Running dmypy as a systemd user service

Feature

Make dmypy look for either its status file or maybe even the socket in a well-known place.

As this well-known place I propose for Linux systems a directory /run/user/$UID/mypy (or the value of the environment variable XDG_RUNTIME_DIR). The name of status file would encode the path for which dmypy is running.

Pitch

mypy can be run as a daemon, which is currently manually started via dmypy start. It can already run in the foreground via dmypy daemon and I have started to use this to start the mypy daemon on login on my Linux system as a systemd user service as this moves the management of the lifecycle of the daemon to the service manager, that I already have running (as does the vast majority of Linux systems), so that I don't have to think about it anymore.

I do this via this service file in ~/.config/systemd/user/

# mypy@.service
[Unit]
Description=mypy daemon

[Service]
Type=simple
WorkingDirectory=%I
ExecStart=dmypy --status-file %t/mypy/%I.json daemon
RuntimeDirectory=mypy

which is instantiated via systemctl enable mypy@$(systemd-escape /path/to/my/project).service. The specifiers %I and %t expand to /path/to/my/project and the value of XDG_RUNTIME_DIR (by default /run/user/$UID). respectively. I then have a wrapper function for dmypy that sets --status-file to the appropriate value for my project directory (/run/user/$UID/mypy/$(systemd-escape $(git rev-parse --show-toplevel)).json to be specific).

I would like to streamline this setup, so that the wrapper is no longer necessary by having mypy look not only for .dmypy.json by default, but also check for the existence of the status file in the above described well-known location. This would allow e.g. for distributions packaging mypy to ship this or a similar service file to users and automatically restart the daemon on upgrades.

The feature itself is extensible. This directory could be used for the dmypy socket directly instead of giving it a random name in /tmp, but I have not yet looked into why this was designed as it is.

I guess a similar feature could be implemented for launchd on MacOS and on Windows, but I have no knowledge of these systems.

Start with the dmypy start and dmypy daemon entry points, then trace how --status-file and the default .dmypy.json location are resolved. Compare that behavior with the proposed XDG_RUNTIME_DIR or /run/user/$UID/mypy location. Done means a systemd user service can run dmypy without a wrapper while existing status-file behavior remains usable.