Add basic mypy linting

Based on pytest setup, though more lax. Currently only the tests are
annotated so only helpful for them.

Fix #721.

.pre-commit-config.yaml

@@ -28,3 +28,12 @@ repos:
         files: ^(CHANGELOG.rst|HOWTORELEASE.rst|README.rst|changelog/.*)$
         language: python
         additional_dependencies: [pygments, restructuredtext_lint]
+-   repo: https://github.com/pre-commit/mirrors-mypy
+    rev: v0.910-1
+    hooks:
+    -   id: mypy
+        files: ^(src/|testing/)
+        args: []
+        additional_dependencies:
+          - pytest>=6.2.0
+          - py>=1.10.0

changelog/721.trivial.rst

@@ -0,0 +1 @@
+Started using type annotations and mypy checking internally. The types are incomplete and not published.

setup.cfg

@@ -58,3 +58,19 @@ setproctitle = setproctitle
 
 [flake8]
 max-line-length = 100
+
+[mypy]
+mypy_path = src
+# TODO: Enable this & fix errors.
+# check_untyped_defs = True
+disallow_any_generics = True
+ignore_missing_imports = True
+no_implicit_optional = True
+show_error_codes = True
+strict_equality = True
+warn_redundant_casts = True
+warn_return_any = True
+warn_unreachable = True
+warn_unused_configs = True
+# TODO: Enable this & fix errors.
+# no_implicit_reexport = True

src/xdist/plugin.py

@@ -250,7 +250,7 @@ def is_xdist_controller(request_or_session) -> bool:
 is_xdist_master = is_xdist_controller
 
 
-def get_xdist_worker_id(request_or_session) -> str:
+def get_xdist_worker_id(request_or_session):
     """Return the id of the current worker ('gw0', 'gw1', etc) or 'master'
     if running on the controller node.
 

src/xdist/remote.py

@@ -236,8 +236,8 @@ def setup_config(config, basetemp):
 
 
 if __name__ == "__channelexec__":
-    channel = channel  # noqa
-    workerinput, args, option_dict, change_sys_path = channel.receive()
+    channel = channel  # type: ignore[name-defined] # noqa: F821
+    workerinput, args, option_dict, change_sys_path = channel.receive()  # type: ignore[name-defined]
 
     if change_sys_path is None:
         importpath = os.getcwd()
@@ -260,7 +260,7 @@ if __name__ == "__channelexec__":
 
     setup_config(config, option_dict.get("basetemp"))
     config._parser.prog = os.path.basename(workerinput["mainargv"][0])
-    config.workerinput = workerinput
-    config.workeroutput = {}
-    interactor = WorkerInteractor(config, channel)
+    config.workerinput = workerinput  # type: ignore[attr-defined]
+    config.workeroutput = {}  # type: ignore[attr-defined]
+    interactor = WorkerInteractor(config, channel)  # type: ignore[name-defined]
     config.hook.pytest_cmdline_main(config=config)