text: #----------------------------------------------------------------------
  # I, Babar K. Zafar, the author or of this code dedicate any and all
  # copyright interest in this code to the public domain. I make this
  # dedication for the benefit of the public at large and to the
  # detriment of our heirs and successors. I intend this dedication to
  # be an overt act of relinquishment in perpetuity of all present and
  # future rights this code under copyright law.
  #
  # Version 0.1 / May 27 2006
  #----------------------------------------------------------------------
  
  import inspect, compiler.ast
  import thread, time
  
  #----------------------------------------------------------------------
  # Module globals.
  #----------------------------------------------------------------------
  
  # Toggle module level debugging mode.
  DEBUG = False
  
  # List of all AST node classes in compiler/ast.py.
  all_ast_nodes = \
      [name for (name, obj) in inspect.getmembers(compiler.ast)
       if inspect.isclass(obj) and issubclass(obj, compiler.ast.Node)]
  
  # List of all builtin functions and types (ignoring exception classes).
  all_builtins = \
      [name for (name, obj) in inspect.getmembers(__builtins__)
       if inspect.isbuiltin(obj) or (inspect.isclass(obj) and \
                                     not issubclass(obj, Exception))]
  
  #----------------------------------------------------------------------
  # Utilties.
  #----------------------------------------------------------------------
  
  def classname(obj):
      return obj.__class__.__name__
  
  def is_valid_ast_node(name):
      return name in all_ast_nodes
  
  def is_valid_builtin(name):
      return name in all_builtins
  
  def get_node_lineno(node):
      return (node.lineno) and node.lineno or 0
         
  #----------------------------------------------------------------------
  # Restricted AST nodes & builtins.
  #----------------------------------------------------------------------
  
  # Deny evaluation of code if the AST contain any of the following nodes:
  unallowed_ast_nodes = [
  #   'Add', 'And',
  #   'AssAttr', 'AssList', 'AssName', 'AssTuple',
  #   'Assert', 'Assign', 'AugAssign',
      'Backquote',
  #   'Bitand', 'Bitor', 'Bitxor', 'Break',
  #   'CallFunc', 'Class', 'Compare', 'Const', 'Continue',
  #   'Decorators', 'Dict', 'Discard', 'Div',
  #   'Ellipsis', 'EmptyNode',
      'Exec',
  #   'Expression', 'FloorDiv',
  #   'For',
      'From',
  #   'Function',
  #   'GenExpr', 'GenExprFor', 'GenExprIf', 'GenExprInner',
  #   'Getattr', 'Global', 'If',
      'Import',
  #   'Invert',
  #   'Keyword', 'Lambda', 'LeftShift',
  #   'List', 'ListComp', 'ListCompFor', 'ListCompIf', 'Mod',
  #   'Module',
  #   'Mul', 'Name', 'Node', 'Not', 'Or', 'Pass', 'Power',
  #   'Print', 'Printnl',
      'Raise',
  #    'Return', 'RightShift', 'Slice', 'Sliceobj',
  #   'Stmt', 'Sub', 'Subscript',
     'TryExcept', 'TryFinally',
  #   'Tuple', 'UnaryAdd', 'UnarySub',
  #   'While','Yield'
  ]
  
  # Deny evaluation of code if it tries to access any of the following builtins:
  unallowed_builtins = [
      '__import__',
  #   'abs', 'apply', 'basestring', 'bool', 'buffer',
  #   'callable', 'chr', 'classmethod', 'cmp', 'coerce',
      'compile',
  #   'complex',
      'delattr',
  #   'dict',
      'dir',
  #   'divmod', 'enumerate',
      'eval', 'execfile', 'file',
  #   'filter', 'float', 'frozenset',
      'getattr', 'globals', 'hasattr',
  #    'hash', 'hex', 'id',
      'input',
  #   'int', 'intern', 'isinstance', 'issubclass', 'iter',
  #   'len', 'list',
      'locals',
  #   'long', 'map', 'max', 'min', 'object', 'oct',
      'open',
  #   'ord', 'pow', 'property', 'range',
      'raw_input',
  #   'reduce',
      'reload',
  #   'repr', 'reversed', 'round', 'set',
      'setattr',
  #   'slice', 'sorted', 'staticmethod',  'str', 'sum', 'super',
  #   'tuple', 'type', 'unichr', 'unicode',
      'vars',
  #    'xrange', 'zip'
  ]
  
  for ast_name in unallowed_ast_nodes:
      assert(is_valid_ast_node(ast_name))
  for name in unallowed_builtins:
      assert(is_valid_builtin(name))
  
  def is_unallowed_ast_node(kind):
      return kind in unallowed_ast_nodes
  
  def is_unallowed_builtin(name):
      return name in unallowed_builtins
  
  #----------------------------------------------------------------------
  # Restricted attributes.
  #----------------------------------------------------------------------
  
  # In addition to these we deny access to all lowlevel attrs (__xxx__).
  unallowed_attr = [
      'im_class', 'im_func', 'im_self',
      'func_code', 'func_defaults', 'func_globals', 'func_name',
      'tb_frame', 'tb_next',
      'f_back', 'f_builtins', 'f_code', 'f_exc_traceback',
      'f_exc_type', 'f_exc_value', 'f_globals', 'f_locals']
  
  def is_unallowed_attr(name):
      return (name[:2] == '__' and name[-2:] == '__') or \
             (name in unallowed_attr)
  
  #----------------------------------------------------------------------
  # SafeEvalVisitor.
  #----------------------------------------------------------------------
  
  class SafeEvalError(object):
      """
      Base class for all which occur while walking the AST.
  
