| 1 | # Copyright 2004-2005 Elemental Security, Inc. All Rights Reserved.
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| 2 | # Licensed to PSF under a Contributor Agreement.
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| 3 |
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| 4 | """Parser engine for the grammar tables generated by pgen.
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| 5 |
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| 6 | The grammar table must be loaded first.
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| 7 |
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| 8 | See Parser/parser.c in the Python distribution for additional info on
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| 9 | how this parsing engine works.
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| 10 |
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| 11 | """
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| 12 |
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| 13 | # Local imports
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| 14 | from . import token
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| 15 |
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| 16 | class ParseError(Exception):
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| 17 | """Exception to signal the parser is stuck."""
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| 18 |
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| 19 | def __init__(self, msg, type, value, context):
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| 20 | Exception.__init__(self, "%s: type=%r, value=%r, context=%r" %
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| 21 | (msg, type, value, context))
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| 22 | self.msg = msg
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| 23 | self.type = type
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| 24 | self.value = value
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| 25 | self.context = context
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| 26 |
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| 27 | class Parser(object):
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| 28 | """Parser engine.
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| 29 |
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| 30 | The proper usage sequence is:
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| 31 |
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| 32 | p = Parser(grammar, [converter]) # create instance
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| 33 | p.setup([start]) # prepare for parsing
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| 34 | <for each input token>:
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| 35 | if p.addtoken(...): # parse a token; may raise ParseError
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| 36 | break
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| 37 | root = p.rootnode # root of abstract syntax tree
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| 38 |
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| 39 | A Parser instance may be reused by calling setup() repeatedly.
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| 40 |
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| 41 | A Parser instance contains state pertaining to the current token
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| 42 | sequence, and should not be used concurrently by different threads
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| 43 | to parse separate token sequences.
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| 44 |
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| 45 | See driver.py for how to get input tokens by tokenizing a file or
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| 46 | string.
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| 47 |
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| 48 | Parsing is complete when addtoken() returns True; the root of the
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| 49 | abstract syntax tree can then be retrieved from the rootnode
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| 50 | instance variable. When a syntax error occurs, addtoken() raises
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| 51 | the ParseError exception. There is no error recovery; the parser
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| 52 | cannot be used after a syntax error was reported (but it can be
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| 53 | reinitialized by calling setup()).
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| 54 |
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| 55 | """
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| 56 |
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| 57 | def __init__(self, grammar, convert=None):
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| 58 | """Constructor.
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| 59 |
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| 60 | The grammar argument is a grammar.Grammar instance; see the
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| 61 | grammar module for more information.
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| 62 |
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| 63 | The parser is not ready yet for parsing; you must call the
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| 64 | setup() method to get it started.
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| 65 |
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| 66 | The optional convert argument is a function mapping concrete
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| 67 | syntax tree nodes to abstract syntax tree nodes. If not
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| 68 | given, no conversion is done and the syntax tree produced is
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| 69 | the concrete syntax tree. If given, it must be a function of
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| 70 | two arguments, the first being the grammar (a grammar.Grammar
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| 71 | instance), and the second being the concrete syntax tree node
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| 72 | to be converted. The syntax tree is converted from the bottom
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| 73 | up.
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| 74 |
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| 75 | A concrete syntax tree node is a (type, value, context, nodes)
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| 76 | tuple, where type is the node type (a token or symbol number),
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| 77 | value is None for symbols and a string for tokens, context is
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| 78 | None or an opaque value used for error reporting (typically a
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| 79 | (lineno, offset) pair), and nodes is a list of children for
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| 80 | symbols, and None for tokens.
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| 81 |
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| 82 | An abstract syntax tree node may be anything; this is entirely
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| 83 | up to the converter function.
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| 84 |
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| 85 | """
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| 86 | self.grammar = grammar
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| 87 | self.convert = convert or (lambda grammar, node: node)
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| 88 |
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| 89 | def setup(self, start=None):
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| 90 | """Prepare for parsing.
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| 91 |
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| 92 | This *must* be called before starting to parse.
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| 93 |
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| 94 | The optional argument is an alternative start symbol; it
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| 95 | defaults to the grammar's start symbol.
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| 96 |
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| 97 | You can use a Parser instance to parse any number of programs;
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| 98 | each time you call setup() the parser is reset to an initial
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| 99 | state determined by the (implicit or explicit) start symbol.
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| 100 |
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| 101 | """
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| 102 | if start is None:
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| 103 | start = self.grammar.start
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| 104 | # Each stack entry is a tuple: (dfa, state, node).
