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# Example of defining an arithmetic expression parser using
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# the operatorPrecedence helper method in pyparsing.
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# Copyright 2006, by Paul McGuire
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from pyparsing import *
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integer = Word(nums).setParseAction(lambda t:int(t[0]))
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variable = Word(alphas,exact=1)
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operand = integer | variable
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# To use the operatorPrecedence helper:
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# 1. Define the "atom" operand term of the grammar.
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# For this simple grammar, the smallest operand is either
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# and integer or a variable. This will be the first argument
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# to the operatorPrecedence method.
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# 2. Define a list of tuples for each level of operator
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# precendence. Each tuple is of the form
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# (opExpr, numTerms, rightLeftAssoc, parseAction), where
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# - opExpr is the pyparsing expression for the operator;
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# may also be a string, which will be converted to a Literal
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# - numTerms is the number of terms for this operator (must
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# - rightLeftAssoc is the indicator whether the operator is
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# right or left associative, using the pyparsing-defined
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# constants opAssoc.RIGHT and opAssoc.LEFT.
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# - parseAction is the parse action to be associated with
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# expressions matching this operator expression (the
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# parse action tuple member may be omitted)
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# 3. Call operatorPrecedence passing the operand expression and
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# the operator precedence list, and save the returned value
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# as the generated pyparsing expression. You can then use
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# this expression to parse input strings, or incorporate it
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# into a larger, more complex grammar.
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expr = operatorPrecedence( operand,
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[("!", 1, opAssoc.LEFT),
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("^", 2, opAssoc.RIGHT),
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(signop, 1, opAssoc.RIGHT),
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(multop, 2, opAssoc.LEFT),
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(plusop, 2, opAssoc.LEFT),]
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print expr.parseString(t)
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docs/examples/simpleArith.py
