pyparsing的递归表达式

小编典典

pyparsing的递归表达式

python

我试图弄清楚如何在可能进行递归（未包含任何内容）的表达式中执行左关联表达式。例如，我想做：

expr + OP + expr

将2种操作解析1 x 2 x 3为(expr OP expr) OP expr结果。

如果我尝试防止expr无限递归进行解析，则可以执行以下操作：

expr -> Group(simple_expr + OP + expr)
      | simple_expr

但是我会得到expr OP (expr OR expr)结果。

如何强制左侧装订？

编辑：我知道关于，operatorPrecedence但当运算符是"IS" + Optional("NOT")或类似时，它似乎不正确匹配。

阅读 247

2021-01-20

共1个答案

小编典典

这是一个解析动作的示例，该动作将采用标记的平面列表并将其嵌套，就像递归左解析一样：

from pyparsing import *

# parse action -maker
def makeLRlike(numterms):
    if numterms is None:
        # None operator can only by binary op
        initlen = 2
        incr = 1
    else:
        initlen = {0:1,1:2,2:3,3:5}[numterms]
        incr = {0:1,1:1,2:2,3:4}[numterms]

    # define parse action for this number of terms,
    # to convert flat list of tokens into nested list
    def pa(s,l,t):
        t = t[0]
        if len(t) > initlen:
            ret = ParseResults(t[:initlen])
            i = initlen
            while i < len(t):
                ret = ParseResults([ret] + t[i:i+incr])
                i += incr
            return ParseResults([ret])
    return pa


# setup a simple grammar for 4-function arithmetic
varname = oneOf(list(alphas))
integer = Word(nums)
operand = integer | varname

# ordinary opPrec definition
arith1 = operatorPrecedence(operand,
    [
    (None, 2, opAssoc.LEFT),
    (oneOf("* /"), 2, opAssoc.LEFT),
    (oneOf("+ -"), 2, opAssoc.LEFT),
    ])

# opPrec definition with parseAction makeLRlike
arith2 = operatorPrecedence(operand,
    [
    (None, 2, opAssoc.LEFT, makeLRlike(None)),
    (oneOf("* /"), 2, opAssoc.LEFT, makeLRlike(2)),
    (oneOf("+ -"), 2, opAssoc.LEFT, makeLRlike(2)),
    ])

# parse a few test strings, using both parsers
for arith in (arith1, arith2):
    print arith.parseString("A+B+C+D+E")[0]
    print arith.parseString("A+B+C*D+E")[0]
    print arith.parseString("12AX+34BY+C*5DZ+E")[0]

印刷品：

（正常）

['A', '+', 'B', '+', 'C', '+', 'D', '+', 'E']
['A', '+', 'B', '+', ['C', '*', 'D'], '+', 'E']
[['12', 'A', 'X'], '+', ['34', 'B', 'Y'], '+', ['C', '*', ['5', 'D', 'Z']], '+', 'E']

（像LR）

[[[['A', '+', 'B'], '+', 'C'], '+', 'D'], '+', 'E']
[[['A', '+', 'B'], '+', ['C', '*', 'D']], '+', 'E']
[[[[['12', 'A'], 'X'], '+', [['34', 'B'], 'Y']], '+', ['C', '*', [['5', 'D'], 'Z']]], '+', 'E']

2021-01-20