Introduce app settings

Following the blueprint from [this blog post](https://overtag.dk/v2/blog/a-settings-pattern-for-reusable-django-apps/). Refactor DJANGO_CELERY_RESULTS_TASK_ID_MAX_LENGTH to use the new setting namespace and document it.

Author: browniebroke

django_celery_results/conf.py

@@ -0,0 +1,41 @@
+"""Application settings."""
+from dataclasses import dataclass
+from typing import Any
+
+from django.conf import settings as django_settings
+
+# All attributes accessed with this prefix are possible
+# to overwrite through django.conf.settings.
+SETTINGS_PREFIX = "DJANGO_CELERY_RESULTS_"
+
+
+@dataclass(frozen=True)
+class AppSettings:
+    """Proxy class to encapsulate all the app settings.
+
+    This instance should be accessed via the singleton
+    ``django_celery_results.conf.app_settings``.
+
+    You shouldn't have to set any of these yourself, the class checks a Django
+    settings with the same name and use these if defined, defaulting to the
+    values documented here.
+    """
+
+    DJANGO_CELERY_RESULTS_TASK_ID_MAX_LENGTH: int = 255
+
+    def __getattribute__(self, __name: str) -> Any:
+        """Check if a Django project settings should override the app default.
+
+        In order to avoid returning any random properties of the Django
+        settings, we first inspect the prefix.
+        """
+        if (
+            __name.startswith(SETTINGS_PREFIX)
+            and hasattr(django_settings, __name)
+        ):
+            return getattr(django_settings, __name)
+
+        return super().__getattribute__(__name)
+
+
+app_settings = AppSettings()

django_celery_results/migrations/0001_initial.py

@@ -1,6 +1,7 @@
-from django.conf import settings
 from django.db import migrations, models
 
+from django_celery_results.conf import app_settings
+
 
 class Migration(migrations.Migration):
 
@@ -18,11 +19,7 @@ class Migration(migrations.Migration):
                                         serialize=False,
                                         verbose_name='ID')),
                 ('task_id', models.CharField(
-                    max_length=getattr(
-                        settings,
-                        'DJANGO_CELERY_RESULTS_TASK_ID_MAX_LENGTH',
-                        255
-                    ),
+                    max_length=app_settings.DJANGO_CELERY_RESULTS_TASK_ID_MAX_LENGTH,  # noqa: E501
                     unique=True,
                     verbose_name='task id'
                 )),

django_celery_results/migrations/0004_auto_20190516_0412.py

@@ -7,6 +7,8 @@
 from django.conf import settings
 from django.db import migrations, models
 
+from django_celery_results.conf import app_settings
+
 
 class Migration(migrations.Migration):
 
@@ -61,11 +63,7 @@ class Migration(migrations.Migration):
             field=models.CharField(
                 db_index=True,
                 help_text='Celery ID for the Task that was run',
-                max_length=getattr(
-                    settings,
-                    'DJANGO_CELERY_RESULTS_TASK_ID_MAX_LENGTH',
-                    255
-                ),
+                max_length=app_settings.DJANGO_CELERY_RESULTS_TASK_ID_MAX_LENGTH,
                 unique=True,
                 verbose_name='Task ID'
             ),
@@ -81,11 +79,7 @@ class Migration(migrations.Migration):
             field=models.CharField(
                 db_index=True,
                 help_text='Name of the Task which was run',
-                max_length=getattr(
-                    settings,
-                    'DJANGO_CELERY_RESULTS_TASK_ID_MAX_LENGTH',
-                    255
-                ),
+                max_length=app_settings.DJANGO_CELERY_RESULTS_TASK_ID_MAX_LENGTH,
                 null=True,
                 verbose_name='Task Name'),
         ),

django_celery_results/migrations/0008_chordcounter.py

@@ -1,8 +1,9 @@
 # Generated by Django 3.0.6 on 2020-05-12 12:05
 
-from django.conf import settings
 from django.db import migrations, models
 
+from django_celery_results.conf import app_settings
+
 
 class Migration(migrations.Migration):
 
@@ -22,11 +23,7 @@ class Migration(migrations.Migration):
                 ('group_id', models.CharField(
                     db_index=True,
                     help_text='Celery ID for the Chord header group',
-                    max_length=getattr(
-                        settings,
-                        'DJANGO_CELERY_RESULTS_TASK_ID_MAX_LENGTH',
-                        255
-                    ),
+                    max_length=app_settings.DJANGO_CELERY_RESULTS_TASK_ID_MAX_LENGTH,  # noqa: E501
                     unique=True,
                     verbose_name='Group ID')),
                 ('sub_tasks', models.TextField(

django_celery_results/migrations/0009_groupresult.py

@@ -1,7 +1,8 @@
 # Generated by Django 3.2 on 2021-04-19 14:55
-from django.conf import settings
 from django.db import migrations, models
 
