Mercurial > hg > expressionparser

annotate expr.py @ 11:e17a3464a0b9

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get precedence from a list position vs magic constants
author Jeff Hammel <jhammel@mozilla.com>
date Fri, 03 Jun 2011 10:51:36 -0700
parents 15fb1081784f
children 835efd8acb04
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1 #!/usr/bin/env python
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2
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3 # Implements a top-down parser/evaluator for simple boolean expressions.
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4 # ideas taken from http://effbot.org/zone/simple-top-down-parsing.htm
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5 #
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6 # Rough grammar:
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7 # expr := literal
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8 # | '(' expr ')'
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9 # | expr '&&' expr
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10 # | expr '||' expr
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11 # | expr '==' expr
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12 # | expr '!=' expr
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13 # literal := BOOL
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14 # | INT
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15 # | STRING
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16 # | IDENT
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17 # BOOL := true|false
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18 # INT := [0-9]+
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19 # STRING := "[^"]*"
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20 # IDENT := [A-Za-z_]\w*
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21
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22 # Identifiers take their values from a mapping dictionary passed as the second
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23 # argument.
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24
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25 __all__ = ['parse', 'ParseError', 'ExpressionParser']
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26 import re
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27
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28 # token classes
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29 class token(object):
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30 pass
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31
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32 class ident_token(object):
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33 def __init__(self, value):
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34 self.value = value
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35 def nud(self, parser):
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36 # identifiers take their value from the value mappings passed
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37 # to the parser
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38 return parser.value(self.value)
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39
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40 class literal_token(object):
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41 def __init__(self, value):
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42 self.value = value
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43 def nud(self, parser):
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44 return self.value
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45
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46 class eq_op_token(object):
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47 "=="
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48 def led(self, parser, left):
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49 return left == parser.expression(self.lbp)
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50
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51 class neq_op_token(object):
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52 "!="
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53 def led(self, parser, left):
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54 return left != parser.expression(self.lbp)
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55
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56 class and_op_token(object):
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57 "&&"
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58 def led(self, parser, left):
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59 right = parser.expression(self.lbp)
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60 return left and right
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61
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62 class or_op_token(object):
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63 "||"
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64 def led(self, parser, left):
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65 right = parser.expression(self.lbp)
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66 return left or right
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67
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68 class lparen_token(object):
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69 "("
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70 def nud(self, parser):
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71 expr = parser.expression()
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72 parser.advance(rparen_token)
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73 return expr
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74
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75 class rparen_token(object):
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76 ")"
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77
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78 class end_token(object):
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79 """always ends parsing"""
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80
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81 ### derived literal tokens
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82
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83 class bool_token(literal_token):
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84 def __init__(self, value):
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85 value = {'true':True, 'false':False}[value]
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86 literal_token.__init__(self, value)
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87
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88 class int_token(literal_token):
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89 def __init__(self, value):
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90 literal_token.__init__(self, int(value))
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91
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92 class string_token(literal_token):
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93 def __init__(self, value):
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94 literal_token.__init__(self, value[1:-1])
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95
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96 precedence = [(end_token, rparen_token),
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97 (or_op_token,),
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98 (and_op_token,),
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99 (eq_op_token, neq_op_token),
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100 (lparen_token,),
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101 ]
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102 for index, rank in enumerate(precedence):
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103 for token in rank:
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104 token.lbp = index # lbp = lowest left binding power
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105
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106 class ParseError(Exception):
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107 """errror parsing conditional expression"""
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108
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109 class ExpressionParser(object):
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110 def __init__(self, text, valuemapping):
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111 """
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112 Initialize the parser with input |text|, and |valuemapping| as
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113 a dict mapping identifier names to values.
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114 """
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115 self.text = text
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116 self.valuemapping = valuemapping
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117
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118 def _tokenize(self):
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119 """
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120 Lex the input text into tokens and yield them in sequence.
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121 """
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122 # scanner callbacks
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123 def bool_(scanner, t): return bool_token(t)
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124 def identifier(scanner, t): return ident_token(t)
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125 def integer(scanner, t): return int_token(t)
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126 def eq(scanner, t): return eq_op_token()
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127 def neq(scanner, t): return neq_op_token()
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128 def or_(scanner, t): return or_op_token()
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129 def and_(scanner, t): return and_op_token()
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130 def lparen(scanner, t): return lparen_token()
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131 def rparen(scanner, t): return rparen_token()
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132 def string_(scanner, t): return string_token(t)
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133
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134 scanner = re.Scanner([
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135 (r"true|false", bool_),
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136 (r"[a-zA-Z_]\w*", identifier),
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137 (r"[0-9]+", integer),
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138 (r'"[^"]*"', string_),
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139 (r"==", eq),
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140 (r"!=", neq),
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141 (r"\|\|", or_),
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142 (r"&&", and_),
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143 (r"\(", lparen),
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144 (r"\)", rparen),
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145 (r"\s+", None), # skip whitespace
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146 ])
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147 tokens, remainder = scanner.scan(self.text)
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148 for t in tokens:
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149 yield t
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150 yield end_token()
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151
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152 def value(self, ident):
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153 """
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154 Look up the value of |ident| in the value mapping passed in the
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155 constructor.
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156 """
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157 return self.valuemapping[ident]
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158
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159 def advance(self, expected):
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160 """
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161 Assert that the next token is an instance of |expected|, and advance
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162 to the next token.
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163 """
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164 if not isinstance(self.token, expected):
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165 raise Exception, "Unexpected token!"
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166 self.token = self.iter.next()
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167
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168 def expression(self, rbp=0):
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169 """
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170 Parse and return the value of an expression until a token with
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171 right binding power greater than rbp is encountered.
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172 """
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173 t = self.token
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174 self.token = self.iter.next()
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175 left = t.nud(self)
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176 while rbp < self.token.lbp:
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177 t = self.token
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178 self.token = self.iter.next()
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179 left = t.led(self, left)
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180 return left
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181
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182 def parse(self):
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183 """
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184 Parse and return the value of the expression in the text
2
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185 passed to the constructor. Raises a ParseError if the expression
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186 could not be parsed.
0
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187 """
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188 try:
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189 self.iter = self._tokenize()
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190 self.token = self.iter.next()
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191 return self.expression()
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192 except:
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193 raise ParseError("could not parse: %s" % self.text)
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194
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195 __call__ = parse
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196
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197 def parse(text, **values):
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198 """
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199 Parse and evaluate a boolean expression in |text|. Use |values| to look
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200 up the value of identifiers referenced in the expression. Returns the final
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201 value of the expression. A ParseError will be raised if parsing fails.
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202 """
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203 return ExpressionParser(text, values).parse()