[1.10.x] Fixed #27186 -- Fixed model form default fallback for MultiWidget, FileInput, SplitDateTimeWidget, SelectDateWidget, and SplitArrayWidget.

Thanks Matt Westcott for the review.

Backport of 3507d4e773aa9ff2336e7230ba231c4ba6eb568f from master

15 files changed, 178 insertions, 13 deletions

diff --git a/django/contrib/postgres/forms/array.py b/django/contrib/postgres/forms/array.py
index dd9ac1178e..bd3daaae0b 100644
--- a/django/contrib/postgres/forms/array.py
+++ b/django/contrib/postgres/forms/array.py
@@ -103,6 +103,12 @@ class SplitArrayWidget(forms.Widget):
         return [self.widget.value_from_datadict(data, files, '%s_%s' % (name, index))
                 for index in range(self.size)]
 
+    def value_omitted_from_data(self, data, files, name):
+        return all(
+            self.widget.value_omitted_from_data(data, files, '%s_%s' % (name, index))
+            for index in range(self.size)
+        )
+
     def id_for_label(self, id_):
         # See the comment for RadioSelect.id_for_label()
         if id_:
diff --git a/django/forms/models.py b/django/forms/models.py
index 1d54ea9676..ce93dead9e 100644
--- a/django/forms/models.py
+++ b/django/forms/models.py
@@ -54,8 +54,8 @@ def construct_instance(form, instance, fields=None, exclude=None):
             continue
         # Leave defaults for fields that aren't in POST data, except for
         # checkbox inputs because they don't appear in POST data if not checked.
-        if (f.has_default() and form.add_prefix(f.name) not in form.data and
-                not getattr(form[f.name].field.widget, 'dont_use_model_field_default_for_empty_data', False)):
+        if (f.has_default() and
+                form[f.name].field.widget.value_omitted_from_data(form.data, form.files, form.add_prefix(f.name))):
             continue
         # Defer saving file-type fields until after the other fields, so a
         # callable upload_to can use the values from other fields.
diff --git a/django/forms/widgets.py b/django/forms/widgets.py
index 735207aa80..965f84f7df 100644
--- a/django/forms/widgets.py
+++ b/django/forms/widgets.py
@@ -237,6 +237,9 @@ class Widget(six.with_metaclass(RenameWidgetMethods)):
         """
         return data.get(name)
 
+    def value_omitted_from_data(self, data, files, name):
+        return name not in data
+
     def id_for_label(self, id_):
         """
         Returns the HTML ID attribute of this Widget for use by a <label>,
@@ -350,6 +353,9 @@ class FileInput(Input):
         "File widgets take data from FILES, not POST"
         return files.get(name)
 
+    def value_omitted_from_data(self, data, files, name):
+        return name not in files
+
 
 FILE_INPUT_CONTRADICTION = object()
 
@@ -480,10 +486,6 @@ def boolean_check(v):
 
 
 class CheckboxInput(Widget):
-    # Don't use model field defaults for fields that aren't in POST data,
-    # because checkboxes don't appear in POST data if not checked.
-    dont_use_model_field_default_for_empty_data = True
-
     def __init__(self, attrs=None, check_test=None):
         super(CheckboxInput, self).__init__(attrs)
         # check_test is a callable that takes a value and returns True
@@ -511,6 +513,11 @@ class CheckboxInput(Widget):
             value = values.get(value.lower(), value)
         return bool(value)
 
+    def value_omitted_from_data(self, data, files, name):
+        # HTML checkboxes don't appear in POST data if not checked, so it's
+        # never known if the value is actually omitted.
+        return False
+
 
 class Select(Widget):
     allow_multiple_selected = False
@@ -871,6 +878,12 @@ class MultiWidget(Widget):
     def value_from_datadict(self, data, files, name):
         return [widget.value_from_datadict(data, files, name + '_%s' % i) for i, widget in enumerate(self.widgets)]
 
