Add forbid-in-block-sequences feature for detecting empty values in block sequences

tests/rules/test_empty_values.py

@@ -258,3 +258,60 @@ def test_in_flow_mappings_comments(self):
                    problem1=(4, 7),
                    problem2=(7, 9),
                    problem3=(10, 5))
+
+    def test_in_list_items_disabled(self):
+        conf = ('empty-values: {forbid-in-block-mappings: false,\n'
+                '               forbid-in-flow-mappings: false,\n'
+                '               forbid-in-list-items: false}\n'
+                'braces: disable\n'
+                'commas: disable\n')
+        self.check('---\n'
+                   'foo:\n'
+                   '  - bar\n'
+                   '  -\n', conf)
+        self.check('---\n'
+                   'foo:\n'
+                   '  -\n', conf)
+
+    def test_in_list_items_primative_item(self):
+        conf = ('empty-values: {forbid-in-block-mappings: false,\n'
+                '               forbid-in-flow-mappings: false,\n'
+                '               forbid-in-list-items: true}\n'
+                'braces: disable\n'
+                'commas: disable\n')
+        self.check('---\n'
+                   'foo:\n'
+                   '  -\n', conf,
+                   problem=(3, 4))
+        self.check('---\n'
+                   'foo:\n'
+                   '  - bar\n'
+                   '  -\n', conf,
+                   problem=(4, 4))
+        self.check('---\n'
+                   'foo:\n'
+                   '  - 1\n'
+                   '  - 2\n'
+                   '  -\n', conf,
+                   problem=(5, 4))
+        self.check('---\n'
+                   'foo:\n'
+                   '  - null\n', conf)
+
+    def test_in_list_items_various_explicit_null(self):
+        conf = ('empty-values: {forbid-in-block-mappings: false,\n'
+                '               forbid-in-flow-mappings: false,\n'
+                '               forbid-in-list-items: true}\n'
+                'braces: disable\n'
+                'commas: disable\n')
+        self.check('---\n'
+                   'foo:\n'
+                   '  - null\n', conf)
+        self.check('---\n'
+                   '- null\n', conf)
+        self.check('---\n'
+                   'foo:\n'
+                   '  - bar: null\n', conf)
+        self.check('---\n'
+                   '- null\n'
+                   '- null\n', conf)

yamllint/rules/empty_values.py

@@ -21,6 +21,7 @@
 
 * Use ``forbid-in-block-mappings`` to prevent empty values in block mappings.
 * Use ``forbid-in-flow-mappings`` to prevent empty values in flow mappings.
+* Use ``forbid-in-list-items`` to prevent empty values in lists.
 
 .. rubric:: Default values (when enabled)
 
@@ -30,6 +31,7 @@
    empty-values:
      forbid-in-block-mappings: true
      forbid-in-flow-mappings: true
+     forbid-in-list-items: false
 
 .. rubric:: Examples
 
@@ -72,6 +74,26 @@
 
     {a: 1, b:, c: 3}
 
+#. With ``empty-values: {forbid-in-list-items: true}``
+
+   the following code snippet would **PASS**:
+   ::
+
+    some-list:
+      - string item
+
+   the following code snippets would **FAIL**:
+   ::
+
+    some-list:
+      -
+
+   ::
+
+    some-list:
+      - string item
+      -
+
 """
 
 import yaml
@@ -82,9 +104,11 @@
 ID = 'empty-values'
 TYPE = 'token'
 CONF = {'forbid-in-block-mappings': bool,
-        'forbid-in-flow-mappings': bool}
+        'forbid-in-flow-mappings': bool,
+        'forbid-in-list-items': bool}
 DEFAULT = {'forbid-in-block-mappings': True,
-           'forbid-in-flow-mappings': True}
+           'forbid-in-flow-mappings': True,
+           'forbid-in-list-items': False}
 
 
 def check(conf, token, prev, next, nextnext, context):
@@ -102,3 +126,10 @@ def check(conf, token, prev, next, nextnext, context):
             yield LintProblem(token.start_mark.line + 1,
                               token.end_mark.column + 1,
                               'empty value in flow mapping')
+
+    if conf['forbid-in-list-items']:
+        if isinstance(token, yaml.BlockEntryToken) and isinstance(next, (
+                yaml.KeyToken, yaml.BlockEndToken, yaml.BlockEntryToken)):
+            yield LintProblem(token.start_mark.line + 1,
+                              token.end_mark.column + 1,
+                              'empty value in list items')