use pytest_mypy_plugins to test mypy plugin

Author: selimb

setup.py

@@ -20,9 +20,11 @@
     "docs": ["sphinx"],
     "tests": [
         "pytest",
-        # typed-ast (a dependency of mypy) fails to install on pypy
+        # re: pypy: typed-ast (a dependency of mypy) fails to install on pypy
         # https://github.com/python/typed_ast/issues/111
-        "mypy; implementation_name != 'pypy'",
+        # re: win32: pytest-mypy-plugins depends on capturer, which isn't supported on
+        # windows
+        "pytest-mypy-plugins; implementation_name != 'pypy' and sys.platform != 'win32'",
     ],
 }
 EXTRAS_REQUIRE["dev"] = (

tests/test_mypy.py

@@ -0,0 +1,26 @@
+# Tests for marshmallow_dataclass.mypy, using pytest-mypy-plugins
+# NOTE: Since pytest-mypy-plugins is not installed on pypy (see setup.py), these tests
+#       will not even be collected by pytest
+- case: NewType_basic
+  mypy_config: |
+    follow_imports = silent
+    plugins = marshmallow_dataclass.mypy
+  main: |
+    from dataclasses import dataclass
+    import marshmallow as ma
+    from marshmallow_dataclass import NewType
+
+    Email = NewType("Email", str, validate=ma.validate.Email)
+    UserID = NewType("UserID", validate=ma.validate.Length(equal=32), typ=str)
+
+    @dataclass
+    class User:
+        id: UserID
+        email: Email
+
+    user = User(id="a"*32, email="[email protected]")
+    reveal_type(user.id)  # N: Revealed type is 'builtins.str'
+    reveal_type(user.email)  # N: Revealed type is 'builtins.str'
+
+    User(id=42, email="[email protected]")  # E: Argument "id" to "User" has incompatible type "int"; expected "str"
+    User(id="a"*32, email=["not", "a", "string"])  # E: Argument "email" to "User" has incompatible type "List[str]"; expected "str"