Fixed #34 -- Added a check for mutable PickledObjectField default.

README.rst

@@ -172,6 +172,7 @@ Changes in version 2.0.0
   ``.save()``, ``.create()``, etc. raise ``IntegrityError`` if `null` is not
   ``True`` and no default value was specified like built-in fields do
   (thanks to Attila-Mihaly Balazs).
+* Added a check for mutable default values to ``PickledObjectField``.
 
 Changes in version 1.1.0
 ========================

picklefield/fields.py

@@ -5,6 +5,7 @@
 from zlib import compress, decompress
 
 import django
+from django.core import checks
 from django.db import models
 from django.utils.encoding import force_text
 
@@ -115,6 +116,33 @@ def get_default(self):
         # If the field doesn't have a default, then we punt to models.Field.
         return super(PickledObjectField, self).get_default()
 
+    def _check_default(self):
+        if self.has_default() and isinstance(self.default, (list, dict, set)):
+            return [
+                checks.Warning(
+                    "%s default should be a callable instead of a mutable instance so "
+                    "that it's not shared between all field instances." % (
+                        self.__class__.__name__,
+                    ),
+                    hint=(
+                        'Use a callable instead, e.g., use `%s` instead of '
+                        '`%r`.' % (
+                            type(self.default).__name__,
+                            self.default,
+                        )
+                    ),
+                    obj=self,
+                    id='picklefield.E001',
+                )
+            ]
+        else:
+            return []
+
+    def check(self, **kwargs):
+        errors = super(PickledObjectField, self).check(**kwargs)
+        errors.extend(self._check_default())
+        return errors
+
     def to_python(self, value):
         """
         B64decode and unpickle the object, optionally decompressing it.

tests/tests.py

@@ -1,10 +1,13 @@
 import json
 from datetime import date
 
-from django.core import serializers
-from django.db import IntegrityError
-from django.test import TestCase
-from picklefield.fields import dbsafe_encode, wrap_conflictual_object
+from django.core import checks, serializers
+from django.db import IntegrityError, models
+from django.test import SimpleTestCase, TestCase
+from django.test.utils import isolate_apps
+from picklefield.fields import (
+    PickledObjectField, dbsafe_encode, wrap_conflictual_object,
+)
 
 from .models import (
     D1, D2, L1, S1, T1, MinimalTestingModel, TestCopyDataType,
@@ -177,3 +180,46 @@ def test_no_copy(self):
     def test_empty_strings_not_allowed(self):
         with self.assertRaises(IntegrityError):
             MinimalTestingModel.objects.create()
+
+
+@isolate_apps('tests')
+class PickledObjectFieldCheckTests(SimpleTestCase):
+    def test_mutable_default_check(self):
+        class Model(models.Model):
+            list_field = PickledObjectField(default=[])
+            dict_field = PickledObjectField(default={})
+            set_field = PickledObjectField(default=set())
+
+        msg = (
+            "PickledObjectField default should be a callable instead of a mutable instance so "
+            "that it's not shared between all field instances."
+        )
+
+        self.assertEqual(Model().check(), [
+            checks.Warning(
+                msg=msg,
+                hint='Use a callable instead, e.g., use `list` instead of `[]`.',
+                obj=Model._meta.get_field('list_field'),
+                id='picklefield.E001',
+            ),
+            checks.Warning(
+                msg=msg,
+                hint='Use a callable instead, e.g., use `dict` instead of `{}`.',
+                obj=Model._meta.get_field('dict_field'),
+                id='picklefield.E001',
+            ),
+            checks.Warning(
+                msg=msg,
+                hint='Use a callable instead, e.g., use `set` instead of `%s`.' % repr(set()),
+                obj=Model._meta.get_field('set_field'),
+                id='picklefield.E001',
+            )
+        ])
+
+    def test_non_mutable_default_check(self):
+        class Model(models.Model):
+            list_field = PickledObjectField(default=list)
+            dict_field = PickledObjectField(default=dict)
+            set_field = PickledObjectField(default=set)
+
+        self.assertEqual(Model().check(), [])