fix: move hardcoded model name to config and unify error handling (#22)

- Extract DEFAULT_EXPLAIN_MODEL constant to config.py; add
  MRTConfig.explain_model field so callers can override the model
  without touching cli.py
- Replace FileNotFoundError in MRTFixture with pytest.fail() so
  missing alembic.ini surfaces as a clean test failure instead of an
  unexpected exception type in a pytest fixture context
- Update test to match new pytest.fail.Exception behaviour

Author: croc100

pytest_mrt/cli.py

@@ -9,6 +9,7 @@
 
 from . import __version__
 from .adapters.django_detector import analyze_django_migrations, is_django_migration
+from .config import DEFAULT_EXPLAIN_MODEL
 from .core.detector import analyze_migrations
 
 app = typer.Typer(
@@ -411,7 +412,7 @@ def explain(
     try:
         client = anthropic.Anthropic()
         message = client.messages.create(
-            model="claude-opus-4-5",
+            model=DEFAULT_EXPLAIN_MODEL,
             max_tokens=1024,
             messages=[
                 {

pytest_mrt/config.py

@@ -6,6 +6,9 @@
 if TYPE_CHECKING:
     pass
 
+# Default model for `mrt explain`. Override via MRTConfig(explain_model=...).
+DEFAULT_EXPLAIN_MODEL = "claude-opus-4-5"
+
 
 @dataclass
 class MRTConfig:
@@ -69,3 +72,7 @@ class MRTConfig:
     # Path to the Django project root. Added to sys.path before import.
     # Required if the project is not on the Python path already.
     django_project_dir: str | None = None
+
+    # Model used by `mrt explain`. Defaults to DEFAULT_EXPLAIN_MODEL.
+    # Override to use a different Claude model, e.g. "claude-3-5-haiku-latest".
+    explain_model: str = DEFAULT_EXPLAIN_MODEL

pytest_mrt/plugin.py

@@ -89,7 +89,7 @@ def __init__(self, config: MRTConfig):
                     "    )\n\n"
                     "  See: https://croc100.github.io/pytest-mrt/quickstart/#django"
                 )
-                raise FileNotFoundError(_hint)
+                pytest.fail(_hint, pytrace=False)
 
             self._runner = MigrationRunner(config.alembic_ini, config.db_url)
             self._seeder = SmartSeeder(self._runner.engine)

tests/test_plugin.py

@@ -614,11 +614,11 @@ def test_auto_detect_django_no_switch_when_explicit_django_settings(tmp_path, mo
 # ── MRTFixture missing alembic.ini ─────────────────────────────────────
 
 def test_mrt_fixture_raises_on_missing_alembic_ini(tmp_path, monkeypatch):
-    """MRTFixture raises FileNotFoundError with Django hint when alembic.ini missing."""
+    """MRTFixture calls pytest.fail with a Django hint when alembic.ini is missing."""
     monkeypatch.delenv("DJANGO_SETTINGS_MODULE", raising=False)
 
     cfg = MRTConfig(alembic_ini=str(tmp_path / "nonexistent.ini"), db_url="sqlite:///test.db")
-    with pytest.raises(FileNotFoundError, match="django_settings"):
+    with pytest.raises(pytest.fail.Exception, match="django_settings"):
         MRTFixture(cfg)