+    def value_omitted_from_data(self, data, files, name):
+        return all(
+            widget.value_omitted_from_data(data, files, name + '_%s' % i)
+            for i, widget in enumerate(self.widgets)
+        )
+
     def format_output(self, rendered_widgets):
         """
         Given a list of rendered widgets (as strings), returns a Unicode string
@@ -1056,6 +1069,12 @@ class SelectDateWidget(Widget):
                 return '%s-%s-%s' % (y, m, d)
         return data.get(name)
 
+    def value_omitted_from_data(self, data, files, name):
+        return not any(
+            ('{}_{}'.format(name, interval) in data)
+            for interval in ('year', 'month', 'day')
+        )
+
     def create_select(self, name, field, value, val, choices, none_value):
         if 'id' in self.attrs:
             id_ = self.attrs['id']
diff --git a/docs/ref/forms/widgets.txt b/docs/ref/forms/widgets.txt
index 0bc23aab66..fde7806b02 100644
--- a/docs/ref/forms/widgets.txt
+++ b/docs/ref/forms/widgets.txt
@@ -271,6 +271,21 @@ foundation for custom widgets.
         customize it and add expensive processing, you should implement some
         caching mechanism yourself.
 
+    .. method:: value_omitted_from_data(data, files, name)
+
+        .. versionadded:: 1.10.2
+
+        Given ``data`` and ``files`` dictionaries and this widget's name,
+        returns whether or not there's data or files for the widget.
+
+        The method's result affects whether or not a field in a model form
+        :ref:`falls back to its default <topics-modelform-save>`.
+
+        A special case is :class:`~django.forms.CheckboxInput`, which always
+        returns ``False`` because an unchecked checkbox doesn't appear in the
+        data of an HTML form submission, so it's unknown whether or not the
+        user actually submitted a value.
+
 ``MultiWidget``
 ---------------
 
diff --git a/docs/releases/1.10.2.txt b/docs/releases/1.10.2.txt
index 8c031d37f5..222323c245 100644
--- a/docs/releases/1.10.2.txt
+++ b/docs/releases/1.10.2.txt
@@ -17,3 +17,8 @@ Bugfixes
 
 * Disabled system check for URL patterns beginning with a '/' when
   ``APPEND_SLASH=False`` (:ticket:`27238`).
+
+* Fixed model form ``default`` fallback for ``MultiWidget``, ``FileInput``,
+  ``SplitDateTimeWidget``, ``SelectDateWidget``, and ``SplitArrayWidget``
+  (:ticket:`27186`). Custom widgets affected by this issue may need to
+  implement a :meth:`~django.forms.Widget.value_omitted_from_data` method.
diff --git a/docs/topics/forms/modelforms.txt b/docs/topics/forms/modelforms.txt
index e2b57fe410..3d5bc746fe 100644
--- a/docs/topics/forms/modelforms.txt
+++ b/docs/topics/forms/modelforms.txt
@@ -299,6 +299,8 @@ to the ``error_messages`` dictionary of the ``ModelForm``’s inner ``Meta`` cla
                 }
             }
 
+.. _topics-modelform-save:
+
 The ``save()`` method
 ---------------------
 
@@ -333,11 +335,11 @@ doesn't validate -- i.e., if ``form.errors`` evaluates to ``True``.
 If an optional field doesn't appear in the form's data, the resulting model
 instance uses the model field :attr:`~django.db.models.Field.default`, if
 there is one, for that field. This behavior doesn't apply to fields that use
-:class:`~django.forms.CheckboxInput` (or any custom widget with
-``dont_use_model_field_default_for_empty_data=True``) since an unchecked
-checkbox doesn't appear in the data of an HTML form submission. Use a custom
-form field or widget if you're designing an API and want the default fallback
-for a ``BooleanField``.
+:class:`~django.forms.CheckboxInput` (or any custom widget whose
+:meth:`~django.forms.Widget.value_omitted_from_data` method always returns
+``False``) since an unchecked checkbox doesn't appear in the data of an HTML
+form submission. Use a custom form field or widget if you're designing an API
+and want the default fallback for a :class:`~django.db.models.BooleanField`.
 