      Attributes:
        errmsg = short decription about the nature of the error
        lineno = line offset to where error occured in source code
      """
      def __init__(self, errmsg, lineno):
          self.errmsg, self.lineno = errmsg, lineno
      def __str__(self):
          return "line %d : %s" % (self.lineno, self.errmsg)
  
  class SafeEvalASTNodeError(SafeEvalError):
      "Expression/statement in AST evaluates to a restricted AST node type."
      pass
  class SafeEvalBuiltinError(SafeEvalError):
      "Expression/statement in tried to access a restricted builtin."
      pass
  class SafeEvalAttrError(SafeEvalError):
      "Expression/statement in tried to access a restricted attribute."
      pass
  
  class SafeEvalVisitor(object):
      """
      Data-driven visitor which walks the AST for some code and makes
      sure it doesn't contain any expression/statements which are
      declared as restricted in 'unallowed_ast_nodes'. We'll also make
      sure that there aren't any attempts to access/lookup restricted
      builtin declared in 'unallowed_builtins'. By default we also won't
      allow access to lowlevel stuff which can be used to dynamically
      access non-local envrioments.
  
      Interface:
        walk(ast) = validate AST and return True if AST is 'safe'
  
      Attributes:
        errors = list of SafeEvalError if walk() returned False
  
      Implementation:
      
      The visitor will automatically generate methods for all of the
      available AST node types and redirect them to self.ok or self.fail
      reflecting the configuration in 'unallowed_ast_nodes'. While
      walking the AST we simply forward the validating step to each of
      node callbacks which take care of reporting errors.
      """
  
      def __init__(self):
          "Initialize visitor by generating callbacks for all AST node types."
          self.errors = []
          for ast_name in all_ast_nodes:
              # Don't reset any overridden callbacks.
              if getattr(self, 'visit' + ast_name, None): continue
              if is_unallowed_ast_node(ast_name):
                  setattr(self, 'visit' + ast_name, self.fail)
              else:
                  setattr(self, 'visit' + ast_name, self.ok)
  
      def walk(self, ast):
          "Validate each node in AST and return True if AST is 'safe'."
          self.visit(ast)
          return self.errors == []
          
      def visit(self, node, *args):
          "Recursively validate node and all of its children."
          fn = getattr(self, 'visit' + classname(node))
          if DEBUG: self.trace(node)
          fn(node, *args)
          for child in node.getChildNodes():
              self.visit(child, *args)
  
      def visitName(self, node, *args):
          "Disallow any attempts to access a restricted builtin/attr."
          name = node.getChildren()[0]
          lineno = get_node_lineno(node)
          if is_unallowed_builtin(name):
              self.errors.append(SafeEvalBuiltinError( \
                  "access to builtin '%s' is denied" % name, lineno))
          elif is_unallowed_attr(name):
              self.errors.append(SafeEvalAttrError( \
                  "access to attribute '%s' is denied" % name, lineno))
                 
      def visitGetattr(self, node, *args):
          "Disallow any attempts to access a restricted attribute."
          name = node.attrname
          lineno = get_node_lineno(node)
          if is_unallowed_attr(name):
              self.errors.append(SafeEvalAttrError( \
                  "access to attribute '%s' is denied" % name, lineno))
              
      def ok(self, node, *args):
          "Default callback for 'harmless' AST nodes."
          pass
      
      def fail(self, node, *args):
          "Default callback for unallowed AST nodes."
          lineno = get_node_lineno(node)
          self.errors.append(SafeEvalASTNodeError( \
              "execution of '%s' statements is denied" % classname(node),
              lineno))
  
      def trace(self, node):
          "Debugging utility for tracing the validation of AST nodes."
          print classname(node)
          for attr in dir(node):
              if attr[:2] != '__':
                  print ' ' * 4, "%-15.15s" % attr, getattr(node, attr)
  
  #----------------------------------------------------------------------
  # Safe 'eval' replacement.
  #----------------------------------------------------------------------
  
  class SafeEvalException(Exception):
      "Base class for all safe-eval related errors."
      pass
  
  class SafeEvalCodeException(SafeEvalException):
      """
      Exception class for reporting all errors which occured while
      validating AST for source code in safe_eval().
  
      Attributes:
        code   = raw source code which failed to validate
        errors = list of SafeEvalError
      """
      def __init__(self, code, errors):
          self.code, self.errors = code, errors
      def __str__(self):
          return '\n'.join([str(err) for err in self.errors])
  
  class SafeEvalContextException(SafeEvalException):
      """
      Exception class for reporting unallowed objects found in the dict
      intended to be used as the local enviroment in safe_eval().
  
