Model fields with .blank or .null now map to required=False. Closes #2017. Closes #2021.

docs/topics/3.0-announcement.md

@@ -506,9 +506,9 @@ We now use the following:
 
 REST framework now has more explicit and clear control over validating empty values for fields.
 
-Previously the meaning of the `required=False` keyword argument was underspecified. In practice its use meant that a field could either be not included in the input, or it could be included, but be `None`.
+Previously the meaning of the `required=False` keyword argument was underspecified. In practice its use meant that a field could either be not included in the input, or it could be included, but be `None` or the empty string.
 
-We now have a better separation, with separate `required` and `allow_none` arguments.
+We now have a better separation, with separate `required`, `allow_none` and `allow_blank` arguments.
 
 The following set of arguments are used to control validation of empty values:
 
@@ -519,7 +519,7 @@ The following set of arguments are used to control validation of empty values:
 
 Typically you'll want to use `required=False` if the corresponding model field has a default value, and additionally set either `allow_none=True` or `allow_blank=True` if required.
 
-The `default` argument is there if you need it, but you'll more typically want defaults to be set on model fields, rather than serializer fields.
+The `default` argument is also available and always implies that the field is not required to be in the input. It is unnecessary to use the `required` argument when a default is specified, and doing so will result in an error.
 
 #### Coercing output types.
 

rest_framework/utils/field_mapping.py

@@ -88,7 +88,7 @@ def get_field_kwargs(field_name, model_field):
         kwargs['read_only'] = True
         return kwargs
 
-    if model_field.has_default():
+    if model_field.has_default() or model_field.blank or model_field.null:
         kwargs['required'] = False
 
     if model_field.flatchoices:

tests/test_model_serializer.py

@@ -90,7 +90,7 @@ class TestRegularFieldMappings(TestCase):
                 email_field = EmailField(max_length=100)
                 float_field = FloatField()
                 integer_field = IntegerField()
-                null_boolean_field = NullBooleanField()
+                null_boolean_field = NullBooleanField(required=False)
                 positive_integer_field = IntegerField()
                 positive_small_integer_field = IntegerField()
                 slug_field = SlugField(max_length=100)
@@ -112,8 +112,8 @@ class TestRegularFieldMappings(TestCase):
                 id = IntegerField(label='ID', read_only=True)
                 value_limit_field = IntegerField(max_value=10, min_value=1)
                 length_limit_field = CharField(max_length=12, min_length=3)
-                blank_field = CharField(allow_blank=True, max_length=10)
+                blank_field = CharField(allow_blank=True, max_length=10, required=False)
-                null_field = IntegerField(allow_null=True)
+                null_field = IntegerField(allow_null=True, required=False)
                 default_field = IntegerField(required=False)
                 descriptive_field = IntegerField(help_text='Some help text', label='A label')
                 choices_field = ChoiceField(choices=[('red', 'Red'), ('blue', 'Blue'), ('green', 'Green')])