Fixed #20765 -- Set small values of `step` using exponential notation.

Browsers parse small factors of 10 as 0 under decimal notation.

Thanks to Trac alias matklad for the report and Claude Paroz for the review.

3 files changed, 20 insertions, 3 deletions

diff --git a/django/forms/fields.py b/django/forms/fields.py
index c4bc3fa88c..7711cd65ad 100644
--- a/django/forms/fields.py
+++ b/django/forms/fields.py
@@ -370,8 +370,14 @@ class DecimalField(IntegerField):
 
     def widget_attrs(self, widget):
         attrs = super(DecimalField, self).widget_attrs(widget)
-        if isinstance(widget, NumberInput) and self.decimal_places:
-            attrs['step'] = '0.%s1' % ('0' * (self.decimal_places - 1))
+        if isinstance(widget, NumberInput):
+            if self.decimal_places is not None:
+                # Use exponential notation for small values since they might
+                # be parsed as 0 otherwise. ref #20765
+                step = str(Decimal('1') / 10 ** self.decimal_places).lower()
+            else:
+                step = 'any'
+            attrs.setdefault('step', step)
         return attrs
 
 
diff --git a/tests/forms_tests/tests/test_fields.py b/tests/forms_tests/tests/test_fields.py
index f7d83503f4..dab67b4366 100644
--- a/tests/forms_tests/tests/test_fields.py
+++ b/tests/forms_tests/tests/test_fields.py
@@ -374,6 +374,17 @@ class FieldsTests(SimpleTestCase):
         self.assertEqual(f.clean('.01'), Decimal(".01"))
         self.assertRaisesMessage(ValidationError, "'Ensure that there are no more than 0 digits before the decimal point.'", f.clean, '1.1')
 
+    def test_decimalfield_widget_attrs(self):
+        f = DecimalField(max_digits=6, decimal_places=2)
+        self.assertEqual(f.widget_attrs(Widget()), {})
+        self.assertEqual(f.widget_attrs(NumberInput()), {'step': '0.01'})
+        f = DecimalField(max_digits=10, decimal_places=0)
+        self.assertEqual(f.widget_attrs(NumberInput()), {'step': '1'})
+        f = DecimalField(max_digits=19, decimal_places=19)
+        self.assertEqual(f.widget_attrs(NumberInput()), {'step': '1e-19'})
+        f = DecimalField(max_digits=20)
+        self.assertEqual(f.widget_attrs(NumberInput()), {'step': 'any'})
+
     def test_decimalfield_localized(self):
         """
         Make sure localized DecimalField's widget renders to a text input with
diff --git a/tests/model_formsets/tests.py b/tests/model_formsets/tests.py
index 09a3ba1778..4411bbf59a 100644
--- a/tests/model_formsets/tests.py
+++ b/tests/model_formsets/tests.py
@@ -576,7 +576,7 @@ class ModelFormsetTest(TestCase):
         formset = AuthorBooksFormSet2(instance=author)
         self.assertEqual(len(formset.forms), 1)
         self.assertHTMLEqual(formset.forms[0].as_p(),
-            '<p><label for="id_bookwithcustompk_set-0-my_pk">My pk:</label> <input id="id_bookwithcustompk_set-0-my_pk" type="number" name="bookwithcustompk_set-0-my_pk" /></p>\n'
+            '<p><label for="id_bookwithcustompk_set-0-my_pk">My pk:</label> <input id="id_bookwithcustompk_set-0-my_pk" type="number" name="bookwithcustompk_set-0-my_pk" step="1" /></p>\n'
             '<p><label for="id_bookwithcustompk_set-0-title">Title:</label> <input id="id_bookwithcustompk_set-0-title" type="text" name="bookwithcustompk_set-0-title" maxlength="100" /><input type="hidden" name="bookwithcustompk_set-0-author" value="1" id="id_bookwithcustompk_set-0-author" /></p>')
 
         data = {