      Attributes:
        keys   = list of keys of the unallowed objects
        errors = list of strings describing the nature of the error
                 for each key in 'keys'
      """
      def __init__(self, keys, errors):
          self.keys, self.errors = keys, errors
      def __str__(self):
          return '\n'.join([str(err) for err in self.errors])
          
  class SafeEvalTimeoutException(SafeEvalException):
      """
      Exception class for reporting that code evaluation execeeded
      the given timelimit.
  
      Attributes:
        timeout = time limit in seconds
      """
      def __init__(self, timeout):
          self.timeout = timeout
      def __str__(self):
          return "Timeout limit execeeded (%s secs) during exec" % self.timeout
  
  def exec_timed(code, context, timeout_secs):
      """
      Dynamically execute 'code' using 'context' as the global enviroment.
      SafeEvalTimeoutException is raised if execution does not finish within
      the given timelimit.
      """
      assert(timeout_secs > 0)
  
      signal_finished = False
      
      def alarm(secs):
          def wait(secs):
              for n in xrange(timeout_secs):
                  time.sleep(1)
                  if signal_finished: break
              else:
                  thread.interrupt_main()
          thread.start_new_thread(wait, (secs,))
  
      try:
          alarm(timeout_secs)
          exec code in context
          signal_finished = True
      except KeyboardInterrupt:
          raise SafeEvalTimeoutException(timeout_secs)
  
  def safe_eval(code, context = {}, timeout_secs = 5):
      """
      Validate source code and make sure it contains no unauthorized
      expression/statements as configured via 'unallowed_ast_nodes' and
      'unallowed_builtins'. By default this means that code is not
      allowed import modules or access dangerous builtins like 'open' or
      'eval'. If code is considered 'safe' it will be executed via
      'exec' using 'context' as the global environment. More details on
      how code is executed can be found in the Python Reference Manual
      section 6.14 (ignore the remark on '__builtins__'). The 'context'
      enviroment is also validated and is not allowed to contain modules
      or builtins. The following exception will be raised on errors:
  
        if 'context' contains unallowed objects = 
          SafeEvalContextException
  
        if code is didn't validate and is considered 'unsafe' = 
          SafeEvalCodeException
  
        if code did not execute within the given timelimit =
          SafeEvalTimeoutException
      """   
      ctx_errkeys, ctx_errors = [], []
      for (key, obj) in context.items():
          if inspect.isbuiltin(obj):
              ctx_errkeys.append(key)
              ctx_errors.append("key '%s' : unallowed builtin %s" % (key, obj))
          if inspect.ismodule(obj):
              ctx_errkeys.append(key)
              ctx_errors.append("key '%s' : unallowed module %s" % (key, obj))
  
      if ctx_errors:
          raise SafeEvalContextException(ctx_errkeys, ctx_errors)
  
      ast = compiler.parse(code)
      checker = SafeEvalVisitor()
  
      if checker.walk(ast):
          exec_timed(code, context, timeout_secs)
      else:
          raise SafeEvalCodeException(code, checker.errors)
         
  #----------------------------------------------------------------------
  # Basic tests.
  #----------------------------------------------------------------------
  
  import unittest
  
  class TestSafeEval(unittest.TestCase):
      def test_builtin(self):
          # attempt to access a unsafe builtin
          self.assertRaises(SafeEvalException,
              safe_eval, "open('test.txt', 'w')")
  
      def test_getattr(self):
          # attempt to get arround direct attr access
          self.assertRaises(SafeEvalException, \
              safe_eval, "getattr(int, '__abs__')")
  
      def test_func_globals(self):
          # attempt to access global enviroment where fun was defined
          self.assertRaises(SafeEvalException, \
              safe_eval, "def x(): pass; print x.func_globals")
  
      def test_lowlevel(self):
          # lowlevel tricks to access 'object'
          self.assertRaises(SafeEvalException, \
              safe_eval, "().__class__.mro()[1].__subclasses__()")
  
      def test_timeout_ok(self):
          # attempt to exectute 'slow' code which finishes within timelimit
          def test(): time.sleep(2)
          env = {'test':test}
          safe_eval("test()", env, timeout_secs = 5)
  
      def test_timeout_exceed(self):
          # attempt to exectute code which never teminates
          self.assertRaises(SafeEvalException, \
              safe_eval, "while 1: pass")
  
      def test_invalid_context(self):
          # can't pass an enviroment with modules or builtins
          env = {'f' : __builtins__.open, 'g' : time}
          self.assertRaises(SafeEvalException, \
              safe_eval, "print 1", env)
  
      def test_callback(self):
          # modify local variable via callback
          self.value = 0
          def test(): self.value = 1
          env = {'test':test}
          safe_eval("test()", env)
          self.assertEqual(self.value, 1)
  
  if __name__ == "__main__":
      unittest.main()
  
  #----------------------------------------------------------------------
  # The End.
  #----------------------------------------------------------------------
  