 .. versionchanged:: 1.10.1
 
@@ -345,6 +347,10 @@ for a ``BooleanField``.
     :class:`~django.forms.CheckboxInput` which means that unchecked checkboxes
     receive a value of ``True`` if that's the model field default.
 
+.. versionchanged:: 1.10.2
+
+    The :meth:`~django.forms.Widget.value_omitted_from_data` method was added.
+
 This ``save()`` method accepts an optional ``commit`` keyword argument, which
 accepts either ``True`` or ``False``. If you call ``save()`` with
 ``commit=False``, then it will return an object that hasn't yet been saved to
diff --git a/tests/forms_tests/widget_tests/test_checkboxinput.py b/tests/forms_tests/widget_tests/test_checkboxinput.py
index 77b03b9ea9..ecfc2105c8 100644
--- a/tests/forms_tests/widget_tests/test_checkboxinput.py
+++ b/tests/forms_tests/widget_tests/test_checkboxinput.py
@@ -85,3 +85,7 @@ class CheckboxInputTest(WidgetTest):
     def test_value_from_datadict_string_int(self):
         value = self.widget.value_from_datadict({'testing': '0'}, {}, 'testing')
         self.assertIs(value, True)
+
+    def test_value_omitted_from_data(self):
+        self.assertIs(self.widget.value_omitted_from_data({'field': 'value'}, {}, 'field'), False)
+        self.assertIs(self.widget.value_omitted_from_data({}, {}, 'field'), False)
diff --git a/tests/forms_tests/widget_tests/test_fileinput.py b/tests/forms_tests/widget_tests/test_fileinput.py
index eb1c9d81ed..6e96dc7b5f 100644
--- a/tests/forms_tests/widget_tests/test_fileinput.py
+++ b/tests/forms_tests/widget_tests/test_fileinput.py
@@ -14,3 +14,7 @@ class FileInputTest(WidgetTest):
         self.check_html(self.widget, 'email', 'test@example.com', html='<input type="file" name="email" />')
         self.check_html(self.widget, 'email', '', html='<input type="file" name="email" />')
         self.check_html(self.widget, 'email', None, html='<input type="file" name="email" />')
+
+    def test_value_omitted_from_data(self):
+        self.assertIs(self.widget.value_omitted_from_data({}, {}, 'field'), True)
+        self.assertIs(self.widget.value_omitted_from_data({}, {'field': 'value'}, 'field'), False)
diff --git a/tests/forms_tests/widget_tests/test_multiwidget.py b/tests/forms_tests/widget_tests/test_multiwidget.py
index bb6f8bfc4f..99aecea1ab 100644
--- a/tests/forms_tests/widget_tests/test_multiwidget.py
+++ b/tests/forms_tests/widget_tests/test_multiwidget.py
@@ -118,6 +118,13 @@ class MultiWidgetTest(WidgetTest):
             '<input id="bar_1" type="text" class="small" value="lennon" name="name_1" />'
         ))
 