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| 105 | # A node is a tuple: (type, value, context, children),
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| 106 | # where children is a list of nodes or None, and context may be None.
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| 107 | newnode = (start, None, None, [])
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| 108 | stackentry = (self.grammar.dfas[start], 0, newnode)
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| 109 | self.stack = [stackentry]
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| 110 | self.rootnode = None
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| 111 | self.used_names = set() # Aliased to self.rootnode.used_names in pop()
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| 112 |
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| 113 | def addtoken(self, type, value, context):
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| 114 | """Add a token; return True iff this is the end of the program."""
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| 115 | # Map from token to label
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| 116 | ilabel = self.classify(type, value, context)
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| 117 | # Loop until the token is shifted; may raise exceptions
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| 118 | while True:
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| 119 | dfa, state, node = self.stack[-1]
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| 120 | states, first = dfa
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| 121 | arcs = states[state]
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| 122 | # Look for a state with this label
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| 123 | for i, newstate in arcs:
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| 124 | t, v = self.grammar.labels[i]
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| 125 | if ilabel == i:
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| 126 | # Look it up in the list of labels
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| 127 | assert t < 256
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| 128 | # Shift a token; we're done with it
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| 129 | self.shift(type, value, newstate, context)
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| 130 | # Pop while we are in an accept-only state
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| 131 | state = newstate
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| 132 | while states[state] == [(0, state)]:
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| 133 | self.pop()
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| 134 | if not self.stack:
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| 135 | # Done parsing!
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| 136 | return True
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| 137 | dfa, state, node = self.stack[-1]
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| 138 | states, first = dfa
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| 139 | # Done with this token
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| 140 | return False
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| 141 | elif t >= 256:
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| 142 | # See if it's a symbol and if we're in its first set
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| 143 | itsdfa = self.grammar.dfas[t]
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| 144 | itsstates, itsfirst = itsdfa
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| 145 | if ilabel in itsfirst:
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| 146 | # Push a symbol
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| 147 | self.push(t, self.grammar.dfas[t], newstate, context)
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| 148 | break # To continue the outer while loop
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| 149 | else:
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| 150 | if (0, state) in arcs:
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| 151 | # An accepting state, pop it and try something else
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| 152 | self.pop()
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| 153 | if not self.stack:
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| 154 | # Done parsing, but another token is input
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| 155 | raise ParseError("too much input",
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| 156 | type, value, context)
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| 157 | else:
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| 158 | # No success finding a transition
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| 159 | raise ParseError("bad input", type, value, context)
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| 160 |
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| 161 | def classify(self, type, value, context):
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| 162 | """Turn a token into a label. (Internal)"""
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| 163 | if type == token.NAME:
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| 164 | # Keep a listing of all used names
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| 165 | self.used_names.add(value)
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| 166 | # Check for reserved words
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| 167 | ilabel = self.grammar.keywords.get(value)
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| 168 | if ilabel is not None:
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| 169 | return ilabel
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| 170 | ilabel = self.grammar.tokens.get(type)
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| 171 | if ilabel is None:
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| 172 | raise ParseError("bad token", type, value, context)
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| 173 | return ilabel
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| 174 |
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| 175 | def shift(self, type, value, newstate, context):
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| 176 | """Shift a token. (Internal)"""
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| 177 | dfa, state, node = self.stack[-1]
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| 178 | newnode = (type, value, context, None)
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| 179 | newnode = self.convert(self.grammar, newnode)
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| 180 | if newnode is not None:
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| 181 | node[-1].append(newnode)
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| 182 | self.stack[-1] = (dfa, newstate, node)
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| 183 |
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| 184 | def push(self, type, newdfa, newstate, context):
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| 185 | """Push a nonterminal. (Internal)"""
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| 186 | dfa, state, node = self.stack[-1]
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| 187 | newnode = (type, None, context, [])
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| 188 | self.stack[-1] = (dfa, newstate, node)
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| 189 | self.stack.append((newdfa, 0, newnode))
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| 190 |
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| 191 | def pop(self):
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| 192 | """Pop a nonterminal. (Internal)"""
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| 193 | popdfa, popstate, popnode = self.stack.pop()
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| 194 | newnode = self.convert(self.grammar, popnode)
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| 195 | if newnode is not None:
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| 196 | if self.stack:
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| 197 | dfa, state, node = self.stack[-1]
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| 198 | node[-1].append(newnode)
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| 199 | else:
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| 200 | self.rootnode = newnode
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| 201 | try:
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| 202 | self.rootnode.used_names = self.used_names
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| 203 | except AttributeError:
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| 204 | # Don't need this hack?
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| 205 | pass
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| 206 |
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