7 another using python 3.7. When Python2.7 sends requests to the daemon, will it send the messages to the daemon that it started.
  • I guess the answer is obviously yes, just don't want to make assumptions here.
• Cancellation,
  • Assume we send a command to lint a large file, and immediately after that we send the same file (but with some changes). Will mypy cancel the previous request or will it simply queue the request. I.e. wait for previous request to complete.
• Multiple requests
  • Assume we send a command to lint a large file
  • While we're waiting for that request to be processed we send another request to the same daemon to process a large file, will the daemon block on the previous request or somehow process this in parallel?

[ilevkivskyi]

Don Jayamanne (@DonJayamanne)

How do we start multiple daemons for different repositories?

I think the simplest way is to set current working directory to the repo root when calling dmypy start command.

[...] I.e. if we were to kill vscode (non-graceful shutdown), will this result in the daemon evetually killing itself, or will it stay alive and never die?

No it will not die. Process created by dmypy start (client) exits immediately, but it leaves the daemon (server) "detached". The daemon however will shut down after a timeout if it receives no requests (configurable, default is 24 hours). Also calling dmypy kill unconditionally kills the server.

Assume we have two instances of vscode, one using Python 2.7 another using python 3.7. When Python2.7 sends requests to the daemon, will it send the messages to the daemon that it started.

Mypy only supports Python 3.5+, it can type check older versions (including 2.7), but you need to have Python 3 installed to run it. If you want to type check a repo in Python 2 mode and then in Python 3 mode, I think the best way is to start two daemons each one configured with --python-version=x.y (you will also need to pass --status-file XXX to each daemon when starting the server, and when communicating with them using dmypy client to distinguish them).

Assume we send a command to lint a large file, and immediately after that we send the same file (but with some changes). Will mypy cancel the previous request or will it simply queue the request. I.e. wait for previous request to complete

Currently this is not possible. dmypy check and dmypy recheck are blocking calls, i.e. they don't exit before the server responds with the checking, and then they print checking results to stdout and exit. From user point of view dmypy check behaves essentially the same as mypy. You can of course issuer another dmypy check (i.e. create another client process) before previous exits, but I am not sure this is going to end well currently. We discussed the possibility to add dmypy cancel to gracefully abort last type checking request, and it may be not so easy to implement. But potentially we can do this if it is important.

Assume we send a command to lint a large file.

It is not how mypy normally works (not only the daemon), see #5587. I actually believe we should figure out that issue first before continuing here.