AppPkg/Applications/Python/Python-2.7.2/Lib/lib2to3/pgen2/parse.py

   1 # Copyright 2004-2005 Elemental Security, Inc. All Rights Reserved.
   2 # Licensed to PSF under a Contributor Agreement.
   3
   4 """Parser engine for the grammar tables generated by pgen.
   5
   6 The grammar table must be loaded first.
   7
   8 See Parser/parser.c in the Python distribution for additional info on
   9 how this parsing engine works.
  10
  11 """
  12
  13 # Local imports
  14 from . import token
  15
  16 class ParseError(Exception):
  17     """Exception to signal the parser is stuck."""
  18
  19     def __init__(self, msg, type, value, context):
  20         Exception.__init__(self, "%s: type=%r, value=%r, context=%r" %
  21                            (msg, type, value, context))
  22         self.msg = msg
  23         self.type = type
  24         self.value = value
  25         self.context = context
  26
  27 class Parser(object):
  28     """Parser engine.
  29
  30     The proper usage sequence is:
  31
  32     p = Parser(grammar, [converter])  # create instance
  33     p.setup([start])                  # prepare for parsing
  34     <for each input token>:
  35         if p.addtoken(...):           # parse a token; may raise ParseError
  36             break
  37     root = p.rootnode                 # root of abstract syntax tree
  38
  39     A Parser instance may be reused by calling setup() repeatedly.
  40
  41     A Parser instance contains state pertaining to the current token
  42     sequence, and should not be used concurrently by different threads
  43     to parse separate token sequences.
  44
  45     See driver.py for how to get input tokens by tokenizing a file or
  46     string.
  47
  48     Parsing is complete when addtoken() returns True; the root of the
  49     abstract syntax tree can then be retrieved from the rootnode
  50     instance variable.  When a syntax error occurs, addtoken() raises
  51     the ParseError exception.  There is no error recovery; the parser
  52     cannot be used after a syntax error was reported (but it can be
  53     reinitialized by calling setup()).
  54
  55     """
  56
  57     def __init__(self, grammar, convert=None):
  58         """Constructor.
  59
  60         The grammar argument is a grammar.Grammar instance; see the
  61         grammar module for more information.
  62
  63         The parser is not ready yet for parsing; you must call the
  64         setup() method to get it started.
  65
  66         The optional convert argument is a function mapping concrete
  67         syntax tree nodes to abstract syntax tree nodes.  If not
  68         given, no conversion is done and the syntax tree produced is
  69         the concrete syntax tree.  If given, it must be a function of
  70         two arguments, the first being the grammar (a grammar.Grammar
  71         instance), and the second being the concrete syntax tree node
  72         to be converted.  The syntax tree is converted from the bottom
  73         up.
  74
  75         A concrete syntax tree node is a (type, value, context, nodes)
  76         tuple, where type is the node type (a token or symbol number),
  77         value is None for symbols and a string for tokens, context is
  78         None or an opaque value used for error reporting (typically a
  79         (lineno, offset) pair), and nodes is a list of children for
  80         symbols, and None for tokens.
  81
  82         An abstract syntax tree node may be anything; this is entirely
  83         up to the converter function.
  84
  85         """
  86         self.grammar = grammar
  87         self.convert = convert or (lambda grammar, node: node)
  88
  89     def setup(self, start=None):
  90         """Prepare for parsing.
  91
  92         This *must* be called before starting to parse.
  93
  94         The optional argument is an alternative start symbol; it
  95         defaults to the grammar's start symbol.
  96
  97         You can use a Parser instance to parse any number of programs;
  98         each time you call setup() the parser is reset to an initial
  99         state determined by the (implicit or explicit) start symbol.
 100
 101         """
 102         if start is None:
 103             start = self.grammar.start
 104         # Each stack entry is a tuple: (dfa, state, node).
 105         # A node is a tuple: (type, value, context, children),
 106         # where children is a list of nodes or None, and context may be None.
 107         newnode = (start, None, None, [])
 108         stackentry = (self.grammar.dfas[start], 0, newnode)
 109         self.stack = [stackentry]
 110         self.rootnode = None
 111         self.used_names = set() # Aliased to self.rootnode.used_names in pop()
 112
 113     def addtoken(self, type, value, context):
 114         """Add a token; return True iff this is the end of the program."""
 115         # Map from token to label
 116         ilabel = self.classify(type, value, context)
 117         # Loop until the token is shifted; may raise exceptions
 118         while True:
 119             dfa, state, node = self.stack[-1]
 120             states, first = dfa
 121             arcs = states[state]
 122             # Look for a state with this label
 123             for i, newstate in arcs:
 124                 t, v = self.grammar.labels[i]
 125                 if ilabel == i:
 126                     # Look it up in the list of labels
 127                     assert t < 256
 128                     # Shift a token; we're done with it
 129                     self.shift(type, value, newstate, context)
 130                     # Pop while we are in an accept-only state
 131                     state = newstate
 132                     while states[state] == [(0, state)]:
 133                         self.pop()
 134                         if not self.stack:
 135                             # Done parsing!
 136                             return True
 137                         dfa, state, node = self.stack[-1]
 138                         states, first = dfa
 139                     # Done with this token
 140                     return False
 141                 elif t >= 256:
 142                     # See if it's a symbol and if we're in its first set
 143                     itsdfa = self.grammar.dfas[t]
 144                     itsstates, itsfirst = itsdfa
 145                     if ilabel in itsfirst:
 146                         # Push a symbol
 147                         self.push(t, self.grammar.dfas[t], newstate, context)
 148                         break # To continue the outer while loop
 149             else:
 150                 if (0, state) in arcs:
 151                     # An accepting state, pop it and try something else
 152                     self.pop()
 153                     if not self.stack:
 154                         # Done parsing, but another token is input
 155                         raise ParseError("too much input",
 156                                          type, value, context)
 157                 else:
 158                     # No success finding a transition
 159                     raise ParseError("bad input", type, value, context)
 160
 161     def classify(self, type, value, context):
 162         """Turn a token into a label.  (Internal)"""
 163         if type == token.NAME:
 164             # Keep a listing of all used names
 165             self.used_names.add(value)
 166             # Check for reserved words
 167             ilabel = self.grammar.keywords.get(value)
 168             if ilabel is not None:
 169                 return ilabel
 170         ilabel = self.grammar.tokens.get(type)
 171         if ilabel is None:
 172             raise ParseError("bad token", type, value, context)
 173         return ilabel
 174
 175     def shift(self, type, value, newstate, context):
 176         """Shift a token.  (Internal)"""
 177         dfa, state, node = self.stack[-1]
 178         newnode = (type, value, context, None)
 179         newnode = self.convert(self.grammar, newnode)
 180         if newnode is not None:
 181             node[-1].append(newnode)
 182         self.stack[-1] = (dfa, newstate, node)
 183
 184     def push(self, type, newdfa, newstate, context):
 185         """Push a nonterminal.  (Internal)"""
 186         dfa, state, node = self.stack[-1]
 187         newnode = (type, None, context, [])
 188         self.stack[-1] = (dfa, newstate, node)
 189         self.stack.append((newdfa, 0, newnode))
 190
 191     def pop(self):
 192         """Pop a nonterminal.  (Internal)"""
 193         popdfa, popstate, popnode = self.stack.pop()
 194         newnode = self.convert(self.grammar, popnode)
 195         if newnode is not None:
 196             if self.stack:
 197                 dfa, state, node = self.stack[-1]
 198                 node[-1].append(newnode)
 199             else:
 200                 self.rootnode = newnode
 201                 self.rootnode.used_names = self.used_names