cpython/Lib/compiler/transformer.py

"""Parse tree transformation module.

Transforms Python source code into an abstract syntax tree (AST)
defined in the ast module.

The simplest ways to invoke this module are via parse and parseFile.
parse(buf) -> AST
parseFile(path) -> AST
"""

# Copyright 1997-1998 Greg Stein and Bill Tutt
#
# Written by Greg Stein (gstein@lyra.org)
#        and Bill Tutt (rassilon@lima.mudlib.org)
# February 1997.
#
# Support for Node subclasses written and other revisions by
#  Jeremy Hylton (jeremy@cnri.reston.va.us)
#
# The output tree has the following nodes:
#
# Source Python line #'s appear at the end of each of all of these nodes
# If a line # doesn't apply, there will be a None instead.
#
# module:     doc, node
# stmt:       [ node1, ..., nodeN ]
# function:   name, argnames, defaults, flags, doc, codeNode
# lambda:     argnames, defaults, flags, codeNode
# classdef:   name, bases, doc, codeNode
# pass:
# break:
# continue:
# for:        assignNode, listNode, bodyNode, elseNode
# while:      testNode, bodyNode, elseNode
# if:         [ (testNode, suiteNode), ... ], elseNode
# exec:       expr1Node, expr2Node, expr3Node
# from:       modname, [ name1, ..., nameN ]
# import:     [ name1, ..., nameN ]
# raise:      expr1Node, expr2Node, expr3Node
# tryfinally: trySuiteNode, finSuiteNode
# tryexcept:  trySuiteNode, [ (exprNode, assgnNode, suiteNode), ... ], elseNode
# return:     valueNode
# const:      value
# print:      [ node1, ..., nodeN ]
# printnl:    [ node1, ..., nodeN ]
# discard:    exprNode
# assign:     [ node1, ..., nodeN ], exprNode
# ass_tuple:  [ node1, ..., nodeN ]
# ass_list:   [ node1, ..., nodeN ]
# ass_name:   name, flags
# ass_attr:   exprNode, attrname, flags
# list:       [ node1, ..., nodeN ]
# dict:       [ (key1, val1), ..., (keyN, valN) ]
# not:        exprNode
# compare:    exprNode, [ (op, node), ..., (op, node) ]
# name:       name
# global:     [ name1, ..., nameN ]
# backquote:  node
# getattr:    exprNode, attrname
# call_func:  node, [ arg1, ..., argN ]
# keyword:    name, exprNode
# subscript:  exprNode, flags, [ sub1, ..., subN ]
# ellipsis:
# sliceobj:   [ node1, ..., nodeN ]
# slice:      exprNode, flags, lowerNode, upperNode
# assert:     expr1, expr2
#
# Compiled as "binary" ops:
# tuple:      [ node1, ..., nodeN ]
# or:         [ node1, ..., nodeN ]
# and:        [ node1, ..., nodeN ]
# bitor:      [ node1, ..., nodeN ]
# bitxor:     [ node1, ..., nodeN ]
# bitand:     [ node1, ..., nodeN ]
#
# Operations easily evaluateable on constants:
# <<:         exprNode, shiftNode
# >>:         exprNode, shiftNode
# +:          leftNode, rightNode
# -:          leftNode, rightNode
# *:          leftNode, rightNode
# /:          leftNode, rightNode
# %:          leftNode, rightNode
# power:      leftNode, rightNode
# unary+:     node
# unary-:     node
# invert:     node
#

import ast
import parser
import symbol
import token
import string

import pprint

error = 'walker.error'

from consts import CO_VARARGS, CO_VARKEYWORDS
from consts import OP_ASSIGN, OP_DELETE, OP_APPLY

def parseFile(path):
    f = open(path)
    src = f.read()
    f.close()
    return parse(src)

def parse(buf):
    return Transformer().parsesuite(buf)

def asList(nodes):
  l = []
  for item in nodes:
    if hasattr(item, "asList"):
      l.append(item.asList())
    else:
      if type(item) is type( (None, None) ):
        l.append(tuple(asList(item)))
      elif type(item) is type( [] ):
        l.append(asList(item))
      else:
        l.append(item)
  return l

def Node(*args):
  kind = args[0]
  if ast.nodes.has_key(kind):
    try:
      return apply(ast.nodes[kind], args[1:])
    except TypeError:
      print ast.nodes[kind], len(args), args
      raise
  else:
    raise error, "Can't find appropriate Node type."
    #return apply(ast.Node, args)

class Transformer:
  """Utility object for transforming Python parse trees.

