diff --git a/py/code/_assertion.py b/py/code/_assertion.py
index 4ed6118bc..2682dc659 100644
--- a/py/code/_assertion.py
+++ b/py/code/_assertion.py
@@ -97,6 +97,42 @@ def enumsubclasses(cls):
     yield cls
 
 
+
+def _format_explanation(explanation):
+    # uck!  See CallFunc for where \n{ and \n} escape sequences are used
+    raw_lines = (explanation or '').split('\n')
+    # escape newlines not followed by { and }
+    lines = [raw_lines[0]]
+    for l in raw_lines[1:]:
+        if l.startswith('{') or l.startswith('}'):
+            lines.append(l)
+        else:
+            lines[-1] += '\\n' + l
+
+    result = lines[:1]
+    stack = [0]
+    stackcnt = [0]
+    for line in lines[1:]:
+        if line.startswith('{'):
+            if stackcnt[-1]:
+                s = 'and   '
+            else:
+                s = 'where '
+            stack.append(len(result))
+            stackcnt[-1] += 1
+            stackcnt.append(0)
+            result.append(' +' + '  '*(len(stack)-1) + s + line[1:])
+        else:
+            assert line.startswith('}')
+            stack.pop()
+            stackcnt.pop()
+            result[stack[-1]] += line[1:]
+    assert len(stack) == 1
+    return '\n'.join(result)
+
+
+
+
 class Interpretable(View):
     """A parse tree node with a few extra methods."""
     explanation = None
@@ -132,36 +168,8 @@ class Interpretable(View):
             raise Failure(self)
 
     def nice_explanation(self):
-        # uck!  See CallFunc for where \n{ and \n} escape sequences are used
-        raw_lines = (self.explanation or '').split('\n')
-        # escape newlines not followed by { and }
-        lines = [raw_lines[0]]
-        for l in raw_lines[1:]:
-            if l.startswith('{') or l.startswith('}'):
-                lines.append(l)
-            else:
-                lines[-1] += '\\n' + l
-                
-        result = lines[:1]
-        stack = [0]
-        stackcnt = [0]
-        for line in lines[1:]:
-            if line.startswith('{'):
-                if stackcnt[-1]:
-                    s = 'and   '
-                else:
-                    s = 'where '
-                stack.append(len(result))
-                stackcnt[-1] += 1
-                stackcnt.append(0)
-                result.append(' +' + '  '*(len(stack)-1) + s + line[1:])
-            else:
-                assert line.startswith('}')
-                stack.pop()
-                stackcnt.pop()
-                result[stack[-1]] += line[1:]
-        assert len(stack) == 1
-        return '\n'.join(result)
+        return _format_explanation(self.explanation)
+
 
 class Name(Interpretable):
     __view__ = ast.Name
@@ -571,16 +579,20 @@ class AssertionError(BuiltinAssertionError):
                     args[0].__class__, id(args[0]))
             
         else: 
-            f = sys._getframe(1)
+            f = py.code.Frame(sys._getframe(1))
             try:
-                source = py.code.Frame(f).statement
+                source = f.statement
                 source = str(source.deindent()).strip()
             except py.error.ENOENT:
                 source = None
                 # this can also occur during reinterpretation, when the
                 # co_filename is set to "<run>".
             if source:
-                self.msg = interpret(source, f, should_fail=True)
+                if sys.version_info >= (2, 6):
+                    from py.__.code._assertionnew import interpret as do_interp
+                else:
+                    do_interp = interpret
+                self.msg = do_interp(source, f, should_fail=True)
                 if not self.args:
                     self.args = (self.msg,)
             else:
diff --git a/py/code/_assertionnew.py b/py/code/_assertionnew.py
new file mode 100644
index 000000000..93af3a57c
--- /dev/null
+++ b/py/code/_assertionnew.py
@@ -0,0 +1,304 @@
+"""
+Like _assertion.py but using builtin AST.  It should replace _assertion.py
+eventually.
+"""
+
+import sys
+import ast
+
+import py
+from py.__.code._assertion import _format_explanation, BuiltinAssertionError
+
+
+class Failure(Exception):
+    """Error found while interpreting AST."""
