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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