  Exposes the following methods:
    tree = transform(ast_tree)
    tree = parsesuite(text)
    tree = parseexpr(text)
    tree = parsefile(fileob | filename)
  """

  def __init__(self):
    self._dispatch = { }
    for value, name in symbol.sym_name.items():
      if hasattr(self, name):
        self._dispatch[value] = getattr(self, name)

  def transform(self, tree):
    """Transform an AST into a modified parse tree."""
    if type(tree) != type(()) and type(tree) != type([]):
      tree = parser.ast2tuple(tree,1)
    return self.compile_node(tree)

  def parsesuite(self, text):
    """Return a modified parse tree for the given suite text."""
    # Hack for handling non-native line endings on non-DOS like OSs.
    text = string.replace(text, '\x0d', '')
    return self.transform(parser.suite(text))

  def parseexpr(self, text):
    """Return a modified parse tree for the given expression text."""
    return self.transform(parser.expr(text))

  def parsefile(self, file):
    """Return a modified parse tree for the contents of the given file."""
    if type(file) == type(''):
      file = open(file)
    return self.parsesuite(file.read())

  # --------------------------------------------------------------
  #
  # PRIVATE METHODS
  #

  def compile_node(self, node):
    ### emit a line-number node?
    n = node[0]
    if n == symbol.single_input:
      return self.single_input(node[1:])
    if n == symbol.file_input:
      return self.file_input(node[1:])
    if n == symbol.eval_input:
      return self.eval_input(node[1:])
    if n == symbol.lambdef:
      return self.lambdef(node[1:])
    if n == symbol.funcdef:
      return self.funcdef(node[1:])
    if n == symbol.classdef:
      return self.classdef(node[1:])

    raise error, ('unexpected node type', n)

  def single_input(self, node):
    ### do we want to do anything about being "interactive" ?

    # NEWLINE | simple_stmt | compound_stmt NEWLINE
    n = node[0][0]
    if n != token.NEWLINE:
      return self.com_stmt(node[0])

    return Node('pass')

  def file_input(self, nodelist):
    doc = self.get_docstring(nodelist, symbol.file_input)
    stmts = [ ]
    for node in nodelist:
      if node[0] != token.ENDMARKER and node[0] != token.NEWLINE:
        self.com_append_stmt(stmts, node)
    return Node('module', doc, Node('stmt', stmts))

  def eval_input(self, nodelist):
    # from the built-in function input()
    ### is this sufficient?
    return self.com_node(nodelist[0])

  def funcdef(self, nodelist):
    # funcdef: 'def' NAME parameters ':' suite
    # parameters: '(' [varargslist] ')'

    lineno = nodelist[1][2]
    name   = nodelist[1][1]
    args   = nodelist[2][2]

    if args[0] == symbol.varargslist:
      names, defaults, flags = self.com_arglist(args[1:])
    else:
      names = defaults = ()
      flags = 0
    doc = self.get_docstring(nodelist[4])

    # code for function
    code = self.com_node(nodelist[4])

    n = Node('function', name, names, defaults, flags, doc, code)
    n.lineno = lineno
    return n

  def lambdef(self, nodelist):
    # lambdef: 'lambda' [varargslist] ':' test
    if nodelist[2][0] == symbol.varargslist:
      names, defaults, flags = self.com_arglist(nodelist[2][1:])
    else:
      names = defaults = ()
      flags = 0

    # code for lambda
    code = self.com_node(nodelist[-1])

    n = Node('lambda', names, defaults, flags, code)
    n.lineno = nodelist[1][2]
    return n

  def classdef(self, nodelist):
    # classdef: 'class' NAME ['(' testlist ')'] ':' suite

    name = nodelist[1][1]
    doc = self.get_docstring(nodelist[-1])
    if nodelist[2][0] == token.COLON:
      bases = []
    else:
      bases = self.com_bases(nodelist[3])

    # code for class
    code = self.com_node(nodelist[-1])

    n = Node('class', name, bases, doc, code)
    n.lineno = nodelist[1][2]
    return n

  def stmt(self, nodelist):
    return self.com_stmt(nodelist[0])

  small_stmt = stmt
  flow_stmt = stmt
  compound_stmt = stmt

  def simple_stmt(self, nodelist):
    # small_stmt (';' small_stmt)* [';'] NEWLINE
    stmts = [ ]
    for i in range(0, len(nodelist), 2):
      self.com_append_stmt(stmts, nodelist[i])
    return Node('stmt', stmts)

