[1.5.x] Fixed #19423 -- Prevented ModelAdmin sharing widgets due to formfield_overrides

Thanks joebuyer at manycycles.com for the report and Simon Charette
for the review.
Backport of 04e6542b5 from master.

django/contrib/admin/options.py

@@ -1,3 +1,4 @@
+import copy
 from functools import update_wrapper, partial
 import warnings
 
@@ -130,7 +131,7 @@ class BaseModelAdmin(six.with_metaclass(forms.MediaDefiningClass)):
         # passed to formfield_for_dbfield override the defaults.
         for klass in db_field.__class__.mro():
             if klass in self.formfield_overrides:
-                kwargs = dict(self.formfield_overrides[klass], **kwargs)
+                kwargs = dict(copy.deepcopy(self.formfield_overrides[klass]), **kwargs)
                 return db_field.formfield(**kwargs)
 
         # For any other type of field, just call its formfield() method.

tests/regressiontests/admin_widgets/models.py

@@ -20,6 +20,7 @@ class Member(models.Model):
 @python_2_unicode_compatible
 class Band(models.Model):
     name = models.CharField(max_length=100)
+    style = models.CharField(max_length=20)
     members = models.ManyToManyField(Member)
 
     def __str__(self):

tests/regressiontests/admin_widgets/tests.py

@@ -10,7 +10,7 @@ from django.contrib.admin import widgets
 from django.contrib.admin.tests import AdminSeleniumWebDriverTestCase
 from django.core.files.storage import default_storage
 from django.core.files.uploadedfile import SimpleUploadedFile
-from django.db.models import DateField
+from django.db.models import CharField, DateField
 from django.test import TestCase as DjangoTestCase
 from django.test.utils import override_settings
 from django.utils import translation
@@ -112,6 +112,23 @@ class AdminFormfieldForDBFieldTests(TestCase):
         self.assertFormfield(models.Event, 'start_date', forms.TextInput,
                              formfield_overrides={DateField: {'widget': forms.TextInput}})
 
+    def testFormfieldOverridesWidgetInstances(self):
+        """
+        Test that widget instances in formfield_overrides are not shared between
+        different fields. (#19423)
+        """
+        class BandAdmin(admin.ModelAdmin):
+            formfield_overrides = {
+                CharField: {'widget': forms.TextInput(attrs={'size':'10'})}
+            }
+        ma = BandAdmin(models.Band, admin.site)
+        f1 = ma.formfield_for_dbfield(models.Band._meta.get_field('name'), request=None)
+        f2 = ma.formfield_for_dbfield(models.Band._meta.get_field('style'), request=None)
+        self.assertNotEqual(f1.widget, f2.widget)
+        self.assertEqual(f1.widget.attrs['maxlength'], '100')
+        self.assertEqual(f2.widget.attrs['maxlength'], '20')
+        self.assertEqual(f2.widget.attrs['size'], '10')
+
     def testFieldWithChoices(self):
         self.assertFormfield(models.Member, 'gender', forms.Select)