Fix field deepcopy to copy error_messages dict

Ensure Field.__deepcopy__ creates an independent copy of error_messages.
Prevents changes to error messages in one form field instance from affecting others.
Addresses shared mutable error_messages issue (#issue).

django/forms/fields.py

@@ -199,6 +199,7 @@ class Field:
         result = copy.copy(self)
         memo[id(self)] = result
         result.widget = copy.deepcopy(self.widget, memo)
+        result.error_messages = self.error_messages.copy()
         result.validators = self.validators[:]
         return result
 

tests/forms_tests/field_tests/test_base.py

@@ -1,3 +1,5 @@
+import copy
+
 from django.forms import ChoiceField, Field, Form, Select
 from django.test import SimpleTestCase
 
@@ -35,6 +37,56 @@ class BasicFieldsTests(SimpleTestCase):
         self.assertEqual(f.fields['field1'].widget.choices, [('1', '1')])
         self.assertEqual(f.fields['field2'].widget.choices, [('2', '2')])
 
+    def test_field_deepcopies_error_messages(self):
+        """
+        Test that __deepcopy__ creates a deep copy of the error_messages dict.
+        Modifying error_messages on one field instance should not affect other
+        instances.
+        """
+        class MyForm(Form):
+            field1 = Field()
+            field2 = Field()
+
+        form1 = MyForm()
+        form2 = MyForm()
+
+        # Store original error message
+        original_error = str(form2.fields['field1'].error_messages['required'])
+
+        # Modify error_messages on form1's field1
+        form1.fields['field1'].error_messages['required'] = 'Custom error for form1'
+
+        # form2's field1 should not be affected
+        self.assertEqual(
+            form2.fields['field1'].error_messages['required'],
+            original_error
+        )
+
+        # form1's field2 should not be affected either
+        self.assertEqual(
+            form1.fields['field2'].error_messages['required'],
+            original_error
+        )
+
+        # Adding a new error message to form1's field1 should not affect form2
+        form1.fields['field1'].error_messages['custom'] = 'New custom error'
+        self.assertNotIn('custom', form2.fields['field1'].error_messages)
+
+    def test_field_direct_deepcopy_error_messages(self):
+        """
+        Test that directly deepcopying a field creates independent error_messages.
+        """
+        field1 = Field(error_messages={'required': 'Original required error'})
+        field2 = copy.deepcopy(field1)
+
+        # Modify field1's error_messages
+        field1.error_messages['required'] = 'Modified required error'
+        field1.error_messages['new_error'] = 'A new error'
+
+        # field2 should not be affected
+        self.assertEqual(field2.error_messages['required'], 'Original required error')
+        self.assertNotIn('new_error', field2.error_messages)
+
 
 class DisabledFieldTests(SimpleTestCase):
     def test_disabled_field_has_changed_always_false(self):