  def parameters(self, nodelist):
    raise error

  def varargslist(self, nodelist):
    raise error

  def fpdef(self, nodelist):
    raise error

  def fplist(self, nodelist):
    raise error

  def dotted_name(self, nodelist):
    raise error

  def comp_op(self, nodelist):
    raise error

  def trailer(self, nodelist):
    raise error

  def sliceop(self, nodelist):
    raise error

  def argument(self, nodelist):
    raise error

  # --------------------------------------------------------------
  #
  # STATEMENT NODES  (invoked by com_node())
  #

  def expr_stmt(self, nodelist):
    # testlist ('=' testlist)*
    exprNode = self.com_node(nodelist[-1])
    if len(nodelist) == 1:
      return Node('discard', exprNode)
    nodes = [ ]
    for i in range(0, len(nodelist) - 2, 2):
      nodes.append(self.com_assign(nodelist[i], OP_ASSIGN))
    n = Node('assign', nodes, exprNode)
    n.lineno = nodelist[1][2]
    return n

  def print_stmt(self, nodelist):
    # print: (test ',')* [test]
    items = [ ]
    for i in range(1, len(nodelist), 2):
      items.append(self.com_node(nodelist[i]))
    if nodelist[-1][0] == token.COMMA:
      n = Node('print', items)
      n.lineno = nodelist[0][2]
      return n
    n = Node('printnl', items)
    n.lineno = nodelist[0][2]
    return n

  def del_stmt(self, nodelist):
    return self.com_assign(nodelist[1], OP_DELETE)

  def pass_stmt(self, nodelist):
    # pass:
    n = Node('pass')
    n.lineno = nodelist[0][2]
    return n

  def break_stmt(self, nodelist):
    # break:
    n = Node('break')
    n.lineno = nodelist[0][2]
    return n

  def continue_stmt(self, nodelist):
    # continue
    n = Node('continue')
    n.lineno = nodelist[0][2]
    return n

  def return_stmt(self, nodelist):
    # return: [testlist]
    if len(nodelist) < 2:
      n = Node('return', Node('const', None))
      n.lineno = nodelist[0][2]
      return n
    n = Node('return', self.com_node(nodelist[1]))
    n.lineno = nodelist[0][2]
    return n

  def raise_stmt(self, nodelist):
    # raise: [test [',' test [',' test]]]
    if len(nodelist) > 5:
      expr3 = self.com_node(nodelist[5])
    else:
      expr3 = None
    if len(nodelist) > 3:
      expr2 = self.com_node(nodelist[3])
    else:
      expr2 = None
    if len(nodelist) > 1:
      expr1 = self.com_node(nodelist[1])
    else:
      expr1 = None
    n = Node('raise', expr1, expr2, expr3)
    n.lineno = nodelist[0][2]
    return n

  def import_stmt(self, nodelist):
    # import: dotted_name (',' dotted_name)* |
    # from: dotted_name 'import' ('*' | NAME (',' NAME)*)
    names = [ ]
    if nodelist[0][1][0] == 'f':
      for i in range(3, len(nodelist), 2):
        # note: nodelist[i] could be (token.STAR, '*') or (token.NAME, name)
        names.append(nodelist[i][1])
      n = Node('from', self.com_dotted_name(nodelist[1]), names)
      n.lineno = nodelist[0][2]
      return n

    for i in range(1, len(nodelist), 2):
      names.append(self.com_dotted_name(nodelist[i]))
    n = Node('import', names)
    n.lineno = nodelist[0][2]
    return n

  def global_stmt(self, nodelist):
    # global: NAME (',' NAME)*
    names = [ ]
    for i in range(1, len(nodelist), 2):
      names.append(nodelist[i][1])
    n = Node('global', names)
    n.lineno = nodelist[0][2]
    return n

  def exec_stmt(self, nodelist):
    # exec_stmt: 'exec' expr ['in' expr [',' expr]]
    expr1 = self.com_node(nodelist[1])
    if len(nodelist) >= 4:
      expr2 = self.com_node(nodelist[3])
      if len(nodelist) >= 6:
        expr3 = self.com_node(nodelist[5])
      else:
        expr3 = None
    else:
      expr2 = expr3 = None

    n = Node('exec', expr1, expr2, expr3)
    n.lineno = nodelist[0][2]
    return n

  def assert_stmt(self, nodelist):
    # 'assert': test, [',' test]
    expr1 = self.com_node(nodelist[1])
    if (len(nodelist) == 4):
      expr2 = self.com_node(nodelist[3])
    else:
      expr2 = Node('name', 'None')
    n = Node('assert', expr1, expr2)
    n.lineno = nodelist[0][2]
    return n

  def if_stmt(self, nodelist):
    # if: test ':' suite ('elif' test ':' suite)* ['else' ':' suite]
    tests = [ ]
    for i in range(0, len(nodelist) - 3, 4):
      testNode = self.com_node(nodelist[i + 1])
      suiteNode = self.com_node(nodelist[i + 3])
      tests.append((testNode, suiteNode))

    if len(nodelist) % 4 == 3:
      elseNode = self.com_node(nodelist[-1])
##      elseNode.lineno = nodelist[-1][1][2]
    else:
      elseNode = None
    n = Node('if', tests, elseNode)
    n.lineno = nodelist[0][2]
    return n

