Output a more clear configuration error

mypy_django_plugin/main.py

@@ -1,9 +1,8 @@
 import configparser
 from functools import partial
-from typing import Callable, Dict, List, Optional, Tuple
+from typing import Callable, Dict, List, NoReturn, Optional, Tuple, cast
 
 from django.db.models.fields.related import RelatedField
-from mypy.errors import Errors
 from mypy.nodes import MypyFile, TypeInfo
 from mypy.options import Options
 from mypy.plugin import (
@@ -52,25 +51,40 @@ def add_new_manager_base(ctx: ClassDefContext) -> None:
 
 
 def extract_django_settings_module(config_file_path: Optional[str]) -> str:
-    errors = Errors()
-    if config_file_path is None:
-        errors.report(0, None, "'django_settings_module' is not set: no mypy config file specified")
-        errors.raise_error()
+
+    def exit(error_type: int) -> NoReturn:
+        """Using mypy's argument parser, raise `SystemExit` to fail hard if validation fails.
+
+        Considering that the plugin's startup duration is around double as long as mypy's, this aims to
+        import and construct objects only when that's required - which happens once and terminates the
+        run. Considering that most of the runs are successful, there's no need for this to linger in the
+        global scope.
+        """
+        from mypy.main import CapturableArgumentParser
+
+        usage = """(config)
+        ...
+        [mypy.plugins.django_stubs]
+            django_settings_module: str (required)
+        ...
+        """.replace("\n" + 8 * " ", "\n")
+        handler = CapturableArgumentParser(prog='(django-stubs) mypy', usage=usage)
+        messages = {1: 'mypy config file is not specified or found',
+                    2: 'no section [mypy.plugins.django-stubs]',
+                    3: 'the setting is not provided'}
+        handler.error("'django_settings_module' is not set: " + messages[error_type])
 
     parser = configparser.ConfigParser()
-    parser.read(config_file_path)  # type: ignore
-
-    if not parser.has_section('mypy.plugins.django-stubs'):
-        errors.report(0, None, "'django_settings_module' is not set: no section [mypy.plugins.django-stubs]",
-                      file=config_file_path)
-        errors.raise_error()
-    if not parser.has_option('mypy.plugins.django-stubs', 'django_settings_module'):
-        errors.report(0, None, "'django_settings_module' is not set: setting is not provided",
-                      file=config_file_path)
-        errors.raise_error()
-
-    django_settings_module = parser.get('mypy.plugins.django-stubs', 'django_settings_module').strip('\'"')
-    return django_settings_module
+    try:
+        parser.read_file(open(cast(str, config_file_path), 'r'), source=config_file_path)
+    except (IsADirectoryError, OSError):
+        exit(1)
+
+    section = 'mypy.plugins.django-stubs'
+    if not parser.has_section(section):
+        exit(2)
+    settings = parser.get(section, 'django_settings_module', fallback=None) or exit(3)
+    return cast(str, settings).strip('\'"')
 
 
 class NewSemanalDjangoPlugin(Plugin):

pytest.ini

@@ -1,5 +1,7 @@
 [pytest]
-testpaths = ./test-data
+testpaths =
+    ./test-plugin
+    ./test-data
 addopts =
     --tb=native
     -s

test-plugin/test_error_handling.py

@@ -0,0 +1,73 @@
+import tempfile
+import typing
+
+import pytest
+
+from mypy_django_plugin.main import extract_django_settings_module
+
+TEMPLATE = """usage: (config)
+...
+[mypy.plugins.django_stubs]
+    django_settings_module: str (required)
+...
+(django-stubs) mypy: error: 'django_settings_module' is not set: {}
+"""
+
+
+@pytest.mark.parametrize(
+    'config_file_contents,message_part',
+    [
+        pytest.param(
+            None,
+            'mypy config file is not specified or found',
+            id='missing-file',
+        ),
+        pytest.param(
+            ['[not-really-django-stubs]'],
+            'no section [mypy.plugins.django-stubs]',
+            id='missing-section',
+        ),
+        pytest.param(
+            ['[mypy.plugins.django-stubs]',
+             '\tnot_django_not_settings_module = badbadmodule'],
+            'the setting is not provided',
+            id='missing-settings-module',
+        ),
+        pytest.param(
+            ['[mypy.plugins.django-stubs]'],
+            'the setting is not provided',
+            id='no-settings-given',
+        ),
+    ],
+)
+def test_misconfiguration_handling(capsys, config_file_contents, message_part):
+    #  type: (typing.Any, typing.List[str], str) -> None
+    """Invalid configuration raises `SystemExit` with a precise error message."""
+    with tempfile.NamedTemporaryFile(mode='w+') as config_file:
+        if not config_file_contents:
+            config_file.close()
+        else:
+            config_file.write('\n'.join(config_file_contents).expandtabs(4))
+            config_file.seek(0)
+
+        with pytest.raises(SystemExit, match='2'):
+            extract_django_settings_module(config_file.name)
+
+    error_message = TEMPLATE.format(message_part)
+    assert error_message == capsys.readouterr().err
+
+
+def test_correct_configuration() -> None:
+    """Django settings module gets extracted given valid configuration."""
+    config_file_contents = [
+        '[mypy.plugins.django-stubs]',
+        '\tsome_other_setting = setting',
+        '\tdjango_settings_module = my.module',
+    ]
+    with tempfile.NamedTemporaryFile(mode='w+') as config_file:
+        config_file.write('\n'.join(config_file_contents).expandtabs(4))
+        config_file.seek(0)
+
+        extracted = extract_django_settings_module(config_file.name)
+
+    assert extracted == 'my.module'