# HG changeset patch
# User Paul Boddie <paul@boddie.org.uk>
# Date 1484953393 -3600
# Node ID 26dcaef4d7a9f33bfa310b32b5bdf5569bca4b14
# Parent  6ab1b9e4be06e6a137a97c05d0653aee6fedbf21
Added a demonstration of getattr involving a visitor-like object dispatching to
methods by using name attributes present on node objects from an expression node
tree to identify those methods.

diff -r 6ab1b9e4be06 -r 26dcaef4d7a9 tests/getattr_visitor.py
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/tests/getattr_visitor.py	Sat Jan 21 00:03:13 2017 +0100
@@ -0,0 +1,104 @@
+class Expr:
+
+    "An expression."
+
+    name = "Expr"
+
+    def __init__(self, ops):
+        self.ops = ops
+
+    def children(self):
+        return self.ops
+
+class Binary:
+
+    "A binary operator."
+
+    name = "Binary"
+
+    def __init__(self, left, op, right):
+        self.left = left
+        self.op = op
+        self.right = right
+
+    def children(self):
+        return self.left, self.right
+
+class Unary:
+
+    "A unary operator."
+
+    name = "Unary"
+
+    def __init__(self, op, operand):
+        self.op = op
+        self.operand = operand
+
+    def children(self):
+        return self.operand,
+
+class Value:
+
+    "A general value."
+
+    name = "Value"
+
+    def __init__(self, value):
+        self.value = value
+
+    def children(self):
+        return ()
+
+class Visitor:
+
+    "Visit nodes in an expression tree."
+
+    def __init__(self):
+        self.indent = 0
+
+    def visit(self, node):
+
+        # Obtain the method for the node name.
+
+        fn = getattr(self, node.name)
+
+        # Call the method.
+
+        fn(node)
+
+        # Visit the node's children.
+
+        self.visitChildren(node)
+
+    def visitChildren(self, node):
+        self.indent += 1
+        for n in node.children():
+            self.visit(n)
+        self.indent -= 1
+
+    def writeIndent(self):
+        i = 0
+        while i < self.indent:
+            print "",
+            i += 1
+
+    def Expr(self, node):
+        self.writeIndent()
+        print "Expression..."
+
+    def Binary(self, node):
+        self.writeIndent()
+        print "Binary operation", node.op
+
+    def Unary(self, node):
+        self.writeIndent()
+        print "Unary operation", node.op
+
+    def Value(self, node):
+        self.writeIndent()
+        print "Value", node.value
+
+# Test the visitor with an example expression.
+
+expr = Expr([Binary(Value(1), "+", Binary(Unary("-", Value(2)), "*", Value(3)))])
+Visitor().visit(expr)