[3.0.x] Fixed #30870 -- Fixed showing that RunPython operations are irreversible by migrate --plan.

Thanks Hasan Ramezani for the initial patch and Kyle Dickerson for the
report.

Backport of 06d34aab7c from master

django/core/management/commands/migrate.py

@@ -343,21 +343,21 @@ class Command(BaseCommand):
     def describe_operation(operation, backwards):
         """Return a string that describes a migration operation for --plan."""
         prefix = ''
+        is_error = False
         if hasattr(operation, 'code'):
             code = operation.reverse_code if backwards else operation.code
-            action = code.__doc__ if code else ''
+            action = (code.__doc__ or '') if code else None
         elif hasattr(operation, 'sql'):
             action = operation.reverse_sql if backwards else operation.sql
         else:
             action = ''
             if backwards:
                 prefix = 'Undo '
-        if action is None:
+        if action is not None:
+            action = str(action).replace('\n', '')
+        elif backwards:
             action = 'IRREVERSIBLE'
             is_error = True
-        else:
-            action = str(action).replace('\n', '')
-            is_error = False
         if action:
             action = ' -> ' + action
         truncated = Truncator(action)

docs/releases/2.2.7.txt

@@ -13,3 +13,7 @@ Bugfixes
   ``has_keys``, or ``has_any_keys`` lookup on
   :class:`~django.contrib.postgres.fields.JSONField`, if the right or left hand
   side of an expression is a key transform (:ticket:`30826`).
+
+* Prevented :option:`migrate --plan` from showing that ``RunPython`` operations
+  are irreversible when ``reverse_code`` callables don't have docstrings or
+  when showing a forward migration plan (:ticket:`30870`).

tests/migrations/test_commands.py

@@ -371,6 +371,28 @@ class MigrateTests(MigrationTestBase):
                 '    Raw SQL operation -> IRREVERSIBLE\n',
                 out.getvalue()
             )
+            out = io.StringIO()
+            call_command('migrate', 'migrations', '0005', plan=True, stdout=out, no_color=True)
+            # Operation is marked as irreversible only in the revert plan.
+            self.assertEqual(
+                'Planned operations:\n'
+                'migrations.0005_fifth\n'
+                '    Raw Python operation\n'
+                '    Raw Python operation\n'
+                '    Raw Python operation -> Feed salamander.\n',
+                out.getvalue()
+            )
+            call_command('migrate', 'migrations', '0005', verbosity=0)
+            out = io.StringIO()
+            call_command('migrate', 'migrations', '0004', plan=True, stdout=out, no_color=True)
+            self.assertEqual(
+                'Planned operations:\n'
+                'migrations.0005_fifth\n'
+                '    Raw Python operation -> IRREVERSIBLE\n'
+                '    Raw Python operation -> IRREVERSIBLE\n'
+                '    Raw Python operation\n',
+                out.getvalue()
+            )
         finally:
             # Cleanup by unmigrating everything: fake the irreversible, then
             # migrate all to zero.

tests/migrations/test_migrations_plan/0005_fifth.py

@@ -0,0 +1,22 @@
+from django.db import migrations
+
+
+def grow_tail(x, y):
+    pass
+
+
+def feed(x, y):
+    """Feed salamander."""
+    pass
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ('migrations', '0004_fourth'),
+    ]
+
+    operations = [
+        migrations.RunPython(migrations.RunPython.noop),
+        migrations.RunPython(grow_tail),
+        migrations.RunPython(feed, migrations.RunPython.noop),
+    ]