Make allowing repeated anchors optional

Add option reuse_anchors as requested by @ingydotnet

lib/yaml/__init__.py

@@ -55,22 +55,22 @@ def load_warning(method):
     warnings.warn(message, YAMLLoadWarning, stacklevel=3)
 
 #------------------------------------------------------------------------------
-def scan(stream, Loader=Loader):
+def scan(stream, Loader=Loader, reuse_anchors=False):
     """
     Scan a YAML stream and produce scanning tokens.
     """
-    loader = Loader(stream)
+    loader = Loader(stream, reuse_anchors=reuse_anchors)
     try:
         while loader.check_token():
             yield loader.get_token()
     finally:
         loader.dispose()
 
-def parse(stream, Loader=Loader):
+def parse(stream, Loader=Loader, reuse_anchors=False):
     """
     Parse a YAML stream and produce parsing events.
     """
-    loader = Loader(stream)
+    loader = Loader(stream, reuse_anchors=reuse_anchors)
     try:
         while loader.check_event():
             yield loader.get_event()
@@ -82,25 +82,25 @@ def compose(stream, Loader=Loader):
     Parse the first YAML document in a stream
     and produce the corresponding representation tree.
     """
-    loader = Loader(stream)
+    loader = Loader(stream, reuse_anchors=reuse_anchors)
     try:
         return loader.get_single_node()
     finally:
         loader.dispose()
 
-def compose_all(stream, Loader=Loader):
+def compose_all(stream, Loader=Loader, reuse_anchors=False):
     """
     Parse all YAML documents in a stream
     and produce corresponding representation trees.
     """
-    loader = Loader(stream)
+    loader = Loader(stream, reuse_anchors=reuse_anchors)
     try:
         while loader.check_node():
             yield loader.get_node()
     finally:
         loader.dispose()
 
-def load(stream, Loader=None):
+def load(stream, Loader=None, reuse_anchors=False):
     """
     Parse the first YAML document in a stream
     and produce the corresponding Python object.
@@ -109,13 +109,13 @@ def load(stream, Loader=None):
         load_warning('load')
         Loader = FullLoader
 
-    loader = Loader(stream)
+    loader = Loader(stream, reuse_anchors=reuse_anchors)
     try:
         return loader.get_single_data()
     finally:
         loader.dispose()
 
-def load_all(stream, Loader=None):
+def load_all(stream, Loader=None, reuse_anchors=False):
     """
     Parse all YAML documents in a stream
     and produce corresponding Python objects.
@@ -124,7 +124,7 @@ def load_all(stream, Loader=None):
         load_warning('load_all')
         Loader = FullLoader
 
-    loader = Loader(stream)
+    loader = Loader(stream, reuse_anchors=reuse_anchors)
     try:
         while loader.check_data():
             yield loader.get_data()

lib/yaml/composer.py

@@ -10,8 +10,9 @@ class ComposerError(MarkedYAMLError):
 
 class Composer:
 
-    def __init__(self):
+    def __init__(self, reuse_anchors=False):
         self.anchors = {}
+        self.reuse_anchors=reuse_anchors
 
     def check_node(self):
         # Drop the STREAM-START event.
@@ -70,6 +71,11 @@ def compose_node(self, parent, index):
             return self.anchors[anchor]
         event = self.peek_event()
         anchor = event.anchor
+        if anchor is not None:
+            if anchor in self.anchors and not self.reuse_anchors:
+                raise ComposerError("found duplicate anchor %r; first occurrence"
+                        % anchor, self.anchors[anchor].start_mark,
+                        "second occurrence", event.start_mark)
         self.descend_resolver(parent, index)
         if self.check_event(ScalarEvent):
             node = self.compose_scalar_node(anchor)

lib/yaml/loader.py

@@ -10,41 +10,41 @@
 
 class BaseLoader(Reader, Scanner, Parser, Composer, BaseConstructor, BaseResolver):
 
-    def __init__(self, stream):
+    def __init__(self, stream, reuse_anchors=False):
         Reader.__init__(self, stream)
         Scanner.__init__(self)
         Parser.__init__(self)
-        Composer.__init__(self)
+        Composer.__init__(self, reuse_anchors=reuse_anchors)
         BaseConstructor.__init__(self)
         BaseResolver.__init__(self)
 
 class FullLoader(Reader, Scanner, Parser, Composer, FullConstructor, Resolver):
 
-    def __init__(self, stream):
+    def __init__(self, stream, reuse_anchors=False):
         Reader.__init__(self, stream)
         Scanner.__init__(self)
         Parser.__init__(self)
-        Composer.__init__(self)
+        Composer.__init__(self, reuse_anchors=reuse_anchors)
         FullConstructor.__init__(self)
         Resolver.__init__(self)
 
 class SafeLoader(Reader, Scanner, Parser, Composer, SafeConstructor, Resolver):
 
-    def __init__(self, stream):
+    def __init__(self, stream, reuse_anchors=False):
         Reader.__init__(self, stream)
         Scanner.__init__(self)
         Parser.__init__(self)
-        Composer.__init__(self)
+        Composer.__init__(self, reuse_anchors=reuse_anchors)
         SafeConstructor.__init__(self)
         Resolver.__init__(self)
 
 class Loader(Reader, Scanner, Parser, Composer, Constructor, Resolver):
 
-    def __init__(self, stream):
+    def __init__(self, stream, reuse_anchors=False):
         Reader.__init__(self, stream)
         Scanner.__init__(self)
         Parser.__init__(self)
-        Composer.__init__(self)
+        Composer.__init__(self, reuse_anchors=reuse_anchors)
         Constructor.__init__(self)
         Resolver.__init__(self)
 
@@ -54,10 +54,10 @@ def __init__(self, stream):
 # to ensure backwards compatibility.
 class UnsafeLoader(Reader, Scanner, Parser, Composer, Constructor, Resolver):
 
-    def __init__(self, stream):
+    def __init__(self, stream, reuse_anchors=False):
         Reader.__init__(self, stream)
         Scanner.__init__(self)
         Parser.__init__(self)
-        Composer.__init__(self)
+        Composer.__init__(self, reuse_anchors=reuse_anchors)
         Constructor.__init__(self)
         Resolver.__init__(self)

tests/lib/canonical.py

@@ -318,12 +318,12 @@ def peek_event(self):
 class CanonicalLoader(CanonicalScanner, CanonicalParser,
         yaml.composer.Composer, yaml.constructor.Constructor, yaml.resolver.Resolver):
 
-    def __init__(self, stream):
+    def __init__(self, stream, reuse_anchors=False):
         if hasattr(stream, 'read'):
             stream = stream.read()
         CanonicalScanner.__init__(self, stream)
         CanonicalParser.__init__(self)
-        yaml.composer.Composer.__init__(self)
+        yaml.composer.Composer.__init__(self, reuse_anchors=reuse_anchors)
         yaml.constructor.Constructor.__init__(self)
         yaml.resolver.Resolver.__init__(self)
 

tests/lib/test_constructor.py

@@ -257,7 +257,7 @@ def test_constructor_types(data_filename, code_filename, verbose=False):
     native1 = None
     native2 = None
     try:
-        native1 = list(yaml.load_all(open(data_filename, 'rb'), Loader=MyLoader))
+        native1 = list(yaml.load_all(open(data_filename, 'rb'), Loader=MyLoader, reuse_anchors=True))
         if len(native1) == 1:
             native1 = native1[0]
         native2 = _load_code(open(code_filename, 'rb').read())