  def while_stmt(self, nodelist):
    # 'while' test ':' suite ['else' ':' suite]

    testNode = self.com_node(nodelist[1])
    bodyNode = self.com_node(nodelist[3])

    if len(nodelist) > 4:
      elseNode = self.com_node(nodelist[6])
    else:
      elseNode = None

    n = Node('while', testNode, bodyNode, elseNode)
    n.lineno = nodelist[0][2]
    return n

  def for_stmt(self, nodelist):
    # 'for' exprlist 'in' exprlist ':' suite ['else' ':' suite]

    assignNode = self.com_assign(nodelist[1], OP_ASSIGN)
    listNode = self.com_node(nodelist[3])
    bodyNode = self.com_node(nodelist[5])

    if len(nodelist) > 8:
      elseNode = self.com_node(nodelist[8])
    else:
      elseNode = None

    n = Node('for', assignNode, listNode, bodyNode, elseNode)
    n.lineno = nodelist[0][2]
    return n

  def try_stmt(self, nodelist):
    # 'try' ':' suite (except_clause ':' suite)+ ['else' ':' suite]
    # | 'try' ':' suite 'finally' ':' suite
    if nodelist[3][0] != symbol.except_clause:
      return self.com_try_finally(nodelist)

    return self.com_try_except(nodelist)

  def suite(self, nodelist):
    # simple_stmt | NEWLINE INDENT NEWLINE* (stmt NEWLINE*)+ DEDENT
    if len(nodelist) == 1:
      return self.com_stmt(nodelist[0])

    stmts = [ ]
    for node in nodelist:
      if node[0] == symbol.stmt:
        self.com_append_stmt(stmts, node)
    return Node('stmt', stmts)

  # --------------------------------------------------------------
  #
  # EXPRESSION NODES  (invoked by com_node())
  #

  def testlist(self, nodelist):
    # testlist: expr (',' expr)* [',']
    # exprlist: expr (',' expr)* [',']
    return self.com_binary('tuple', nodelist)

  exprlist = testlist

  def test(self, nodelist):
    # and_test ('or' and_test)* | lambdef
    if len(nodelist) == 1 and nodelist[0][0] == symbol.lambdef:
      return self.lambdef(nodelist[0])
    return self.com_binary('or', nodelist)

  def and_test(self, nodelist):
    # not_test ('and' not_test)*
    return self.com_binary('and', nodelist)

  def not_test(self, nodelist):
    # 'not' not_test | comparison
    result = self.com_node(nodelist[-1])
    if len(nodelist) == 2:
      n = Node('not', result)
      n.lineno = nodelist[0][2]
      return n
    return result

  def comparison(self, nodelist):
    # comparison: expr (comp_op expr)*
    node = self.com_node(nodelist[0])
    if len(nodelist) == 1:
      return node

    results = [ ]
    for i in range(2, len(nodelist), 2):
      nl = nodelist[i-1]

      # comp_op: '<' | '>' | '=' | '>=' | '<=' | '<>' | '!=' | '=='
      #          | 'in' | 'not' 'in' | 'is' | 'is' 'not'
      n = nl[1]
      if n[0] == token.NAME:
        type = n[1]
        if len(nl) == 3:
          if type == 'not':
            type = 'not in'
          else:
            type = 'is not'
      else:
        type = _cmp_types[n[0]]

      lineno = nl[1][2]
      results.append((type, self.com_node(nodelist[i])))

    # we need a special "compare" node so that we can distinguish
    #   3 < x < 5   from    (3 < x) < 5
    # the two have very different semantics and results (note that the
    # latter form is always true)

    n = Node('compare', node, results)
    n.lineno = lineno
    return n

  def expr(self, nodelist):
    # xor_expr ('|' xor_expr)*
    return self.com_binary('bitor', nodelist)

  def xor_expr(self, nodelist):
    # xor_expr ('^' xor_expr)*
    return self.com_binary('bitxor', nodelist)

  def and_expr(self, nodelist):
    # xor_expr ('&' xor_expr)*
    return self.com_binary('bitand', nodelist)

  def shift_expr(self, nodelist):
    # shift_expr ('<<'|'>>' shift_expr)*
    node = self.com_node(nodelist[0])
    for i in range(2, len(nodelist), 2):
      right = self.com_node(nodelist[i])
      if nodelist[i-1][0] == token.LEFTSHIFT:
        node = Node('<<', [node, right])
        node.lineno = nodelist[1][2]
      else:
        node = Node('>>', [node, right])
        node.lineno = nodelist[1][2]
    return node