+
+    def __init__(self, explanation=""):
+        self.cause = sys.exc_info()
+        self.explanation = explanation
+
+
+def interpret(source, frame, should_fail=False):
+    mod = ast.parse(source)
+    visitor = DebugInterpreter(frame)
+    try:
+        visitor.visit(mod)
+    except Failure as failure:
+        return getfailure(failure)
+    if should_fail:
+        return ("(assertion failed, but when it was re-run for "
+                "printing intermediate values, it did not fail.  Suggestions: "
+                "compute assert expression before the assert or use --nomagic)")
+
+def run(offending_line, frame=None):
+    if frame is None:
+        frame = py.code.Frame(sys._getframe(1))
+    return interpret(offending_line, frame)
+
+def getfailure(failure):
+    explanation = _format_explanation(failure.explanation)
+    value = failure.cause[1]
+    if str(value):
+        lines = explanation.splitlines()
+        lines[0] += " << {0}".format(value)
+        explanation = "\n".join(lines)
+    text = "{0}: {1}".format(failure.cause[0].__name__, explanation)
+    if text.startswith("AssertionError: assert "):
+        text = text[16:]
+    return text
+
+
+operator_map = {
+    ast.BitOr : "|",
+    ast.BitXor : "^",
+    ast.BitAnd : "&",
+    ast.LShift : "<<",
+    ast.RShift : ">>",
+    ast.Add : "+",
+    ast.Sub : "-",
+    ast.Mult : "*",
+    ast.Div : "/",
+    ast.FloorDiv : "//",
+    ast.Mod : "%",
+    ast.Eq : "==",
+    ast.NotEq : "!=",
+    ast.Lt : "<",
+    ast.LtE : "<=",
+    ast.Gt : ">",
+    ast.GtE : ">=",
+}
+
+unary_map = {
+    ast.Not : "not {0}",
+    ast.Invert : "~{0}",
+    ast.USub : "-{0}",
+    ast.UAdd : "+{0}"
+}
+
+
+class DebugInterpreter(ast.NodeVisitor):
+    """Interpret AST nodes to gleam useful debugging information."""
+
+    def __init__(self, frame):
+        self.frame = frame
+
+    def generic_visit(self, node):
+        # Fallback when we don't have a special implementation.
+        if isinstance(node, ast.expr):
+            mod = ast.Expression(node)
+            co = self._compile(mod)
+            try:
+                result = self.frame.eval(co)
+            except Exception:
+                raise Failure()
+            explanation = self.frame.repr(result)
+            return explanation, result
+        elif isinstance(node, ast.stmt):
+            mod = ast.Module([node])
+            co = self._compile(mod, "exec")
+            try:
+                frame.exec_(co)
+            except Exception:
+                raise Failure()
+            return None, None
+        else:
+            raise AssertionError("can't handle {0}".format(node))
+
+    def _compile(self, source, mode="eval"):
+        return compile(source, "<assertion interpretation>", mode)
+
+    def visit_Module(self, mod):
+        for stmt in mod.body:
+            self.visit(stmt)
+
+    def visit_Name(self, name):
+        explanation, result = self.generic_visit(name)
+        # See if the name is local.
+        source = "{0!r} in locals() is not globals()".format(name.id)
+        co = self._compile(source)
+        try:
+            local = self.frame.eval(co)
+        except Exception, e:
+            # have to assume it isn't
+            local = False
+        if not local:
+            return name.id, result
+        return explanation, result
+
+    def visit_Compare(self, comp):
+        left = comp.left
+        left_explanation, left_result = self.visit(left)
+        got_result = False
+        for op, next_op in zip(comp.ops, comp.comparators):
+            if got_result and not result:
+                break
+            next_explanation, next_result = self.visit(next_op)
+            op_symbol = operator_map[op.__class__]
+            explanation = "{0} {1} {2}".format(left_explanation, op_symbol,
+                                               next_explanation)
+            source = "__exprinfo_left {0} __exprinfo_right".format(op_symbol)
+            co = self._compile(source)
+            try:
+                result = self.frame.eval(co, __exprinfo_left=left_result,
+                                         __exprinfo_right=next_result)
+            except Exception:
+                raise Failure(explanation)
+            else:
+                got_result = True
+            left_explanation, left_result = next_explanation, next_result
+        return explanation, result
+
+    def visit_BoolOp(self, boolop):
+        is_or = isinstance(boolop.op, ast.Or)
+        explanations = []
+        for operand in boolop.values:
+            explanation, result = self.visit(operand)
+            explanations.append(explanation)
+            if result == is_or:
+                break
+        name = " or " if is_or else " and "
+        explanation = "(" + name.join(explanations) + ")"
+        return explanation, result
+
+    def visit_UnaryOp(self, unary):
+        pattern = unary_map[unary.op.__class__]
+        operand_explanation, operand_result = self.visit(unary.operand)
+        explanation = pattern.format(operand_explanation)
+        co = self._compile(pattern.format("__exprinfo_expr"))
+        try:
+            result = self.frame.eval(co, __exprinfo_expr=operand_result)
+        except Exception:
+            raise Failure(explanation)
+        return explanation, result
+
+    def visit_BinOp(self, binop):
+        left_explanation, left_result = self.visit(binop.left)
+        right_explanation, right_result = self.visit(binop.right)
+        symbol = operator_map[binop.op.__class__]
+        explanation = "{0} {1} {2}".format(left_explanation, symbol,
