mypy_daemon.rst

.. program:: dmypy

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 command-line interface of mypy daemon may change in future mypy releases.

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 type check 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 -- prog.py pkg/*.py

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

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 :ref:`remote caching <remote-cache>` to speed up the initial run. The speedup can be significant if you have a large codebase.)

Note

Mypy 0.780 added support for following imports in dmypy (enabled by default). This functionality is still experimental. You can use --follow-imports=skip or --follow-imports=error to fall back to the stable functionality. See :ref:`follow-imports` for details on how these work.

Note

The mypy daemon requires --local-partial-types and automatically enables it.

Daemon client commands

While dmypy run is sufficient for most uses, some workflows (ones using :ref:`remote caching <remote-cache>`, 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 recheck checks the same set of files as the most recent check or recheck command. (You can also use the :option:`--update` and :option:`--remove` options to alter the set of files, and to define which files should be processed.)
• 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.

Additional daemon flags

.. option:: --status-file FILE

   Use ``FILE`` as the status file for storing daemon runtime state. This is
   normally a JSON file that contains information about daemon process and
   connection. The default path is ``.dmypy.json`` in the current working
   directory.

.. option:: --log-file FILE

   Direct daemon stdout/stderr to ``FILE``. This is useful for debugging daemon
   crashes, since the server traceback is not always printed by the client.
   This is available for the ``start``, ``restart``, and ``run`` commands.

.. option:: --timeout TIMEOUT

   Automatically shut down server after ``TIMEOUT`` seconds of inactivity.
   This is available for the ``start``, ``restart``, and ``run`` commands.

.. option:: --update FILE

   Re-check ``FILE``, or add it to the set of files being
   checked (and check it). This option may be repeated, and it's only available for
   the ``recheck`` command.  By default, mypy finds and checks all files changed
   since the previous run and files that depend on them.  However, if you use this option
   (and/or :option:`--remove`), mypy assumes that only the explicitly
   specified files have changed. This is only useful to
   speed up mypy if you type check a very large number of files, and use an
   external, fast file system watcher, such as `watchman`_ or
   `watchdog`_, to determine which files got edited or deleted.
   *Note:* This option is never required and is only available for
   performance tuning.

.. option:: --remove FILE

   Remove ``FILE`` from the set of files being checked. This option may be
   repeated. This is only available for the
   ``recheck`` command. See :option:`--update` above for when this may be useful.
   *Note:* This option is never required and is only available for performance
   tuning.

.. option:: --fswatcher-dump-file FILE

   Collect information about the current internal file state. This is
   only available for the ``status`` command. This will dump JSON to
   ``FILE`` in the format ``{path: [modification_time, size,
   content_hash]}``. This is useful for debugging the built-in file
   system watcher. *Note:* This is an internal flag and the format may
   change.

.. option:: --perf-stats-file FILE

   Write performance profiling information to ``FILE``. This is only available
   for the ``check``, ``recheck``, and ``run`` commands.

.. option:: --export-types

   Store all expression types in memory for future use. This is useful to speed
   up future calls to ``dmypy inspect`` (but uses more memory). Only valid for
   ``check``, ``recheck``, and ``run`` command.

Static inference of annotations

The mypy daemon supports (as an experimental feature) statically inferring draft function and method type annotations. Use dmypy suggest FUNCTION to generate a draft signature in the format (param_type_1, param_type_2, ...) -> ret_type (types are included for all arguments, including keyword-only arguments, *args and **kwargs).

This is a low-level feature intended to be used by editor integrations, IDEs, and other tools (for example, the mypy plugin for PyCharm), to automatically add annotations to source files, or to propose function signatures.

In this example, the function format_id() has no annotation:

def format_id(user):
    return f"User: {user}"

root = format_id(0)

dmypy suggest uses call sites, return statements, and other heuristics (such as looking for signatures in base classes) to infer that format_id() accepts an int argument and returns a str. Use dmypy suggest module.format_id to print the suggested signature for the function.

More generally, the target function may be specified in two ways:

• By its fully qualified name, i.e. [package.]module.[class.]function.
• By its location in a source file, i.e. /path/to/file.py:line. The path can be absolute or relative, and line can refer to any line number within the function body.

This command can also be used to find a more precise alternative for an existing, imprecise annotation with some Any types.

The following flags customize various aspects of the dmypy suggest command.

.. option:: --json

   Output the signature as JSON, so that `PyAnnotate`_ can read it and add
   the signature to the source file. Here is what the JSON looks like:

   .. code-block:: python

      [{"func_name": "example.format_id",
        "line": 1,
        "path": "/absolute/path/to/example.py",
        "samples": 0,
        "signature": {"arg_types": ["int"], "return_type": "str"}}]

.. option:: --no-errors

   Only produce suggestions that cause no errors in the checked code. By default,
   mypy will try to find the most precise type, even if it causes some type errors.

.. option:: --no-any

   Only produce suggestions that don't contain ``Any`` types. By default mypy
   proposes the most precise signature found, even if it contains ``Any`` types.

