Add mypy as a flycheck checker

Rather than allowing the user to specify each argument, allow the user
to set a path to a mypy configuration file.

Output parsing is based on
https://github.com/lbolla/emacs-flycheck-mypy and adds support for
warnings too.

CHANGES.rst

@@ -21,6 +21,7 @@
   - MarkdownLint CLI with ``markdownlint`` [GH-1366]
   - Rust with ``rust-clippy`` [GH-1385]
   - VHDL with ``ghdl`` [GH-1160]
+  - mypy with ``python-mypy`` [GH-1354]
 
 - New features:
 

doc/languages.rst

@@ -900,6 +900,23 @@ to view the docstring of the syntax checker.  Likewise, you may use
 
       .. syntax-checker-config-file:: flycheck-flake8rc
 
+   .. syntax-checker:: python-mypy
+
+      Type check python with  `mypy <http://www.mypy-lang.org/>`_.
+
+      .. note::
+
+         This syntax checker requires mypy 0.580 or newer.
+
+      .. syntax-checker-config-file:: flycheck-python-mypy-ini
+
+      .. defcustom:: flycheck-python-mypy-cache-dir
+
+         Directory used to write ``.mypy_cache`` directories.
+
+         Set to ``null-device`` to disable writing cache directories
+         entirely.
+
    .. syntax-checker:: python-pylint
 
       Check syntax and lint with `Pylint <https://pylint.org/>`_.

flycheck.el

@@ -225,6 +225,7 @@ attention to case differences."
     python-flake8
     python-pylint
     python-pycompile
+    python-mypy
     r-lintr
     racket
     rpm-rpmlint
@@ -8950,6 +8951,35 @@ See URL `https://docs.python.org/3.4/library/py_compile.html'."
           line ", " column ", " (one-or-more not-newline) line-end))
   :modes python-mode)
 
+(flycheck-def-config-file-var flycheck-python-mypy-ini python-mypy
+                              "mypy.ini"
+  :safe #'stringp)
+
+(flycheck-def-option-var flycheck-python-mypy-cache-dir nil python-mypy
+  "Directory used to write .mypy_cache directories."
+  :safe #'stringp
+  :type '(choice
+          (const :tag "Write to the working directory" nil)
+          (const :tag "Never write .mypy_cache directories" null-device)
+          (string :tag "Path"))
+  :package-version '(flycheck . "32"))
+
+(flycheck-define-checker python-mypy
+  "Mypy syntax and type checker.  Requires mypy>=0.580.
+
+See URL `http://mypy-lang.org/'."
+  :command ("mypy"
+            (config-file "--config-file" flycheck-python-mypy-ini)
+            (option "--cache-dir" flycheck-python-mypy-cache-dir)
+            source-original)
+  :error-patterns
+  ((error line-start (file-name) ":" line ": error:" (message) line-end)
+   (warning line-start (file-name) ":" line ": warning:" (message) line-end))
+  :modes python-mode
+  ;; Ensure the file is saved, to work around https://github.com/python/mypy/issues/4746.
+  :predicate flycheck-buffer-saved-p
+  :next-checkers '(t . python-flake8))
+
 (flycheck-def-option-var flycheck-lintr-caching t r-lintr
   "Whether to enable caching in lintr.
 

test/flycheck-test.el

@@ -3764,6 +3764,15 @@ Why not:
    '(22 1 error "undefined name 'antigravity'" :id "F821"
         :checker python-flake8)))
 
+(flycheck-ert-def-checker-test python-mypy python nil
+  (let ((flycheck-disabled-checkers '(python-flake8))
+        (flycheck-checkers '(python-mypy))
+        (flycheck-python-mypy-cache-dir null-device))
+    (flycheck-ert-should-syntax-check
+     "language/python/invalid_type.py" 'python-mode
+     '(2 nil error "Incompatible return value type (got \"str\", expected \"int\")"
+         :checker python-mypy))))
+
 (flycheck-ert-def-checker-test python-pylint python syntax-error
   (let ((flycheck-disabled-checkers '(python-flake8))
         (python-indent-guess-indent-offset nil)) ; Silence Python Mode

test/resources/language/python/invalid_type.py

@@ -0,0 +1,2 @@
+def foo(x: str) -> int:
+    return x