Fixed #15511 -- Allow optional fields on ``MultiValueField` subclasses.

The `MultiValueField` class gets a new ``require_all_fields`` argument that
defaults to ``True``. If set to ``False``, individual fields can be made
optional, and a new ``incomplete`` validation error will be raised if any
required fields have empty values.

The ``incomplete`` error message can be defined on a `MultiValueField`
subclass or on each individual field. Skip duplicate errors.

1 files changed, 24 insertions, 8 deletions

diff --git a/django/forms/fields.py b/django/forms/fields.py
index b07ebe84ee..e995187682 100644
--- a/django/forms/fields.py
+++ b/django/forms/fields.py
@@ -955,15 +955,20 @@ class MultiValueField(Field):
     """
     default_error_messages = {
         'invalid': _('Enter a list of values.'),
+        'incomplete': _('Enter a complete value.'),
     }
 
     def __init__(self, fields=(), *args, **kwargs):
+        self.require_all_fields = kwargs.pop('require_all_fields', True)
         super(MultiValueField, self).__init__(*args, **kwargs)
-        # Set 'required' to False on the individual fields, because the
-        # required validation will be handled by MultiValueField, not by those
-        # individual fields.
         for f in fields:
-            f.required = False
+            f.error_messages.setdefault('incomplete',
+                                        self.error_messages['incomplete'])
+            if self.require_all_fields:
+                # Set 'required' to False on the individual fields, because the
+                # required validation will be handled by MultiValueField, not
+                # by those individual fields.
+                f.required = False
         self.fields = fields
 
     def validate(self, value):
@@ -993,15 +998,26 @@ class MultiValueField(Field):
                 field_value = value[i]
             except IndexError:
                 field_value = None
-            if self.required and field_value in self.empty_values:
-                raise ValidationError(self.error_messages['required'], code='required')
+            if field_value in self.empty_values:
+                if self.require_all_fields:
+                    # Raise a 'required' error if the MultiValueField is
+                    # required and any field is empty.
+                    if self.required:
+                        raise ValidationError(self.error_messages['required'], code='required')
+                elif field.required:
+                    # Otherwise, add an 'incomplete' error to the list of
+                    # collected errors and skip field cleaning, if a required
+                    # field is empty.
+                    if field.error_messages['incomplete'] not in errors:
+                        errors.append(field.error_messages['incomplete'])
+                    continue
             try:
                 clean_data.append(field.clean(field_value))
             except ValidationError as e:
                 # Collect all validation errors in a single list, which we'll
                 # raise at the end of clean(), rather than raising a single
-                # exception for the first error we encounter.
-                errors.extend(e.error_list)
+                # exception for the first error we encounter. Skip duplicates.
+                errors.extend(m for m in e.error_list if m not in errors)
         if errors:
             raise ValidationError(errors)