fix: keep --format json/html output free of status preamble (#97)

mrt check printed human-readable status lines (Detected: Django migrations,
--since/--min-revision/--check-compat notices) to stdout regardless of
--format. With --format json this corrupted the output: consumers piping to a
JSON parser hit 'Expecting value: line 1 column 1'. The bug was latent for
Alembic and became easy to hit now that Django + --check-compat + --format
json is a supported combination.

Gate all informational console.print calls on fmt == 'table'. Error warnings
that precede typer.Exit are unaffected (they abort before any JSON is
emitted). Add regression tests for Alembic and Django --check-compat json.

Author: croc100

CHANGELOG.md

@@ -12,6 +12,9 @@ Versioning: [Semantic Versioning](https://semver.org/spec/v2.0.0.html)
 ### Added
 - **`mrt check --check-compat` now supports Django migrations.** Rolling-deploy compatibility checks (MRT7xx) were previously Alembic-only. Django operations are now mapped to the same patterns: `RemoveField` → MRT701 (DROP COLUMN), `RenameField` → MRT702 (RENAME COLUMN), `DeleteModel`/`RenameModel`/`AlterModelTable` → MRT703 (DROP/RENAME TABLE), and `AddField` NOT NULL without a default → MRT704. Per-line `# noqa: MRTxxx` suppression works for these too. MRT705 (column type change) remains Alembic-only — Django's `AlterField` carries the full field with no reference to the previous type, so a type change cannot be detected statically.
 
+### Fixed
+- **`mrt check --format json` leaked human-readable status lines into stdout**, making the output unparseable whenever `--since`, `--min-revision`, `--check-compat`, or Django auto-detection printed an informational line (e.g. `Detected: Django migrations`). These lines are now suppressed for non-`table` formats, so `--format json` emits pure JSON.
+
 ### Removed
 - The "`--check-compat` is not yet supported for Django migrations" warning, now that it is supported.
 

pytest_mrt/commands/check.py

@@ -163,15 +163,16 @@ def check(
             )
             raise typer.Exit(1)
 
-        console.print(
-            f"[dim]--since {since}: checking {len(since_set)} migration(s) after this point[/dim]"
-        )
-        console.print(
-            "[dim]Note: graph checks (orphan, data-hole detection) skipped — "
-            "run without --since for full analysis.[/dim]"
-        )
+        if fmt == "table":
+            console.print(
+                f"[dim]--since {since}: checking {len(since_set)} migration(s) after this point[/dim]"
+            )
+            console.print(
+                "[dim]Note: graph checks (orphan, data-hole detection) skipped — "
+                "run without --since for full analysis.[/dim]"
+            )
 
-    if is_django:
+    if is_django and fmt == "table":
         console.print("[dim]Detected: Django migrations[/dim]")
 
     if min_revision:
@@ -190,11 +191,12 @@ def check(
                 "Check the revision ID and try again.[/yellow]"
             )
             raise typer.Exit(1)
-        console.print(
-            f"[dim]--min-revision {min_revision}: checking {len(min_set)} newer migration(s), older ones skipped[/dim]"
-        )
+        if fmt == "table":
+            console.print(
+                f"[dim]--min-revision {min_revision}: checking {len(min_set)} newer migration(s), older ones skipped[/dim]"
+            )
 
-    if check_compat:
+    if check_compat and fmt == "table":
         console.print(
             "[dim]--check-compat: rolling-deploy compatibility checks enabled (MRT7xx)[/dim]"
         )

tests/test_cli.py

@@ -293,6 +293,43 @@ class Migration(migrations.Migration):
     assert "Django" in result.output or result.exit_code == 0
 
 
+def test_check_compat_json_output_is_pure_json_alembic(tmp_path, versions_dir):
+    """--check-compat --format json must not leak the status preamble into stdout."""
+    import json
+
+    _risky_migration(versions_dir)
+    result = runner.invoke(
+        app, ["check", str(versions_dir), "--check-compat", "--format", "json"]
+    )
+    data = json.loads(result.output)  # would raise if preamble leaked
+    assert "findings" in data
+
+
+def test_check_compat_json_output_is_pure_json_django(tmp_path):
+    """Django + --check-compat + --format json emits parseable JSON with MRT7xx codes."""
+    import json
+
+    d = tmp_path / "myapp" / "migrations"
+    d.mkdir(parents=True)
+    (d / "0001_initial.py").write_text(
+        "from django.db import migrations\n"
+        "class Migration(migrations.Migration):\n"
+        "    dependencies = []\n"
+        "    operations = []\n"
+    )
+    (d / "0002_rename.py").write_text(
+        "from django.db import migrations\n"
+        "class Migration(migrations.Migration):\n"
+        "    dependencies = [('myapp', '0001_initial')]\n"
+        "    operations = [migrations.RenameField("
+        "model_name='u', old_name='a', new_name='b')]\n"
+    )
+    result = runner.invoke(app, ["check", str(d), "--check-compat", "--format", "json"])
+    data = json.loads(result.output)
+    assert any(f["rule"] == "MRT702" for f in data["findings"])
+    assert result.exit_code == 2
+
+
 def test_check_empty_directory_exits_0(tmp_path):
     empty = tmp_path / "empty"
     empty.mkdir()