Parsing logical sentence very slow with pyparsing

问题

I try to use pyparsing to parse logical expressions such as these

x
FALSE
NOT x
(x + y <= 5) AND (y >= 10) OR NOT (z < 100 OR w)

(A=True OR NOT (G < 8) => S = J) => ((P = A) AND not (P = 1) AND (B = O)) => (S = T)

((P = T) AND NOT (K =J) AND (B = F)) => (S = O) AND
 ((P = T) OR (k and b => (8 + z <= 10)) AND NOT (a + 9 <= F)) => (7 = a + z)

The code I wrote below seems to work OK -- but it is very slow (e.g. the last example above takes a few seconds). Did I structure the grammar in some inefficient way? may be recursion should be used instead of operatorPrecedence ? Is there a way to speed it up ?

identifier = Group(Word(alphas, alphanums + "_")  +  Optional("'"))
num = Regex(r"[+-]?\d+(:?\.\d*)?(:?[eE][+-]?\d+)?")
operator = Regex(">=|<=|!=|>|<|=")
operand = identifier |  num  
aexpr = operatorPrecedence(operand,
                           [('*',2,opAssoc.LEFT,),
                            ('+',2,opAssoc.LEFT,),
                            (operator,2,opAssoc.LEFT,)
                            ])

op_prec = [(CaselessLiteral('not'),1,opAssoc.RIGHT,),
           (CaselessLiteral('and'),2,opAssoc.LEFT ,),
           (CaselessLiteral('or'), 2,opAssoc.LEFT ,),
           ('=>', 2,opAssoc.LEFT ,),
           ]
sentence = operatorPrecedence(aexpr,op_prec)
return sentence

回答1:

I had the same problem. Found a solution here (parserElement.enablePackrat()): https://github.com/pyparsing/pyparsing

The following code is now parsed instantly (vs 60 sec before)

ParserElement.enablePackrat()

integer  = Word(nums).setParseAction(lambda t:int(t[0]))('int')
operand  = integer | variable('var')

# Left precedence
eq    = Literal("==")('eq')
gt    = Literal(">")('gt')
gtEq  = Literal(">=")('gtEq')
lt    = Literal("<")('lt')
ltEq  = Literal("<=")('ltEq')
notEq = Literal("!=")('notEq')
mult  = oneOf('* /')('mult')
plus  = oneOf('+ -')('plus')

_and  = oneOf('&& and')('and')
_or   = oneOf('|| or')('or')

# Right precedence
sign     = oneOf('+ -')('sign')
negation = Literal('!')('negation')

# Operator groups per presedence
right_op = negation | sign 

# Highest precedence
left_op_1 = mult 
left_op_2 = plus 
left_op_3 = gtEq | ltEq | lt | gt
left_op_4 = eq   | notEq
left_op_5 = _and
left_op_6 = _or
# Lowest precedence

condition = operatorPrecedence( operand, [
     (right_op,   1, opAssoc.RIGHT),
     (left_op_1,  2, opAssoc.LEFT),
     (left_op_2,  2, opAssoc.LEFT),
     (left_op_3,  2, opAssoc.LEFT),
     (left_op_4,  2, opAssoc.LEFT),
     (left_op_5,  2, opAssoc.LEFT),
     (left_op_6,  2, opAssoc.LEFT)
    ]
)('computation')

回答2:

I put your code into a small program

from sys import argv
from pyparsing import *

def parsit(aexpr):
    identifier = Group(Word(alphas, alphanums + "_")  +  Optional("'"))
    num = Regex(r"[+-]?\d+(:?\.\d*)?(:?[eE][+-]?\d+)?")
    operator = Regex(">=|<=|!=|>|<|=")
    operand = identifier |  num
    aexpr = operatorPrecedence(operand,
                               [('*',2,opAssoc.LEFT,),
                                ('+',2,opAssoc.LEFT,),
                                (operator,2,opAssoc.LEFT,)
                                ])

    op_prec = [(CaselessLiteral('not'),1,opAssoc.RIGHT,),
               (CaselessLiteral('and'),2,opAssoc.LEFT ,),
               (CaselessLiteral('or'), 2,opAssoc.LEFT ,),
               ('=>', 2,opAssoc.LEFT ,),
               ]
    sentence = operatorPrecedence(aexpr,op_prec)
    return sentence

def demo02(arg):
    sent = parsit(arg)
    print arg, ":", sent.parseString(arg)

def demo01():
    for arg in ["x", "FALSE", "NOT x",
                  "(x + y <= 5) AND (y >= 10) OR NOT (z < 100 OR w)",
                  "(A=True OR NOT (G < 8) => S = J) => ((P = A) AND not (P = 1) AND (B = O)) => (S = T)",
                  "((P = T) AND NOT (K =J) AND (B = F)) => (S = O) AND ((P = T) OR (k and b => (8 + z <= 10)) AND NOT (a + 9 <= F)) => (7 = a + z)"
                  ]:
        demo02(arg)


if len(argv) <= 1:
    demo01()
else:
    for arg in argv[1:]:
        demo02(arg)

and ran through cProfile

$ python -m cProfile pyparsetest.py

You will find many parseImpl calls, but in the middle of the output there is

2906500/8   26.374    0.000   72.667    9.083 pyparsing.py:913(_parseNoCache)
212752/300    1.045    0.000   72.608    0.242 pyparsing.py:985(tryParse)

the 72.667 beeing the comulated time from 72 total.

Therefore I would venture the guess that "caching" would offer a good lever.

Just enabling http://pyparsing-public.wikispaces.com/FAQs did not help, thoug. I added the lines

import pyparsing
pyparsing.usePackrat = True

and the runtime was the same.

The Number-Regex also looks fine to me -- quite standard, I guess. For example replacing it with

#num = Regex(r"[+-]?\d+(:?\.\d*)?(:?[eE][+-]?\d+)?")
num = Regex(r"8|1|10|100|5")

also did not help. There is no "empty match" in my simple variant, which I guessed might be an issue -- but it seems not.

Last try is to look at the result parser with:

....
sentence = operatorPrecedence(aexpr,op_prec)
print sentence 
return sentence
....

And... whow... long!

Well, and not using your first operatorPrecedence is a lot faster, but doesn't work anymore for arithmetics.

Thus, I would venture the guess that, yes, try to seperate the two kinds of expressions (boolean and arithmetic) more. Maybe that will improve it. I will look into it too, it interests me as well.

来源：https://stackoverflow.com/questions/13277207/parsing-logical-sentence-very-slow-with-pyparsing

标签

python

pyparsing