fix: mypy plugin loading under pre-commit

Author: dairiki

.pre-commit-config.yaml

@@ -24,6 +24,7 @@ repos:
       - marshmallow-enum
       - typeguard
       - types-setuptools
+      - typing-inspect
     args: [--show-error-codes]
 - repo: https://github.com/asottile/blacken-docs
   rev: v1.12.1

mypy_plugin.py

@@ -0,0 +1,48 @@
+"""Shim to load the marshmallow_dataclass.mypy plugin.
+
+This shim is needed when running mypy from pre-commit.
+
+Pre-commit runs mypy from its own venv (into which we do not want
+to install marshmallow_dataclass). Because of this, loading the plugin
+by module name, e.g.
+
+    [tool.mypy]
+    plugins = "marshmallow_dataclass.mypy"
+
+does not work. Mypy also supports specifying a path to the plugin
+module source, which would normally get us out of this bind, however,
+the fact that our plugin is in a file named "mypy.py" causes issues.
+
+If we set
+
+    [tool.mypy]
+    plugins = "marshmallow_dataclass/mypy.py"
+
+mypy `attempts to load`__ the plugin module by temporarily prepending
+ ``marshmallow_dataclass`` to ``sys.path`` then importing the ``mypy``
+module. Sadly, mypy's ``mypy`` module has already been imported,
+so this doesn't end well.
+
+__ https://github.com/python/mypy/blob/914901f14e0e6223077a8433388c367138717451/mypy/build.py#L450
+
+
+Our solution, here, is to manually load the plugin module (with a better
+``sys.path``, and import the ``plugin`` from the real plugin module into this one.
+
+Now we can configure mypy to load this file, by path.
+
+    [tool.mypy]
+    plugins = "mypy_plugin.py"
+
+"""
+import importlib
+import sys
+from os import fspath
+from pathlib import Path
+
+src = fspath(Path(__file__).parent)
+sys.path.insert(0, src)
+plugin_module = importlib.import_module("marshmallow_dataclass.mypy")
+del sys.path[0]
+
+plugin = plugin_module.plugin

pyproject.toml

@@ -12,15 +12,10 @@ packages = [
     "marshmallow_dataclass",
     "tests",
 ]
-# XXX: Can not load our plugin when running mypy from pre-commit.
-# Pre-commit runs mypy from its own venv (into which we do not want
-# to install marshmallow_dataclass).
-# The fact that our plugin is in a file named "mypy.py" causes issues
-# (I think) if we try to load it by path.  In that case mypy adds
-# the containing directory to sys.path then calls import_module("mypy"),
-# which, in turn, finds the already imported sys.modules['mypy'].
-#
-# plugins = "marshmallow_dataclass.mypy"
+# XXX: Specifying the marshmallow_dataclass.mypy plugin directly by
+# module name or by path does not work when running mypy from pre-commit.
+# (See the docstring in mypy_plugin.py for more.)
+plugins = "mypy_plugin.py"
 
 warn_redundant_casts = true
 warn_unused_configs = true