Make read-only visitor more visitor-like.

By moving the descend logic into generic_visit, read-only
visitors have the ability to do pre and post descend logic.
This is also matching what python's ast.Visitor does.

Author: Pike

fluent.syntax/fluent/syntax/ast.py

@@ -10,8 +10,8 @@ class Visitor(object):
     To generally define which nodes not to descend in to, overload
     `generic_visit`.
     To handle specific node types, add methods like `visit_Pattern`.
-    The boolean value of the returned value determines if the visitor
-    descends into the children of the given AST node.
+    If you want to still descend into the children of the node, call
+    `generic_visit` of the superclass.
     '''
     def visit(self, node):
         if isinstance(node, list):
@@ -22,14 +22,11 @@ def visit(self, node):
             return
         nodename = type(node).__name__
         visit = getattr(self, 'visit_{}'.format(nodename), self.generic_visit)
-        should_descend = visit(node)
-        if not should_descend:
-            return
-        for propname, propvalue in vars(node).items():
-            self.visit(propvalue)
+        visit(node)
 
     def generic_visit(self, node):
-        return True
+        for propname, propvalue in vars(node).items():
+            self.visit(propvalue)
 
 
 class Transformer(Visitor):

fluent.syntax/tests/syntax/test_visitor.py

@@ -18,11 +18,10 @@ def __init__(self):
 
     def generic_visit(self, node):
         self.calls[type(node).__name__] += 1
-        return super(MockVisitor, self).generic_visit(node)
+        super(MockVisitor, self).generic_visit(node)
 
     def visit_Pattern(self, node):
         self.pattern_calls += 1
-        return False
 
 
 class TestVisitor(unittest.TestCase):
@@ -87,11 +86,11 @@ def __init__(self):
         self.word_count = 0
 
     def generic_visit(self, node):
-        return not isinstance(node, (ast.Span, ast.Annotation))
+        if not isinstance(node, (ast.Span, ast.Annotation)):
+            super(VisitorCounter, self).generic_visit(node)
 
     def visit_TextElement(self, node):
         self.word_count += len(node.value.split())
-        return False
 
 
 class ReplaceText(object):