+from django_celery_results.conf import app_settings
+
 
 class FakeAddIndex(migrations.AddIndex):
     """Fake AddIndex to correct for duplicate index
@@ -35,11 +36,7 @@ class Migration(migrations.Migration):
                     verbose_name='ID')),
                 ('group_id', models.CharField(
                     help_text='Celery ID for the Group that was run',
-                    max_length=getattr(
-                        settings,
-                        'DJANGO_CELERY_RESULTS_TASK_ID_MAX_LENGTH',
-                        255
-                    ),
+                    max_length=app_settings.DJANGO_CELERY_RESULTS_TASK_ID_MAX_LENGTH,  # noqa: E501
                     unique=True,
                     verbose_name='Group ID')),
                 ('date_created', models.DateTimeField(
@@ -81,11 +78,7 @@ class Migration(migrations.Migration):
             name='group_id',
             field=models.CharField(
                 help_text='Celery ID for the Chord header group',
-                max_length=getattr(
-                    settings,
-                    'DJANGO_CELERY_RESULTS_TASK_ID_MAX_LENGTH',
-                    255
-                ),
+                max_length=app_settings.DJANGO_CELERY_RESULTS_TASK_ID_MAX_LENGTH,  # noqa: E501
                 unique=True,
                 verbose_name='Group ID'),
         ),
@@ -128,11 +121,7 @@ class Migration(migrations.Migration):
             name='task_id',
             field=models.CharField(
                 help_text='Celery ID for the Task that was run',
-                max_length=getattr(
-                    settings,
-                    'DJANGO_CELERY_RESULTS_TASK_ID_MAX_LENGTH',
-                    255
-                ),
+                max_length=app_settings.DJANGO_CELERY_RESULTS_TASK_ID_MAX_LENGTH,  # noqa: E501
                 unique=True,
                 verbose_name='Task ID'),
         ),
@@ -141,11 +130,7 @@ class Migration(migrations.Migration):
             name='task_name',
             field=models.CharField(
                 help_text='Name of the Task which was run',
-                max_length=getattr(
-                    settings,
-                    'DJANGO_CELERY_RESULTS_TASK_ID_MAX_LENGTH',
-                    255
-                ),
+                max_length=app_settings.DJANGO_CELERY_RESULTS_TASK_ID_MAX_LENGTH,  # noqa: E501
                 null=True,
                 verbose_name='Task Name'),
         ),

django_celery_results/models.py

@@ -5,11 +5,11 @@
 from celery import states
 from celery.result import GroupResult as CeleryGroupResult
 from celery.result import result_from_tuple
-from django.conf import settings
 from django.db import models
 from django.utils.translation import gettext_lazy as _
 
 from . import managers
+from .conf import app_settings
 
 ALL_STATES = sorted(states.ALL_STATES)
 TASK_STATE_CHOICES = sorted(zip(ALL_STATES, ALL_STATES))
@@ -19,11 +19,7 @@ class TaskResult(models.Model):
     """Task result/status."""
 
     task_id = models.CharField(
-        max_length=getattr(
-            settings,
-            'DJANGO_CELERY_RESULTS_TASK_ID_MAX_LENGTH',
-            255
-        ),
+        max_length=app_settings.DJANGO_CELERY_RESULTS_TASK_ID_MAX_LENGTH,
         unique=True,
         verbose_name=_('Task ID'),
         help_text=_('Celery ID for the Task that was run'))
@@ -32,11 +28,8 @@ class TaskResult(models.Model):
         verbose_name=_('Periodic Task Name'),
         help_text=_('Name of the Periodic Task which was run'))
     task_name = models.CharField(
-        null=True, max_length=getattr(
-            settings,
-            'DJANGO_CELERY_RESULTS_TASK_ID_MAX_LENGTH',
-            255
-        ),
+        null=True,
+        max_length=app_settings.DJANGO_CELERY_RESULTS_TASK_ID_MAX_LENGTH,
         verbose_name=_('Task Name'),
         help_text=_('Name of the Task which was run'))
     task_args = models.TextField(
@@ -140,10 +133,7 @@ class ChordCounter(models.Model):
     """Chord synchronisation."""
 
     group_id = models.CharField(
-        max_length=getattr(
-            settings,
-            "DJANGO_CELERY_RESULTS_TASK_ID_MAX_LENGTH",
-            255),
+        max_length=app_settings.DJANGO_CELERY_RESULTS_TASK_ID_MAX_LENGTH,
         unique=True,
         verbose_name=_("Group ID"),
         help_text=_("Celery ID for the Chord header group"),
@@ -181,11 +171,7 @@ class GroupResult(models.Model):
     """Task Group result/status."""
 
     group_id = models.CharField(
-        max_length=getattr(
-            settings,
-            "DJANGO_CELERY_RESULTS_TASK_ID_MAX_LENGTH",
-            255
-        ),
+        max_length=app_settings.DJANGO_CELERY_RESULTS_TASK_ID_MAX_LENGTH,
         unique=True,
         verbose_name=_("Group ID"),
         help_text=_("Celery ID for the Group that was run"),

docs/configuration.rst

@@ -0,0 +1,15 @@
+Configuration
+=============
+
+These are the available settings that can be configured in your Django
+project's settings module by defining a setting with the same name.
+
+.. _settings-task_id_max_length:
+
+* ``DJANGO_CELERY_RESULTS_TASK_ID_MAX_LENGTH`` (Default: ``255``)
+
+  The max length, as an integer, of the ``task_id`` and ``task_name``
+  fields on the ``TaskResult`` model. Defaults to 255.
+
+  Also used for the max length of the ``group_id`` fields on the
+  ``GroupResult`` and ``ChordCounter`` models.

docs/index.rst

@@ -12,6 +12,7 @@ Contents
 
     getting_started
     injecting_metadata
+    configuration
     copyright
 
 .. toctree::

setup.cfg

@@ -9,7 +9,7 @@ markers =
 [flake8]
 # classes can be lowercase, arguments and variables can be uppercase
 # whenever it makes the code more readable.
-ignore = N806, N802, N801, N803
+ignore = N806, N802, N801, N803, W503
 
 [pep257]
 convention=google