+    def test_value_omitted_from_data(self):
+        widget = MyMultiWidget(widgets=(TextInput(), TextInput()))
+        self.assertIs(widget.value_omitted_from_data({}, {}, 'field'), True)
+        self.assertIs(widget.value_omitted_from_data({'field_0': 'x'}, {}, 'field'), False)
+        self.assertIs(widget.value_omitted_from_data({'field_1': 'y'}, {}, 'field'), False)
+        self.assertIs(widget.value_omitted_from_data({'field_0': 'x', 'field_1': 'y'}, {}, 'field'), False)
+
     def test_needs_multipart_true(self):
         """
         needs_multipart_form should be True if any widgets need it.
diff --git a/tests/forms_tests/widget_tests/test_selectdatewidget.py b/tests/forms_tests/widget_tests/test_selectdatewidget.py
index 6efa240b66..fcce916f76 100644
--- a/tests/forms_tests/widget_tests/test_selectdatewidget.py
+++ b/tests/forms_tests/widget_tests/test_selectdatewidget.py
@@ -477,3 +477,11 @@ class SelectDateWidgetTest(WidgetTest):
             w.value_from_datadict({'date_year': '1899', 'date_month': '8', 'date_day': '13'}, {}, 'date'),
             '13-08-1899',
         )
+
+    def test_value_omitted_from_data(self):
+        self.assertIs(self.widget.value_omitted_from_data({}, {}, 'field'), True)
+        self.assertIs(self.widget.value_omitted_from_data({'field_month': '12'}, {}, 'field'), False)
+        self.assertIs(self.widget.value_omitted_from_data({'field_year': '2000'}, {}, 'field'), False)
+        self.assertIs(self.widget.value_omitted_from_data({'field_day': '1'}, {}, 'field'), False)
+        data = {'field_day': '1', 'field_month': '12', 'field_year': '2000'}
+        self.assertIs(self.widget.value_omitted_from_data(data, {}, 'field'), False)
diff --git a/tests/forms_tests/widget_tests/test_widget.py b/tests/forms_tests/widget_tests/test_widget.py
new file mode 100644
index 0000000000..3911627a0c
--- /dev/null
+++ b/tests/forms_tests/widget_tests/test_widget.py
@@ -0,0 +1,12 @@
+from __future__ import unicode_literals
+
+from django.forms import Widget
+from django.test import SimpleTestCase
+
+
+class WidgetTests(SimpleTestCase):
+
+    def test_value_omitted_from_data(self):
+        widget = Widget()
+        self.assertIs(widget.value_omitted_from_data({}, {}, 'field'), True)
+        self.assertIs(widget.value_omitted_from_data({'field': 'value'}, {}, 'field'), False)
diff --git a/tests/model_forms/models.py b/tests/model_forms/models.py
index dad245ba78..9898022cca 100644
--- a/tests/model_forms/models.py
+++ b/tests/model_forms/models.py
@@ -120,9 +120,11 @@ class PublicationDefaults(models.Model):
     CATEGORY_CHOICES = ((1, 'Games'), (2, 'Comics'), (3, 'Novel'))
     title = models.CharField(max_length=30)
     date_published = models.DateField(default=datetime.date.today)
+    datetime_published = models.DateTimeField(default=datetime.datetime(2000, 1, 1))
     mode = models.CharField(max_length=2, choices=MODE_CHOICES, default=default_mode)
     category = models.IntegerField(choices=CATEGORY_CHOICES, default=default_category)
     active = models.BooleanField(default=True)
+    file = models.FileField(default='default.txt')
 
 
 class Author(models.Model):
diff --git a/tests/model_forms/tests.py b/tests/model_forms/tests.py
index 593e18fdaa..f2931262c2 100644
--- a/tests/model_forms/tests.py
+++ b/tests/model_forms/tests.py
@@ -601,6 +601,58 @@ class ModelFormBaseTest(TestCase):
         m1 = mf1.save(commit=False)
         self.assertEqual(m1.mode, mode)
 