  def arith_expr(self, nodelist):
    node = self.com_node(nodelist[0])
    for i in range(2, len(nodelist), 2):
      right = self.com_node(nodelist[i])
      if nodelist[i-1][0] == token.PLUS:
        node = Node('+', [node, right])
        node.lineno = nodelist[1][2]
      else:
        node = Node('-', [node, right])
        node.lineno = nodelist[1][2]
    return node

  def term(self, nodelist):
    node = self.com_node(nodelist[0])
    for i in range(2, len(nodelist), 2):
      right = self.com_node(nodelist[i])
      if nodelist[i-1][0] == token.STAR:
        node = Node('*', [node, right])
        node.lineno = nodelist[1][2]
      elif nodelist[i-1][0] == token.SLASH:
        node = Node('/', [node, right])
        node.lineno = nodelist[1][2]
      else:
        node = Node('%', [node, right])
        node.lineno = nodelist[1][2]
    return node

  def factor(self, nodelist):
    t = nodelist[0][0]
    node = self.com_node(nodelist[-1])
    if t == token.PLUS:
      node = Node('unary+', node)
      node.lineno = nodelist[0][2]
    elif t == token.MINUS:
      node = Node('unary-', node)
      node.lineno = nodelist[0][2]
    elif t == token.TILDE:
      node = Node('invert', node)
      node.lineno = nodelist[0][2]
    return node

  def power(self, nodelist):
    # power: atom trailer* ('**' factor)*
    node = self.com_node(nodelist[0])
    for i in range(1, len(nodelist)):
      if nodelist[i][0] == token.DOUBLESTAR:
        n = Node('power', [node, self.com_node(nodelist[i+1])])
        n.lineno = nodelist[i][2]
        return n

      node = self.com_apply_trailer(node, nodelist[i])

    return node

  def atom(self, nodelist):
    t = nodelist[0][0]
    if t == token.LPAR:
      if nodelist[1][0] == token.RPAR:
        n = Node('tuple', ())
        n.lineno = nodelist[0][2]
        return n
      return self.com_node(nodelist[1])

    if t == token.LSQB:
      if nodelist[1][0] == token.RSQB:
        n = Node('list', ())
        n.lineno = nodelist[0][2]
        return n
      return self.com_list_constructor(nodelist[1])

    if t == token.LBRACE:
      if nodelist[1][0] == token.RBRACE:
        return Node('dict', ())
      return self.com_dictmaker(nodelist[1])

    if t == token.BACKQUOTE:
      n = Node('backquote', self.com_node(nodelist[1]))
      n.lineno = nodelist[0][2]
      return n

    if t == token.NUMBER:
      ### need to verify this matches compile.c
      k = eval(nodelist[0][1])
      n = Node('const', k)
      n.lineno = nodelist[0][2]
      return n

    if t == token.STRING:
      ### need to verify this matches compile.c
      k = ''
      for node in nodelist:
        k = k + eval(node[1])
      n = Node('const', k)
      n.lineno = nodelist[0][2]
      return n

    if t == token.NAME:
      ### any processing to do?
      n = Node('name', nodelist[0][1])
      n.lineno = nodelist[0][2]
      return n

    raise error, "unknown node type"

  # --------------------------------------------------------------
  #
  # INTERNAL PARSING UTILITIES
  #

  def com_node(self, node):
    # Note: compile.c has handling in com_node for del_stmt, pass_stmt,
    #       break_stmt, stmt, small_stmt, flow_stmt, simple_stmt,
    #       and compound_stmt.
    #       We'll just dispatch them.

    #
    # A ';' at the end of a line can make a NEWLINE token appear here,
    # Render it harmless. (genc discards ('discard', ('const', xxxx)) Nodes)
    # 
    if node[0] == token.NEWLINE:
      return Node('discard', Node('const', None))

    if node[0] not in _legal_node_types:
      raise error, 'illegal node passed to com_node: %s' % node[0]

    return self._dispatch[node[0]](node[1:])

  def com_arglist(self, nodelist):
    # varargslist:
    #   (fpdef ['=' test] ',')* ('*' NAME [',' ('**'|'*' '*') NAME]
    #  | fpdef ['=' test] (',' fpdef ['=' test])* [',']
    #  | ('**'|'*' '*') NAME)
    # fpdef: NAME | '(' fplist ')'
    # fplist: fpdef (',' fpdef)* [',']
    names = [ ]
    defaults = [ ]
    flags = 0

    i = 0
    while i < len(nodelist):
      node = nodelist[i]
      if node[0] == token.STAR or node[0] == token.DOUBLESTAR:
        if node[0] == token.STAR:
          node = nodelist[i+1]
          if node[0] == token.NAME:
            names.append(node[1])
            flags = flags | CO_VARARGS
            i = i + 3