.. option:: --flex-any FRACTION

   Only allow some fraction of types in the suggested signature to be ``Any`` types.
   The fraction ranges from ``0`` (same as ``--no-any``) to ``1``.

.. option:: --callsites

   Only find call sites for a given function instead of suggesting a type.
   This will produce a list with line numbers and types of actual
   arguments for each call: ``/path/to/file.py:line: (arg_type_1, arg_type_2, ...)``.

.. option:: --use-fixme NAME

   Use a dummy name instead of plain ``Any`` for types that cannot
   be inferred. This may be useful to emphasize to a user that a given type
   couldn't be inferred and needs to be entered manually.

.. option:: --max-guesses NUMBER

   Set the maximum number of types to try for a function (default: ``64``).

Statically inspect expressions

The daemon allows to get declared or inferred type of an expression (or other information about an expression, such as known attributes or definition location) using dmypy inspect LOCATION command. The location of the expression should be specified in the format path/to/file.py:line:column[:end_line:end_column]. Both line and column are 1-based. Both start and end position are inclusive. These rules match how mypy prints the error location in error messages.

If a span is given (i.e. all 4 numbers), then only an exactly matching expression is inspected. If only a position is given (i.e. 2 numbers, line and column), mypy will inspect all expressions, that include this position, starting from the innermost one.

Consider this Python code snippet:

def foo(x: int, longer_name: str) -> None:
    x
    longer_name

Here to find the type of x one needs to call dmypy inspect src.py:2:5:2:5 or dmypy inspect src.py:2:5. While for longer_name one needs to call dmypy inspect src.py:3:5:3:15 or, for example, dmypy inspect src.py:3:10. Please note that this command is only valid after daemon had a successful type check (without parse errors), so that types are populated, e.g. using dmypy check. In case where multiple expressions match the provided location, their types are returned separated by a newline.

Important note: it is recommended to check files with :option:`--export-types` since otherwise most inspections will not work without :option:`--force-reload`.

.. option:: --show INSPECTION

   What kind of inspection to run for expression(s) found. Currently the supported
   inspections are:

   * ``type`` (default): Show the best known type of a given expression.
   * ``attrs``: Show which attributes are valid for an expression (e.g. for
     auto-completion). Format is ``{"Base1": ["name_1", "name_2", ...]; "Base2": ...}``.
     Names are sorted by method resolution order. If expression refers to a module,
     then module attributes will be under key like ``"<full.module.name>"``.
   * ``definition`` (experimental): Show the definition location for a name
     expression or member expression. Format is ``path/to/file.py:line:column:Symbol``.
     If multiple definitions are found (e.g. for a Union attribute), they are
     separated by comma.

.. option:: --verbose

   Increase verbosity of types string representation (can be repeated).
   For example, this will print fully qualified names of instance types (like
   ``"builtins.str"``), instead of just a short name (like ``"str"``).

.. option:: --limit NUM

   If the location is given as ``line:column``, this will cause daemon to
   return only at most ``NUM`` inspections of innermost expressions.
   Value of 0 means no limit (this is the default). For example, if one calls
   ``dmypy inspect src.py:4:10 --limit=1`` with this code

   .. code-block:: python

      def foo(x: int) -> str: ..
      def bar(x: str) -> None: ...
      baz: int
      bar(foo(baz))

   This will output just one type ``"int"`` (for ``baz`` name expression).
   While without the limit option, it would output all three types: ``"int"``,
   ``"str"``, and ``"None"``.

.. option:: --include-span

   With this option on, the daemon will prepend each inspection result with
   the full span of corresponding expression, formatted as ``1:2:1:4 -> "int"``.
   This may be useful in case multiple expressions match a location.

.. option:: --include-kind

   With this option on, the daemon will prepend each inspection result with
   the kind of corresponding expression, formatted as ``NameExpr -> "int"``.
   If both this option and :option:`--include-span` are on, the kind will
   appear first, for example ``NameExpr:1:2:1:4 -> "int"``.

.. option:: --include-object-attrs

   This will make the daemon include attributes of ``object`` (excluded by
   default) in case of an ``atts`` inspection.

.. option:: --union-attrs

   Include attributes valid for some of possible expression types (by default
   an intersection is returned). This is useful for union types of type variables
   with values. For example, with this code:

   .. code-block:: python

      from typing import Union

      class A:
          x: int
          z: int
      class B:
          y: int
          z: int
      var: Union[A, B]
      var

   The command ``dmypy inspect --show attrs src.py:10:1`` will return
   ``{"A": ["z"], "B": ["z"]}``, while with ``--union-attrs`` it will return
   ``{"A": ["x", "z"], "B": ["y", "z"]}``.

.. option:: --force-reload

   Force re-parsing and re-type-checking file before inspection. By default
   this is done only when needed (for example file was not loaded from cache
   or daemon was initially run without ``--export-types`` mypy option),
   since reloading may be slow (up to few seconds for very large files).