+    def test_default_splitdatetime_field(self):
+        class PubForm(forms.ModelForm):
+            datetime_published = forms.SplitDateTimeField(required=False)
+
+            class Meta:
+                model = PublicationDefaults
+                fields = ('datetime_published',)
+
+        mf1 = PubForm({})
+        self.assertEqual(mf1.errors, {})
+        m1 = mf1.save(commit=False)
+        self.assertEqual(m1.datetime_published, datetime.datetime(2000, 1, 1))
+
+        mf2 = PubForm({'datetime_published_0': '2010-01-01', 'datetime_published_1': '0:00:00'})
+        self.assertEqual(mf2.errors, {})
+        m2 = mf2.save(commit=False)
+        self.assertEqual(m2.datetime_published, datetime.datetime(2010, 1, 1))
+
+    def test_default_filefield(self):
+        class PubForm(forms.ModelForm):
+            class Meta:
+                model = PublicationDefaults
+                fields = ('file',)
+
+        mf1 = PubForm({})
+        self.assertEqual(mf1.errors, {})
+        m1 = mf1.save(commit=False)
+        self.assertEqual(m1.file.name, 'default.txt')
+
+        mf2 = PubForm({}, {'file': SimpleUploadedFile('name', b'foo')})
+        self.assertEqual(mf2.errors, {})
+        m2 = mf2.save(commit=False)
+        self.assertEqual(m2.file.name, 'name')
+
+    def test_selectdatewidget(self):
+        class PubForm(forms.ModelForm):
+            date_published = forms.DateField(required=False, widget=forms.SelectDateWidget)
+
+            class Meta:
+                model = PublicationDefaults
+                fields = ('date_published',)
+
+        mf1 = PubForm({})
+        self.assertEqual(mf1.errors, {})
+        m1 = mf1.save(commit=False)
+        self.assertEqual(m1.date_published, datetime.date.today())
+
+        mf2 = PubForm({'date_published_year': '2010', 'date_published_month': '1', 'date_published_day': '1'})
+        self.assertEqual(mf2.errors, {})
+        m2 = mf2.save(commit=False)
+        self.assertEqual(m2.date_published, datetime.date(2010, 1, 1))
+
 
 class FieldOverridesByFormMetaForm(forms.ModelForm):
     class Meta:
diff --git a/tests/postgres_tests/models.py b/tests/postgres_tests/models.py
index d94eb90d4a..7d2dabcd01 100644
--- a/tests/postgres_tests/models.py
+++ b/tests/postgres_tests/models.py
@@ -40,7 +40,7 @@ class PostgreSQLModel(models.Model):
 
 
 class IntegerArrayModel(PostgreSQLModel):
-    field = ArrayField(models.IntegerField())
+    field = ArrayField(models.IntegerField(), default=[], blank=True)
 
 
 class NullableIntegerArrayModel(PostgreSQLModel):
diff --git a/tests/postgres_tests/test_array.py b/tests/postgres_tests/test_array.py
index 6a305556d4..5929c565d6 100644
--- a/tests/postgres_tests/test_array.py
+++ b/tests/postgres_tests/test_array.py
@@ -20,7 +20,9 @@ from .models import (
 
 try:
     from django.contrib.postgres.fields import ArrayField
-    from django.contrib.postgres.forms import SimpleArrayField, SplitArrayField
+    from django.contrib.postgres.forms import (
+        SimpleArrayField, SplitArrayField, SplitArrayWidget,
+    )
 except ImportError:
     pass
 
@@ -714,3 +716,26 @@ class TestSplitFormField(PostgreSQLTestCase):
             'Item 0 in the array did not validate: Ensure this value has at most 2 characters (it has 3).',
             'Item 2 in the array did not validate: Ensure this value has at most 2 characters (it has 4).',
         ])
+
+    def test_splitarraywidget_value_omitted_from_data(self):
+        class Form(forms.ModelForm):
+            field = SplitArrayField(forms.IntegerField(), required=False, size=2)
+
+            class Meta:
+                model = IntegerArrayModel
+                fields = ('field',)
+
+        form = Form({'field_0': '1', 'field_1': '2'})
+        self.assertEqual(form.errors, {})
+        obj = form.save(commit=False)
+        self.assertEqual(obj.field, [1, 2])
+
+
+class TestSplitFormWidget(PostgreSQLTestCase):
+
+    def test_value_omitted_from_data(self):
+        widget = SplitArrayWidget(forms.TextInput(), size=2)
+        self.assertIs(widget.value_omitted_from_data({}, {}, 'field'), True)
+        self.assertIs(widget.value_omitted_from_data({'field_0': 'value'}, {}, 'field'), False)
+        self.assertIs(widget.value_omitted_from_data({'field_1': 'value'}, {}, 'field'), False)
+        self.assertIs(widget.value_omitted_from_data({'field_0': 'value', 'field_1': 'value'}, {}, 'field'), False)