        if i < len(nodelist):
          # should be DOUBLESTAR or STAR STAR
          if nodelist[i][0] == token.DOUBLESTAR:
            node = nodelist[i+1]
          else:
            node = nodelist[i+2]
          names.append(node[1])
          flags = flags | CO_VARKEYWORDS

        break

      # fpdef: NAME | '(' fplist ')'
      names.append(self.com_fpdef(node))

      i = i + 1
      if i >= len(nodelist):
        break

      if nodelist[i][0] == token.EQUAL:
        defaults.append(self.com_node(nodelist[i + 1]))
        i = i + 2
      elif len(defaults):
        # Treat "(a=1, b)" as "(a=1, b=None)"
        defaults.append(Node('const', None))

      i = i + 1

    return names, defaults, flags

  def com_fpdef(self, node):
    # fpdef: NAME | '(' fplist ')'
    if node[1][0] == token.LPAR:
      return self.com_fplist(node[2])
    return node[1][1]

  def com_fplist(self, node):
    # fplist: fpdef (',' fpdef)* [','] 
    if len(node) == 2:
      return self.com_fpdef(node[1])
    list = [ ]
    for i in range(1, len(node), 2):
      list.append(self.com_fpdef(node[i]))
    return tuple(list)

  def com_dotted_name(self, node):
    # String together the dotted names and return the string
    name = ""
    for n in node:
      if type(n) == type(()) and n[0] == 1:
        name = name + n[1] + '.'
    return name[:-1]

  def com_bases(self, node):
    bases = [ ]
    for i in range(1, len(node), 2):
      bases.append(self.com_node(node[i]))
    return bases

  def com_try_finally(self, nodelist):
    # try_fin_stmt: "try" ":" suite "finally" ":" suite
    n = Node('tryfinally', self.com_node(nodelist[2]), self.com_node(nodelist[5]))
    n.lineno = nodelist[0][2]
    return n

  def com_try_except(self, nodelist):
    # try_except: 'try' ':' suite (except_clause ':' suite)* ['else' suite]
    #tryexcept:  [TryNode, [except_clauses], elseNode)]
    stmt = self.com_node(nodelist[2])
    clauses = []
    elseNode = None
    for i in range(3, len(nodelist), 3):
      node = nodelist[i]
      if node[0] == symbol.except_clause:
        # except_clause: 'except' [expr [',' expr]] */
        if len(node) > 2:
          expr1 = self.com_node(node[2])
          if len(node) > 4:
            expr2 = self.com_assign(node[4], OP_ASSIGN)
          else:
            expr2 = None
        else:
          expr1 = expr2 = None
        clauses.append((expr1, expr2, self.com_node(nodelist[i+2])))

      if node[0] == token.NAME:
        elseNode = self.com_node(nodelist[i+2])
    n = Node('tryexcept', self.com_node(nodelist[2]), clauses, elseNode)
    n.lineno = nodelist[0][2]
    return n

  def com_assign(self, node, assigning):
    # return a node suitable for use as an "lvalue"
    # loop to avoid trivial recursion
    while 1:
      t = node[0]
      if t == symbol.exprlist or t == symbol.testlist:
        if len(node) > 2:
          return self.com_assign_tuple(node, assigning)
        node = node[1]
      elif t in _assign_types:
        if len(node) > 2:
          raise SyntaxError, "can't assign to operator"
        node = node[1]
      elif t == symbol.power:
        if node[1][0] != symbol.atom:
          raise SyntaxError, "can't assign to operator"
        if len(node) > 2:
          primary = self.com_node(node[1])
          for i in range(2, len(node)-1):
            ch = node[i]
            if ch[0] == token.DOUBLESTAR:
              raise SyntaxError, "can't assign to operator"
            primary = self.com_apply_trailer(primary, ch)
          return self.com_assign_trailer(primary, node[-1], assigning)
        node = node[1]
      elif t == symbol.atom:
        t = node[1][0]
        if t == token.LPAR:
          node = node[2]
          if node[0] == token.RPAR:
            raise SyntaxError, "can't assign to ()"
        elif t == token.LSQB:
          node = node[2]
          if node[0] == token.RSQB:
            raise SyntaxError, "can't assign to []"
          return self.com_assign_list(node, assigning)
        elif t == token.NAME:
          return self.com_assign_name(node[1], assigning)
        else:
          raise SyntaxError, "can't assign to literal"
      else:
        raise SyntaxError, "bad assignment"

  def com_assign_tuple(self, node, assigning):
    assigns = [ ]
    for i in range(1, len(node), 2):
      assigns.append(self.com_assign(node[i], assigning))
    return Node('ass_tuple', assigns)

