PyYAML patch to discard values

pyyaml.patch

diff --git a/ext/_yaml.pyx b/ext/_yaml.pyx
index ff4efe8..876a6c6 100644
--- a/ext/_yaml.pyx
+++ b/ext/_yaml.pyx
@@ -258,8 +258,11 @@ cdef class CParser:
     cdef int stream_cache_len
     cdef int stream_cache_pos
     cdef int unicode_source
+    cdef object filter
+    cdef object path
+    cdef int discarding
 
-    def __init__(self, stream):
+    def __init__(self, stream, filter):
         cdef is_readable
         if yaml_parser_initialize(&self.parser) == 0:
             raise MemoryError
@@ -306,6 +309,9 @@ cdef class CParser:
         self.current_token = None
         self.current_event = None
         self.anchors = {}
+        self.filter = filter
+        self.path = []
+        self.discarding = 0
 
     def __dealloc__(self):
         yaml_parser_delete(&self.parser)
@@ -848,7 +854,10 @@ cdef class CParser:
         index = 0
         self._parse_next_event()
         while self.parsed_event.type != YAML_SEQUENCE_END_EVENT:
-            value.append(self._compose_node(node, index))
+            if self.discarding:
+                self._compose_node(node, index)
+            else:
+                value.append(self._compose_node(node, index))
             index = index+1
             self._parse_next_event()
         node.end_mark = Mark(self.stream_name,
@@ -887,8 +896,18 @@ cdef class CParser:
         self._parse_next_event()
         while self.parsed_event.type != YAML_MAPPING_END_EVENT:
             item_key = self._compose_node(node, None)
-            item_value = self._compose_node(node, item_key)
-            value.append((item_key, item_value))
+            if self.discarding:
+                self._compose_node(node, item_key)
+            else:
+                self.path.append(item_key.value)
+                if self.filter(self.path):
+                    item_value = self._compose_node(node, item_key)
+                    value.append((item_key, item_value))
+                else:
+                    self.discarding = 1
+                    self._compose_node(node, item_key)
+                    self.discarding = 0
+                self.path.pop(-1)
             self._parse_next_event()
         node.end_mark = Mark(self.stream_name,
                 self.parsed_event.end_mark.index,
diff --git a/lib3/yaml/cyaml.py b/lib3/yaml/cyaml.py
index 1e606c7..97328d4 100644
--- a/lib3/yaml/cyaml.py
+++ b/lib3/yaml/cyaml.py
@@ -22,8 +22,8 @@ class CBaseLoader(CParser, BaseConstructor, BaseResolver):
 
 class CSafeLoader(CParser, SafeConstructor, Resolver):
 
-    def __init__(self, stream):
-        CParser.__init__(self, stream)
+    def __init__(self, stream, filter=lambda _: True):
+        CParser.__init__(self, stream, filter)
         SafeConstructor.__init__(self)
         Resolver.__init__(self)