ANTLR: Generating Parse Trees using Python

Question

I'm new to ANTLR and currently I'm trying to use ANTLR 3.1.3 with Python. I've already installed successfully ANTLR runtime for Python. But I don't know how to print out the parse tree of a particular input.

Grammar

grammar Expr;

options {
    language=Python;
    output=AST;
}

expr    :   atom (op atom)* NEWLINE ;
op  :   '+' | '-';
atom    :   ID | INT;
ID  :   ('a'..'z'|'A'..'Z')+ ;  
INT     :   ('0'..'9')+ ;
NEWLINE :   '\r'? '\n' ;
WS  :   (' '|'\t'|'\n'|'\r')+ {self.skip()} ;

Python code

import sys
import antlr3
from antlr3 import *
from ExprLexer import ExprLexer
from ExprParser import ExprParser

# test the parser with an input
char_stream = antlr3.ANTLRStringStream('3+5\n')
lexer = ExprLexer(char_stream)
tokens = antlr3.CommonTokenStream(lexer)
parser = ExprParser(tokens)

# print the parse tree
t = parser.expr().tree
print t.toStringTree()

Although what I want when the code's executed is the parse tree. The code will print out '3+5' only.

Would you please tell me how to modify the code to get the parse tree printed?

Alessandro Mariotti Alessandro Mariotti · Accepted Answer · 2014-07-03T08:32:03

Some weeks ago, I wanted to print out the ANTLR AST (not the parse tree, but it depends on how you implemented your grammar..) for debugging purposes.

You can do something very easy like:

def print_tree(tree, lev):
    print (" " * lev) + "` " + str(tree)
    for c in tree.getChildren():
        print_tree(c, lev + 1)

print_tree(parser.expr().tree, 0)

NB: I've written the code directly in SO, so it is untested

ANTLR: Generating Parse Trees using Python

1 Answers