  def com_assign_list(self, node, assigning):
    assigns = [ ]
    for i in range(1, len(node), 2):
      assigns.append(self.com_assign(node[i], assigning))
    return Node('ass_list', assigns)

  def com_assign_name(self, node, assigning):
    n = Node('ass_name', node[1], assigning)
    n.lineno = node[2]
    return n

  def com_assign_trailer(self, primary, node, assigning):
    t = node[1][0]
    if t == token.LPAR:
      raise SyntaxError, "can't assign to function call"
    if t == token.DOT:
      return self.com_assign_attr(primary, node[2], assigning)
    if t == token.LSQB:
      return self.com_subscriptlist(primary, node[2], assigning)
    raise SyntaxError, "unknown trailer type: %s" % t

  def com_assign_attr(self, primary, node, assigning):
    return Node('ass_attr', primary, node[1], assigning)

  def com_binary(self, type, nodelist):
    "Compile 'NODE (OP NODE)*' into (type, [ node1, ..., nodeN ])."
    if len(nodelist) == 1:
      return self.com_node(nodelist[0])
    items = [ ]
    for i in range(0, len(nodelist), 2):
      items.append(self.com_node(nodelist[i]))
    return Node(type, items)

  def com_stmt(self, node):
    #pprint.pprint(node)
    result = self.com_node(node)
    try:
      result[0]
    except:
      print node[0]
    if result[0] == 'stmt':
      return result
    return Node('stmt', [ result ])

  def com_append_stmt(self, stmts, node):
    result = self.com_node(node)
    try:
      result[0]
    except:
      print node
    if result[0] == 'stmt':
      stmts[len(stmts):] = result[1]
    else:
      stmts.append(result)

  def com_list_constructor(self, nodelist):
    values = [ ]
    for i in range(1, len(nodelist), 2):
      values.append(self.com_node(nodelist[i]))
    return Node('list', values)

  def com_dictmaker(self, nodelist):
    # dictmaker: test ':' test (',' test ':' value)* [',']
    items = [ ]
    for i in range(1, len(nodelist), 4):
      items.append((self.com_node(nodelist[i]), self.com_node(nodelist[i+2])))
    return Node('dict', items)

  def com_apply_trailer(self, primaryNode, nodelist):
    t = nodelist[1][0]
    if t == token.LPAR:
      return self.com_call_function(primaryNode, nodelist[2])
    if t == token.DOT:
      return self.com_select_member(primaryNode, nodelist[2])
    if t == token.LSQB:
      return self.com_subscriptlist(primaryNode, nodelist[2], OP_APPLY)

    raise SyntaxError, 'unknown node type: %s' % t

  def com_select_member(self, primaryNode, nodelist):
    if nodelist[0] != token.NAME:
      raise SyntaxError, "member must be a name"
    n = Node('getattr', primaryNode, nodelist[1])
    n.lineno = nodelist[2]
    return n

  def com_call_function(self, primaryNode, nodelist):
    if nodelist[0] == token.RPAR:
      return Node('call_func', primaryNode, [ ])
    args = [ ]
    kw = 0
    for i in range(1, len(nodelist), 2):
      kw, result = self.com_argument(nodelist[i], kw)
      args.append(result)
    return Node('call_func', primaryNode, args)

  def com_argument(self, nodelist, kw):
    if len(nodelist) == 2:
      if kw:
        raise SyntaxError, "non-keyword arg after keyword arg"
      return 0, self.com_node(nodelist[1])
    result = self.com_node(nodelist[3])
    n = nodelist[1]
    while len(n) == 2 and n[0] != token.NAME:
      n = n[1]
    if n[0] != token.NAME:
      raise SyntaxError, "keyword can't be an expression (%s)"%n[0]
    node = Node('keyword', n[1], result)
    node.lineno = n[2]
    return 1, node

  def com_subscriptlist(self, primary, nodelist, assigning):
    # slicing:      simple_slicing | extended_slicing
    # simple_slicing:   primary "[" short_slice "]"
    # extended_slicing: primary "[" slice_list "]" 
    # slice_list:   slice_item ("," slice_item)* [","]

    # backwards compat slice for '[i:j]'
    if len(nodelist) == 2:
      sub = nodelist[1]
      if (sub[1][0] == token.COLON or \
              (len(sub) > 2 and sub[2][0] == token.COLON)) and \
             sub[-1][0] != symbol.sliceop:
        return self.com_slice(primary, sub, assigning)

    subscripts = [ ]
    for i in range(1, len(nodelist), 2):
      subscripts.append(self.com_subscript(nodelist[i]))

    return Node('subscript', primary, assigning, subscripts)

  def com_subscript(self, node):
    # slice_item: expression | proper_slice | ellipsis
    ch = node[1]
    if ch[0] == token.DOT and node[2][0] == token.DOT:
      return Node('ellipsis')
    if ch[0] == token.COLON or len(node) > 2:
      return self.com_sliceobj(node)
    return self.com_node(ch)

  def com_sliceobj(self, node):
    # proper_slice: short_slice | long_slice
    # short_slice:  [lower_bound] ":" [upper_bound]
    # long_slice:   short_slice ":" [stride]
    # lower_bound:  expression
    # upper_bound:  expression
    # stride:       expression
    #
    # Note: a stride may be further slicing...

