202 lines
8.1 KiB
Python
202 lines
8.1 KiB
Python
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# Copyright 2004-2005 Elemental Security, Inc. All Rights Reserved.
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# Licensed to PSF under a Contributor Agreement.
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"""Parser engine for the grammar tables generated by pgen.
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The grammar table must be loaded first.
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See Parser/parser.c in the Python distribution for additional info on
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how this parsing engine works.
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"""
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# Local imports
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from . import token
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class ParseError(Exception):
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"""Exception to signal the parser is stuck."""
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def __init__(self, msg, type, value, context):
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Exception.__init__(self, "%s: type=%r, value=%r, context=%r" %
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(msg, type, value, context))
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self.msg = msg
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self.type = type
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self.value = value
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self.context = context
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class Parser(object):
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"""Parser engine.
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The proper usage sequence is:
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p = Parser(grammar, [converter]) # create instance
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p.setup([start]) # prepare for parsing
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<for each input token>:
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if p.addtoken(...): # parse a token; may raise ParseError
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break
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root = p.rootnode # root of abstract syntax tree
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A Parser instance may be reused by calling setup() repeatedly.
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A Parser instance contains state pertaining to the current token
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sequence, and should not be used concurrently by different threads
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to parse separate token sequences.
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See driver.py for how to get input tokens by tokenizing a file or
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string.
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Parsing is complete when addtoken() returns True; the root of the
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abstract syntax tree can then be retrieved from the rootnode
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instance variable. When a syntax error occurs, addtoken() raises
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the ParseError exception. There is no error recovery; the parser
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cannot be used after a syntax error was reported (but it can be
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reinitialized by calling setup()).
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"""
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def __init__(self, grammar, convert=None):
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"""Constructor.
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The grammar argument is a grammar.Grammar instance; see the
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grammar module for more information.
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The parser is not ready yet for parsing; you must call the
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setup() method to get it started.
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The optional convert argument is a function mapping concrete
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syntax tree nodes to abstract syntax tree nodes. If not
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given, no conversion is done and the syntax tree produced is
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the concrete syntax tree. If given, it must be a function of
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two arguments, the first being the grammar (a grammar.Grammar
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instance), and the second being the concrete syntax tree node
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to be converted. The syntax tree is converted from the bottom
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up.
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A concrete syntax tree node is a (type, value, context, nodes)
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tuple, where type is the node type (a token or symbol number),
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value is None for symbols and a string for tokens, context is
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None or an opaque value used for error reporting (typically a
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(lineno, offset) pair), and nodes is a list of children for
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symbols, and None for tokens.
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An abstract syntax tree node may be anything; this is entirely
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up to the converter function.
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"""
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self.grammar = grammar
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self.convert = convert or (lambda grammar, node: node)
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def setup(self, start=None):
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"""Prepare for parsing.
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This *must* be called before starting to parse.
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The optional argument is an alternative start symbol; it
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defaults to the grammar's start symbol.
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You can use a Parser instance to parse any number of programs;
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each time you call setup() the parser is reset to an initial
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state determined by the (implicit or explicit) start symbol.
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"""
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if start is None:
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start = self.grammar.start
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# Each stack entry is a tuple: (dfa, state, node).
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# A node is a tuple: (type, value, context, children),
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# where children is a list of nodes or None, and context may be None.
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newnode = (start, None, None, [])
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stackentry = (self.grammar.dfas[start], 0, newnode)
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self.stack = [stackentry]
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self.rootnode = None
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self.used_names = set() # Aliased to self.rootnode.used_names in pop()
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def addtoken(self, type, value, context):
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"""Add a token; return True iff this is the end of the program."""
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# Map from token to label
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ilabel = self.classify(type, value, context)
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# Loop until the token is shifted; may raise exceptions
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while True:
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dfa, state, node = self.stack[-1]
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states, first = dfa
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arcs = states[state]
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# Look for a state with this label
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for i, newstate in arcs:
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t, v = self.grammar.labels[i]
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if ilabel == i:
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# Look it up in the list of labels
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assert t < 256
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# Shift a token; we're done with it
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self.shift(type, value, newstate, context)
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# Pop while we are in an accept-only state
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state = newstate
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while states[state] == [(0, state)]:
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self.pop()
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if not self.stack:
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# Done parsing!
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return True
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dfa, state, node = self.stack[-1]
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states, first = dfa
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# Done with this token
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return False
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elif t >= 256:
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# See if it's a symbol and if we're in its first set
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itsdfa = self.grammar.dfas[t]
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itsstates, itsfirst = itsdfa
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if ilabel in itsfirst:
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# Push a symbol
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self.push(t, self.grammar.dfas[t], newstate, context)
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break # To continue the outer while loop
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else:
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if (0, state) in arcs:
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# An accepting state, pop it and try something else
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self.pop()
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if not self.stack:
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# Done parsing, but another token is input
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raise ParseError("too much input",
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type, value, context)
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else:
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# No success finding a transition
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raise ParseError("bad input", type, value, context)
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def classify(self, type, value, context):
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"""Turn a token into a label. (Internal)"""
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if type == token.NAME:
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# Keep a listing of all used names
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self.used_names.add(value)
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# Check for reserved words
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ilabel = self.grammar.keywords.get(value)
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if ilabel is not None:
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return ilabel
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ilabel = self.grammar.tokens.get(type)
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if ilabel is None:
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raise ParseError("bad token", type, value, context)
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return ilabel
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def shift(self, type, value, newstate, context):
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"""Shift a token. (Internal)"""
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dfa, state, node = self.stack[-1]
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newnode = (type, value, context, None)
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newnode = self.convert(self.grammar, newnode)
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if newnode is not None:
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node[-1].append(newnode)
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self.stack[-1] = (dfa, newstate, node)
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def push(self, type, newdfa, newstate, context):
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"""Push a nonterminal. (Internal)"""
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dfa, state, node = self.stack[-1]
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newnode = (type, None, context, [])
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self.stack[-1] = (dfa, newstate, node)
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self.stack.append((newdfa, 0, newnode))
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def pop(self):
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"""Pop a nonterminal. (Internal)"""
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popdfa, popstate, popnode = self.stack.pop()
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newnode = self.convert(self.grammar, popnode)
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if newnode is not None:
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if self.stack:
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dfa, state, node = self.stack[-1]
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node[-1].append(newnode)
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else:
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self.rootnode = newnode
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self.rootnode.used_names = self.used_names
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