Sunday, 31 March 2019

NodeVisitor class for PEG parser in Python

Imagine the following types of strings:

if ((a1 and b) or (a2 and c)) or (c and d) or (e and f)

Now, I'd like to get the expressions in parentheses, so I wrote a PEG parser with the following grammar:

from parsimonious.grammar import Grammar

grammar = Grammar(
    r"""
    program     = if expr+
    expr        = term (operator term)*
    term        = (factor operator factor) / factor
    factor      = (lpar word operator word rpar) / (lpar expr rpar)

    if          = "if" ws
    and         = "and"
    or          = "or"
    operator    = ws? (and / or) ws?

    word        = ~"\w+"
    lpar        = "("
    rpar        = ")"

    ws          = ~"\s*"
    """)

which parses just fine with

tree = grammar.parse(string)

Now the question arises: how to write a NodeVisitor class for this tree to get only the factors? My problem here is the second branch which can be deeply nested.


I tried with
def walk(node, level = 0):
    if node.expr.name == "factor":
        print(level * "-", node.text)

    for child in node.children:
        walk(child, level + 1)

walk(tree)

but to no avail, really (factors bubble up in duplicates).
Note: This question is based on another one on StackOverflow.



from NodeVisitor class for PEG parser in Python

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