+                                           right_explanation)
+        source = "__exprinfo_left {0} __exprinfo_right".format(symbol)
+        co = self._compile(source)
+        try:
+            result = self.frame.eval(co, __exprinfo_left=left_result,
+                                     __exprinfo_right=right_result)
+        except Exception:
+            raise Failure(explanation)
+        return explanation, result
+
+    def visit_Call(self, call):
+        func_explanation, func = self.visit(call.func)
+        arg_explanations = []
+        ns = {"__exprinfo_func" : func}
+        arguments = []
+        for arg in call.args:
+            arg_explanation, arg_result = self.visit(arg)
+            arg_name = "__exprinfo_{0}".format(len(ns))
+            ns[arg_name] = arg_result
+            arguments.append(arg_name)
+            arg_explanations.append(arg_explanation)
+        for keyword in call.keywords:
+            arg_explanation, arg_result = self.visit(keyword.value)
+            arg_name = "__exprinfo_{0}".format(len(ns))
+            ns[arg_name] = arg_result
+            keyword_source = "{0}={{0}}".format(keyword.id)
+            arguments.append(keyword_source.format(arg_name))
+            arg_explanations.append(keyword_source.format(arg_explanation))
+        if call.starargs:
+            arg_explanation, arg_result = self.visit(call.starargs)
+            arg_name = "__exprinfo_star"
+            ns[arg_name] = arg_result
+            arguments.append("*{0}".format(arg_name))
+            arg_explanations.append("*{0}".format(arg_explanation))
+        if call.kwargs:
+            arg_explanation, arg_result = self.visit(call.kwargs)
+            arg_name = "__exprinfo_kwds"
+            ns[arg_name] = arg_result
+            arguments.append("**{0}".format(arg_name))
+            arg_explanations.append("**{0}".format(arg_explanation))
+        args_explained = ", ".join(arg_explanations)
+        explanation = "{0}({1})".format(func_explanation, args_explained)
+        args = ", ".join(arguments)
+        source = "__exprinfo_func({0})".format(args)
+        co = self._compile(source)
+        try:
+            result = self.frame.eval(co, **ns)
+        except Exception:
+            raise Failure(explanation)
+        # Only show result explanation if it's not a builtin call or returns a
+        # bool.
+        if not isinstance(call.func, ast.Name) or \
+                not self._is_builtin_name(call.func):
+            source = "isinstance(__exprinfo_value, bool)"
+            co = self._compile(source)
+            try:
+                is_bool = self.frame.eval(co, __exprinfo_value=result)
+            except Exception:
+                is_bool = False
+            if not is_bool:
+                pattern = "{0}\n{{{0} = {1}\n}}"
+                rep = self.frame.repr(result)
+                explanation = pattern.format(rep, explanation)
+        return explanation, result
+
+    def _is_builtin_name(self, name):
+        pattern = "{0!r} not in globals() and {0!r} not in locals()"
+        source = pattern.format(name.id)
+        co = self._compile(source)
+        try:
+            return self.frame.eval(co)
+        except Exception:
+            return False
+
+    def visit_Attribute(self, attr):
+        if not isinstance(attr.ctx, ast.Load):
+            return self.generic_visit(attr)
+        source_explanation, source_result = self.visit(attr.value)
+        explanation = "{0}.{1}".format(source_explanation, attr.attr)
+        source = "__exprinfo_expr.{0}".format(attr.attr)
+        co = self._compile(source)
+        try:
+            result = self.frame.eval(co, __exprinfo_expr=source_result)
+        except Exception:
+            raise Failure(explanation)
+        # Check if the attr is from an instance.
+        source = "{0!r} in getattr(__exprinfo_expr, '__dict__', {{}})"
+        source = source.format(attr.attr)
+        co = self._compile(source)
+        try:
+            from_instance = self.frame.eval(co, __exprinfo_expr=source_result)
+        except Exception:
+            from_instance = True
+        if from_instance:
+            rep = self.frame.repr(result)
+            pattern = "{0}\n{{{0} = {1}\n}}"
+            explanation = pattern.format(rep, explanation)
+        return explanation, result
+
+    def visit_Assert(self, assrt):
+        test_explanation, test_result = self.visit(assrt.test)
+        if test_explanation.startswith("False\n{False =") and \
+                test_explanation.endswith("\n"):
+            test_explanation = test_explanation[15:-2]
+        explanation = "assert {0}".format(test_explanation)
+        if not test_result:
+            try:
+                raise BuiltinAssertionError
+            except Exception:
+                raise Failure(explanation)
+        return explanation, test_result
+
+    def visit_Assign(self, assign):
+        value_explanation, value_result = self.visit(assign.value)
+        explanation = "... = {0}".format(value_explanation)
+        name = ast.Name("__exprinfo_expr", ast.Load(), assign.value.lineno,
+                        assign.value.col_offset)
+        new_assign = ast.Assign(assign.targets, name, assign.lineno,
+                                assign.col_offset)
+        mod = ast.Module([new_assign])
+        co = self._compile(mod, "exec")
+        try:
+            self.frame.exec_(co, __exprinfo_expr=value_result)
+        except Exception:
+            raise Failure(explanation)
+        return explanation, value_result