1 files changed, 143 insertions, 0 deletions
diff --git a/Lib/fontTools/misc/visitor.py b/Lib/fontTools/misc/visitor.py
new file mode 100644
index 00000000..3d28135f
--- /dev/null
+++ b/Lib/fontTools/misc/visitor.py
@@ -0,0 +1,143 @@
+"""Generic visitor pattern implementation for Python objects."""
+
+import enum
+
+
+class Visitor(object):
+
+    defaultStop = False
+
+    @classmethod
+    def _register(celf, clazzes_attrs):
+        assert celf != Visitor, "Subclass Visitor instead."
+        if "_visitors" not in celf.__dict__:
+            celf._visitors = {}
+
+        def wrapper(method):
+            assert method.__name__ == "visit"
+            for clazzes, attrs in clazzes_attrs:
+                if type(clazzes) != tuple:
+                    clazzes = (clazzes,)
+                if type(attrs) == str:
+                    attrs = (attrs,)
+                for clazz in clazzes:
+                    _visitors = celf._visitors.setdefault(clazz, {})
+                    for attr in attrs:
+                        assert attr not in _visitors, (
+                            "Oops, class '%s' has visitor function for '%s' defined already."
+                            % (clazz.__name__, attr)
+                        )
+                        _visitors[attr] = method
+            return None
+
+        return wrapper
+
+    @classmethod
+    def register(celf, clazzes):
+        if type(clazzes) != tuple:
+            clazzes = (clazzes,)
+        return celf._register([(clazzes, (None,))])
+
+    @classmethod
+    def register_attr(celf, clazzes, attrs):
+        clazzes_attrs = []
+        if type(clazzes) != tuple:
+            clazzes = (clazzes,)
+        if type(attrs) == str:
+            attrs = (attrs,)
+        for clazz in clazzes:
+            clazzes_attrs.append((clazz, attrs))
+        return celf._register(clazzes_attrs)
+
+    @classmethod
+    def register_attrs(celf, clazzes_attrs):
+        return celf._register(clazzes_attrs)
+
+    @classmethod
+    def _visitorsFor(celf, thing, _default={}):
+        typ = type(thing)
+
+        for celf in celf.mro():
+
+            _visitors = getattr(celf, "_visitors", None)
+            if _visitors is None:
+                break
+
+            m = celf._visitors.get(typ, None)
+            if m is not None:
+                return m
+
+        return _default
+
+    def visitObject(self, obj, *args, **kwargs):
+        """Called to visit an object. This function loops over all non-private
+        attributes of the objects and calls any user-registered (via
+        @register_attr() or @register_attrs()) visit() functions.
+
+        If there is no user-registered visit function, of if there is and it
+        returns True, or it returns None (or doesn't return anything) and
+        visitor.defaultStop is False (default), then the visitor will proceed
+        to call self.visitAttr()"""
+
+        keys = sorted(vars(obj).keys())
+        _visitors = self._visitorsFor(obj)
+        defaultVisitor = _visitors.get("*", None)
+        for key in keys:
+            if key[0] == "_":
+                continue
+            value = getattr(obj, key)
+            visitorFunc = _visitors.get(key, defaultVisitor)
+            if visitorFunc is not None:
+                ret = visitorFunc(self, obj, key, value, *args, **kwargs)
+                if ret == False or (ret is None and self.defaultStop):
+                    continue
+            self.visitAttr(obj, key, value, *args, **kwargs)
+
+    def visitAttr(self, obj, attr, value, *args, **kwargs):
+        """Called to visit an attribute of an object."""
+        self.visit(value, *args, **kwargs)
+
+    def visitList(self, obj, *args, **kwargs):
+        """Called to visit any value that is a list."""
+        for value in obj:
+            self.visit(value, *args, **kwargs)
+
+    def visitDict(self, obj, *args, **kwargs):
+        """Called to visit any value that is a dictionary."""
+        for value in obj.values():
+            self.visit(value, *args, **kwargs)
+
+    def visitLeaf(self, obj, *args, **kwargs):
+        """Called to visit any value that is not an object, list,
+        or dictionary."""
+        pass
+
+    def visit(self, obj, *args, **kwargs):
+        """This is the main entry to the visitor. The visitor will visit object
+        obj.
+
+        The visitor will first determine if there is a registered (via
+        @register()) visit function for the type of object. If there is, it
+        will be called, and (visitor, obj, *args, **kwargs) will be passed to
+        the user visit function.
+
+        If there is no user-registered visit function, of if there is and it
+        returns True, or it returns None (or doesn't return anything) and
+        visitor.defaultStop is False (default), then the visitor will proceed
+        to dispatch to one of self.visitObject(), self.visitList(),
+        self.visitDict(), or self.visitLeaf() (any of which can be overriden in
+        a subclass)."""
+
+        visitorFunc = self._visitorsFor(obj).get(None, None)
+        if visitorFunc is not None:
+            ret = visitorFunc(self, obj, *args, **kwargs)
+            if ret == False or (ret is None and self.defaultStop):
+                return
+        if hasattr(obj, "__dict__") and not isinstance(obj, enum.Enum):
+            self.visitObject(obj, *args, **kwargs)
+        elif isinstance(obj, list):
+            self.visitList(obj, *args, **kwargs)
+        elif isinstance(obj, dict):
+            self.visitDict(obj, *args, **kwargs)
+        else:
+            self.visitLeaf(obj, *args, **kwargs)