Mypy daemon (mypy server)

Instead of running mypy as a command-line tool, you can also run it as a long-running daemon (server) process and use a command-line client to send type-checking requests to the server. This way mypy can perform type checking much faster, since program state cached from previous runs is kept in memory and doesn’t have to be read from the file system on each run. The server also uses finer-grained dependency tracking to reduce the amount of work that needs to be done.

If you have a large codebase to check, running mypy using the mypy daemon can be 10 or more times faster than the regular command-line mypy tool, especially if your workflow involves running mypy repeatedly after small edits – which is often a good idea, as this way you’ll find errors sooner.

Note

The mypy daemon is experimental. In particular, the command-line interface may change in future mypy releases.

Note

The mypy daemon currently supports macOS and Linux only.

Note

Each mypy daemon process supports one user and one set of source files, and it can only process one type checking request at a time. You can run multiple mypy daemon processes to type check multiple repositories.

Basic usage

The client utility dmypy is used to control the mypy daemon. Use dmypy run -- <flags> <files> to typecheck a set of files (or directories). This will launch the daemon if it is not running. You can use almost arbitrary mypy flags after --. The daemon will always run on the current host. Example:

dmypy run -- --follow-imports=error prog.py pkg1/ pkg2/

Note

You’ll need to use either the --follow-imports=error or the --follow-imports=skip option with dmypy because the current implementation can’t follow imports. See Following imports or not? for details on how these work. You can also define these using a configuration file.

dmypy run will automatically restart the daemon if the configuration or mypy version changes.

You need to provide all files or directories you want to type check (other than stubs) as arguments. This is a result of the --follow-imports restriction mentioned above.

The initial run will process all the code and may take a while to finish, but subsequent runs will be quick, especially if you’ve only changed a few files. You can use remote caching to speed up the initial run. The speedup can be significant if you have a large codebase.

Additional features

While dmypy run is sufficient for most uses, some workflows (ones using remote caching, perhaps), require more precise control over the lifetime of the daemon process:

  • dmypy stop stops the daemon.
  • dmypy start -- <flags> starts the daemon but does not check any files. You can use almost arbitrary mypy flags after --.
  • dmypy restart -- <flags> restarts the daemon. The flags are the same as with dmypy start. This is equivalent to a stop command followed by a start.
  • Use dmypy run --timeout SECONDS -- <flags> (or start or restart) to automatically shut down the daemon after inactivity. By default, the daemon runs until it’s explicitly stopped.
  • dmypy check <files> checks a set of files using an already running daemon.
  • dmypy status checks whether a daemon is running. It prints a diagnostic and exits with 0 if there is a running daemon.

Use dmypy --help for help on additional commands and command-line options not discussed here, and dmypy <command> --help for help on command-specific options.

Limitations

  • You have to use either the --follow-imports=error or the --follow-imports=skip option because of an implementation limitation. This can be defined through the command line or through a configuration file.
  • Windows is not supported.