dict field: support allow_empty option

docs/api-guide/fields.md

@@ -471,9 +471,10 @@ We can now reuse our custom `StringListField` class throughout our application,
 
 A field class that validates a dictionary of objects. The keys in `DictField` are always assumed to be string values.
 
-**Signature**: `DictField(child=<A_FIELD_INSTANCE>)`
+**Signature**: `DictField(child=<A_FIELD_INSTANCE>, allow_empty=True)`
 
 - `child` - A field instance that should be used for validating the values in the dictionary. If this argument is not provided then values in the mapping will not be validated.
+- `allow_empty` - Designates if empty dictionaries are allowed.
 
 For example, to create a field that validates a mapping of strings to strings, you would write something like this:
 

rest_framework/fields.py

@@ -1663,11 +1663,13 @@ class DictField(Field):
     child = _UnvalidatedField()
     initial = {}
     default_error_messages = {
-        'not_a_dict': _('Expected a dictionary of items but got type "{input_type}".')
+        'not_a_dict': _('Expected a dictionary of items but got type "{input_type}".'),
+        'empty': _('This dictionary may not be empty.'),
     }
 
     def __init__(self, *args, **kwargs):
         self.child = kwargs.pop('child', copy.deepcopy(self.child))
+        self.allow_empty = kwargs.pop('allow_empty', True)
 
         assert not inspect.isclass(self.child), '`child` has not been instantiated.'
         assert self.child.source is None, (
@@ -1693,6 +1695,9 @@ class DictField(Field):
             data = html.parse_html_dict(data)
         if not isinstance(data, dict):
             self.fail('not_a_dict', input_type=type(data).__name__)
+        if not self.allow_empty and len(data) == 0:
+            self.fail('empty')
+
         return self.run_child_validation(data)
 
     def to_representation(self, value):

tests/test_fields.py

@@ -1982,6 +1982,7 @@ class TestDictField(FieldValues):
     """
     valid_inputs = [
         ({'a': 1, 'b': '2', 3: 3}, {'a': '1', 'b': '2', '3': '3'}),
+        ({}, {}),
     ]
     invalid_inputs = [
         ({'a': 1, 'b': None, 'c': None}, {'b': ['This field may not be null.'], 'c': ['This field may not be null.']}),
@@ -2009,6 +2010,16 @@ class TestDictField(FieldValues):
         output = field.run_validation(None)
         assert output is None
 
+    def test_allow_empty_disallowed(self):
+        """
+        If allow_empty is False then an empty dict is not a valid input.
+        """
+        field = serializers.DictField(allow_empty=False)
+        with pytest.raises(serializers.ValidationError) as exc_info:
+            field.run_validation({})
+
+        assert exc_info.value.detail == ['This dictionary may not be empty.']
+
 
 class TestNestedDictField(FieldValues):
     """