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# eval_arith.py
#
# Copyright 2009, 2011 Paul McGuire
#
# Expansion on the pyparsing example simpleArith.py, to include evaluation
# of the parsed tokens.
#
# Added support for exponentiation, using right-to-left evaluation of
# operands
#
from pyparsing import (
Word,
nums,
alphas,
Combine,
oneOf,
opAssoc,
infixNotation,
Literal,
ParserElement,
)
ParserElement.enablePackrat()
class EvalConstant:
"Class to evaluate a parsed constant or variable"
vars_ = {}
def __init__(self, tokens):
self.value = tokens[0]
def eval(self):
if self.value in EvalConstant.vars_:
return EvalConstant.vars_[self.value]
else:
return float(self.value)
class EvalSignOp:
"Class to evaluate expressions with a leading + or - sign"
def __init__(self, tokens):
self.sign, self.value = tokens[0]
def eval(self):
mult = {"+": 1, "-": -1}[self.sign]
return mult * self.value.eval()
def operatorOperands(tokenlist):
"generator to extract operators and operands in pairs"
it = iter(tokenlist)
while 1:
try:
yield (next(it), next(it))
except StopIteration:
break
class EvalPowerOp:
"Class to evaluate multiplication and division expressions"
def __init__(self, tokens):
self.value = tokens[0]
def eval(self):
res = self.value[-1].eval()
for val in self.value[-3::-2]:
res = val.eval() ** res
return res
class EvalMultOp:
"Class to evaluate multiplication and division expressions"
def __init__(self, tokens):
self.value = tokens[0]
def eval(self):
prod = self.value[0].eval()
for op, val in operatorOperands(self.value[1:]):
if op == "*":
prod *= val.eval()
if op == "/":
prod /= val.eval()
return prod
class EvalAddOp:
"Class to evaluate addition and subtraction expressions"
def __init__(self, tokens):
self.value = tokens[0]
def eval(self):
sum = self.value[0].eval()
for op, val in operatorOperands(self.value[1:]):
if op == "+":
sum += val.eval()
if op == "-":
sum -= val.eval()
return sum
class EvalComparisonOp:
"Class to evaluate comparison expressions"
opMap = {
"<": lambda a, b: a < b,
"<=": lambda a, b: a <= b,
">": lambda a, b: a > b,
">=": lambda a, b: a >= b,
"!=": lambda a, b: a != b,
"=": lambda a, b: a == b,
"LT": lambda a, b: a < b,
"LE": lambda a, b: a <= b,
"GT": lambda a, b: a > b,
"GE": lambda a, b: a >= b,
"NE": lambda a, b: a != b,
"EQ": lambda a, b: a == b,
"<>": lambda a, b: a != b,
}
def __init__(self, tokens):
self.value = tokens[0]
def eval(self):
val1 = self.value[0].eval()
for op, val in operatorOperands(self.value[1:]):
fn = EvalComparisonOp.opMap[op]
val2 = val.eval()
if not fn(val1, val2):
break
val1 = val2
else:
return True
return False
# define the parser
integer = Word(nums)
real = Combine(Word(nums) + "." + Word(nums))
variable = Word(alphas, exact=1)
operand = real | integer | variable
signop = oneOf("+ -")
multop = oneOf("* /")
plusop = oneOf("+ -")
expop = Literal("**")
# use parse actions to attach EvalXXX constructors to sub-expressions
operand.setParseAction(EvalConstant)
arith_expr = infixNotation(
operand,
[
(signop, 1, opAssoc.RIGHT, EvalSignOp),
(expop, 2, opAssoc.LEFT, EvalPowerOp),
(multop, 2, opAssoc.LEFT, EvalMultOp),
(plusop, 2, opAssoc.LEFT, EvalAddOp),
],
)
comparisonop = oneOf("< <= > >= != = <> LT GT LE GE EQ NE")
comp_expr = infixNotation(
arith_expr,
[
(comparisonop, 2, opAssoc.LEFT, EvalComparisonOp),
],
)
# sample expressions posted on comp.lang.python, asking for advice
# in safely evaluating them
rules = [
"( A - B ) = 0",
"( B - C + B ) = 0",
"(A + B + C + D + E + F + G + H + I) = J",
"(A + B + C + D + E + F + G + H) = I",
"(A + B + C + D + E + F) = G",
"(A + B + C + D + E) = (F + G + H + I + J)",
"(A + B + C + D + E) = (F + G + H + I)",
"(A + B + C + D + E) = F",
"(A + B + C + D) = (E + F + G + H)",
"(A + B + C) = D",
"(A + B + C) = (D + E + F)",
"(A + B) = (C + D + E + F)",
"(A + B) = (C + D)",
"(A + B) = (C - D + E - F - G + H + I + J)",
"(A + B) = C",
"(A + B) = 0",
"(A+B+C+D+E) = (F+G+H+I+J)",
"(A+B+C+D) = (E+F+G+H)",
"(A+B+C+D)=(E+F+G+H)",
"(A+B+C)=(D+E+F)",
"(A+B)=(C+D)",
"(A+B)=C",
"(A-B)=C",
"(A/(B+C))",
"(B/(C+D))",
"(G + H) = I",
"-0.99 LE ((A+B+C)-(D+E+F+G)) LE 0.99",
"-0.99 LE (A-(B+C)) LE 0.99",
"-1000.00 LE A LE 0.00",
"-5000.00 LE A LE 0.00",
"A < B",
"A < 7000",
"A = -(B)",
"A = C",
"A = 0",
"A GT 0",
"A GT 0.00",
"A GT 7.00",
"A LE B",
"A LT -1000.00",
"A LT -5000",
"A LT 0",
"G=(B+C+D)",
"A=B",
"I = (G + H)",
"0.00 LE A LE 4.00",
"4.00 LT A LE 7.00",
"0.00 LE A LE 4.00 LE E > D",
"2**2**(A+3)",
]
vars_ = {
"A": 0,
"B": 1.1,
"C": 2.2,
"D": 3.3,
"E": 4.4,
"F": 5.5,
"G": 6.6,
"H": 7.7,
"I": 8.8,
"J": 9.9,
}
# define tests from given rules
tests = []
for t in rules:
t_orig = t
t = t.replace("=", "==")
t = t.replace("EQ", "==")
t = t.replace("LE", "<=")
t = t.replace("GT", ">")
t = t.replace("LT", "<")
t = t.replace("GE", ">=")
t = t.replace("LE", "<=")
t = t.replace("NE", "!=")
t = t.replace("<>", "!=")
tests.append((t_orig, eval(t, vars_)))
# copy vars_ to EvalConstant lookup dict
EvalConstant.vars_ = vars_
failed = 0
for test, expected in tests:
ret = comp_expr.parseString(test)[0]
parsedvalue = ret.eval()
print(test, expected, parsedvalue)
if abs(parsedvalue - expected) > 1e-6:
print("<<< FAIL")
failed += 1
else:
print("")
print("")
if failed:
raise Exception("could not parse")
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