    items = [ ]

    if node[1][0] == token.COLON:
      items.append(Node('const', None))
      i = 2
    else:
      items.append(self.com_node(node[1]))
      # i == 2 is a COLON
      i = 3

    if i < len(node) and node[i][0] == symbol.test:
      items.append(self.com_node(node[i]))
      i = i + 1
    else:
      items.append(Node('const', None))

    # a short_slice has been built. look for long_slice now by looking
    # for strides...
    for j in range(i, len(node)):
      ch = node[j]
      if len(ch) == 2:
        items.append(Node('const', None))
      else:
        items.append(self.com_node(ch[2]))

    return Node('sliceobj', items)

  def com_slice(self, primary, node, assigning):
    # short_slice:  [lower_bound] ":" [upper_bound]
    lower = upper = None
    if len(node) == 3:
      if node[1][0] == token.COLON:
        upper = self.com_node(node[2])
      else:
        lower = self.com_node(node[1])
    elif len(node) == 4:
      lower = self.com_node(node[1])
      upper = self.com_node(node[3])
    return Node('slice', primary, assigning, lower, upper)

  def get_docstring(self, node, n=None):
    if n is None:
      n = node[0]
      node = node[1:]
    if n == symbol.suite:
      if len(node) == 1:
        return self.get_docstring(node[0])
      for sub in node:
        if sub[0] == symbol.stmt:
          return self.get_docstring(sub)
      return None
    if n == symbol.file_input:
      for sub in node:
        if sub[0] == symbol.stmt:
          return self.get_docstring(sub)
      return None
    if n == symbol.atom:
      if node[0][0] == token.STRING:
        s = ''
        for t in node:
          s = s + eval(t[1])
        return s
      return None
    if n == symbol.stmt or n == symbol.simple_stmt or n == symbol.small_stmt:
      return self.get_docstring(node[0])
    if n in _doc_nodes and len(node) == 1:
      return self.get_docstring(node[0])
    return None


_doc_nodes = [
  symbol.expr_stmt,
  symbol.testlist,
  symbol.test,
  symbol.and_test,
  symbol.not_test,
  symbol.comparison,
  symbol.expr,
  symbol.xor_expr,
  symbol.and_expr,
  symbol.shift_expr,
  symbol.arith_expr,
  symbol.term,
  symbol.factor,
  symbol.power,
  ]

# comp_op: '<' | '>' | '=' | '>=' | '<=' | '<>' | '!=' | '=='
#             | 'in' | 'not' 'in' | 'is' | 'is' 'not'
_cmp_types = {
  token.LESS : '<',
  token.GREATER : '>',
  token.EQEQUAL : '==',
  token.EQUAL : '==',
  token.LESSEQUAL : '<=',
  token.GREATEREQUAL : '>=',
  token.NOTEQUAL : '!=',
  }

_legal_node_types = [
  symbol.funcdef,
  symbol.classdef,
  symbol.stmt,
  symbol.small_stmt,
  symbol.flow_stmt,
  symbol.simple_stmt,
  symbol.compound_stmt,
  symbol.expr_stmt,
  symbol.print_stmt,
  symbol.del_stmt,
  symbol.pass_stmt,
  symbol.break_stmt,
  symbol.continue_stmt,
  symbol.return_stmt,
  symbol.raise_stmt,
  symbol.import_stmt,
  symbol.global_stmt,
  symbol.exec_stmt,
  symbol.assert_stmt,
  symbol.if_stmt,
  symbol.while_stmt,
  symbol.for_stmt,
  symbol.try_stmt,
  symbol.suite,
  symbol.testlist,
  symbol.test,
  symbol.and_test,
  symbol.not_test,
  symbol.comparison,
  symbol.exprlist,
  symbol.expr,
  symbol.xor_expr,
  symbol.and_expr,
  symbol.shift_expr,
  symbol.arith_expr,
  symbol.term,
  symbol.factor,
  symbol.power,
  symbol.atom,
  ]

_assign_types = [
  symbol.test,
  symbol.and_test,
  symbol.not_test,
  symbol.comparison,
  symbol.expr,
  symbol.xor_expr,
  symbol.and_expr,
  symbol.shift_expr,
  symbol.arith_expr,
  symbol.term,
  symbol.factor,
  ]