Allow required lists to be empty

cleancat/base.py

@@ -368,19 +368,19 @@ class List(Field):
     base_type = list
     blank_value = []
 
-    def __init__(self, field_instance, max_length=None, **kwargs):
+    def __init__(
+        self, field_instance, max_length=None, allow_empty=False, **kwargs
+    ):
         self.max_length = max_length
+        self.allow_empty = allow_empty
         super(List, self).__init__(**kwargs)
         self.field_instance = field_instance
 
-    def has_value(self, value):
-        return bool(value)
-
     def clean(self, value):
         value = super(List, self).clean(value)
 
         item_cnt = len(value)
-        if self.required and not item_cnt:
+        if not self.allow_empty and not item_cnt:
             raise ValidationError('List must not be empty.')
 
         if self.max_length and item_cnt > self.max_length:

tests/test_base.py

@@ -470,18 +470,24 @@ def test_it_enforces_max_length(self, value, valid):
             with pytest.raises(ValidationError, match=expected_err_msg):
                 field.clean(value)
 
-    @pytest.mark.parametrize('value', [None, []])
-    def test_it_enforces_required_flag(self, value):
+    def test_it_enforces_required_flag(self):
         expected_err_msg = 'This field is required.'
         with pytest.raises(ValidationError, match=expected_err_msg):
-            List(String()).clean(value)
+            List(String()).clean(None)
 
-    @pytest.mark.parametrize('value', [None, []])
-    def test_it_can_be_optional(self, value):
+    def test_it_enforces_allow_empty_flag(self):
+        expected_err_msg = 'List must not be empty.'
+        with pytest.raises(ValidationError, match=expected_err_msg):
+            List(String()).clean([])
+
+    def test_it_can_be_optional(self):
         with pytest.raises(StopValidation) as e:
-            List(String(), required=False).clean(value)
+            List(String(), required=False).clean(None)
         assert e.value.args[0] == []
 
+    def test_it_can_be_empty(self):
+        assert List(String(), allow_empty=True).clean([]) == []
+
 
 class TestSortedSetField:
     def test_it_dedupes_valid_values(self):