Fix empty HTML values when a default is provided.

docs/api-guide/fields.md

@@ -112,6 +112,8 @@ Two options are currently used in HTML form generation, `'input_type'` and `'bas
 
 A boolean representation.
 
+When using HTML encoded form input be aware that omitting a value will always be treated as setting a field to `False`, even if it has a `default=True` option specified. This is because HTML checkbox inputs represent the unchecked state by omitting the value, so REST framework treats omission as if it is an empty checkbox input.
+
 Corresponds to `django.db.models.fields.BooleanField`.
 
 **Signature:** `BooleanField()`

rest_framework/fields.py

@@ -185,8 +185,13 @@ def __init__(self, read_only=False, write_only=False,
         self.allow_null = allow_null
 
         if allow_null and self.default_empty_html is empty:
+            # HTML input cannot represent `None` values, so we need to
+            # forcibly coerce empty HTML values to `None` if `allow_null=True`.
             self.default_empty_html = None
 
+        if default is not empty:
+            self.default_empty_html = default
+
         if validators is not None:
             self.validators = validators[:]
 

tests/test_fields.py

@@ -215,6 +215,26 @@ class MockHTMLDict(dict):
         assert serializer.validated_data == {'archived': False}
 
 
+class TestCharHTMLInput:
+    def setup(self):
+        class TestSerializer(serializers.Serializer):
+            message = serializers.CharField(default='happy')
+        self.Serializer = TestSerializer
+
+    def test_empty_html_checkbox(self):
+        """
+        HTML checkboxes do not send any value, but should be treated
+        as `False` by BooleanField.
+        """
+        # This class mocks up a dictionary like object, that behaves
+        # as if it was returned for multipart or urlencoded data.
+        class MockHTMLDict(dict):
+            getlist = None
+        serializer = self.Serializer(data=MockHTMLDict())
+        assert serializer.is_valid()
+        assert serializer.validated_data == {'message': 'happy'}
+
+
 class TestCreateOnlyDefault:
     def setup(self):
         default = serializers.CreateOnlyDefault('2001-01-01')