Python 3

Requirements

You will need to install Python and Pip, version 3.6 or better. See https://www.python.org/downloads/ and https://www.geeksforgeeks.org/how-to-install-pip-on-windows/.

A simple example targeting Python3

An example of a parser for the Python3 target consists of the following files.

An Antlr4 grammar, e.g., Expr.g4:

antlr

grammar Expr;
start_ : expr (';' expr)* EOF;
expr : atom | ('+' | '-') expr | expr '**' expr | expr ('*' | '/') expr | expr ('+' | '-') expr | '(' expr ')' | atom ;
atom : INT ;
INT : [0-9]+ ;
WS : [ \t\n\r]+ -> skip ;

Driver.py: The driver code opens a file, creates a lexer, token stream, and parser, then calls the parser.

python

import sys
from antlr4 import *
from ExprLexer import ExprLexer
from ExprParser import ExprParser
from VisitorInterp import VisitorInterp

def main(argv):
    input_stream = FileStream(argv[1])
    lexer = ExprLexer(input_stream)
    stream = CommonTokenStream(lexer)
    parser = ExprParser(stream)
    tree = parser.start_()

if __name__ == '__main__':
    main(sys.argv)

requirements.txt: This file contains a list of the required packages for the program. Required packages are downloaded by pip. The file must include a reference to the Antlr Python3 runtime.
```
antlr4-python3-runtime==4.13.0
```
A build script, e.g., build.sh: You should provide a script that builds the program.
```
pip install -r requirements.txt
antlr4 -Dlanguage=Python3 Expr.g4
```

It is vital that the versions for the Antlr tool used to generate the parser and the Antlr Python3 runtime match. E.g., 4.13.0. Using build files will help eliminate common errors from happening.

For a list of antlr4 tool options, please visit ANTLR Tool Command Line Options.

Input, e.g., input.txt:
```
-(1 + 2)/3;
1;
2+3;
8*9
```
A run script, which runs your program.
```
python Driver.py input.txt
```

Parse tree traversal

Tree traversal is used to implement static or dynamic program analysis. Antlr generates two types of tree traversals: visitors and listeners.

Understanding when to choose a visitor versus a listener is a good idea. For further information, see https://tomassetti.me/listeners-and-visitors/.

A visitor is the best choice when computing only a single synthesized attribute or when you want to control the order of parse tree nodes visited. Alternatively, a listener is the best choice when computing both synthesized and inherited attributes.

In many situations, they are interchangeable.

Visitors

Antlr visitors generally implement a post-order tree walk. If you write visit... methods, the method must contain code to visit the children in the order you want. For a post-order tree walk, visit the children first.

To implement a visitor, add the -visitor option to the antlr4 command. Create a class that inherits from the generated visitor, then add visit methods that implement the analysis. Your driver code should call the visit() method for the root of the parse tree.

For example, the following code implements an expression evaluator for the Expr.g4 grammar using a visitor.

Driver.py:

python

import sys
from antlr4 import *
from ExprLexer import ExprLexer
from ExprParser import ExprParser
from VisitorInterp import VisitorInterp

def main(argv):
    input_stream = FileStream(argv[1])
    lexer = ExprLexer(input_stream)
    stream = CommonTokenStream(lexer)
    parser = ExprParser(stream)
    tree = parser.start_()
    if parser.getNumberOfSyntaxErrors() > 0:
        print("syntax errors")
    else:
        vinterp = VisitorInterp()
        vinterp.visit(tree)

if __name__ == '__main__':
    main(sys.argv)

VisitorInterp.py:

python

import sys
from antlr4 import *
from ExprParser import ExprParser
from ExprVisitor import ExprVisitor

class VisitorInterp(ExprVisitor):
    def visitAtom(self, ctx:ExprParser.AtomContext):
        return int(ctx.getText())

    def visitExpr(self, ctx:ExprParser.ExprContext):
        if ctx.getChildCount() == 3:
            if ctx.getChild(0).getText() == "(":
                return self.visit(ctx.getChild(1))
            op = ctx.getChild(1).getText()
            v1 = self.visit(ctx.getChild(0))
            v2 = self.visit(ctx.getChild(2))
            if op == "+":
                return v1 + v2
            if op == "-":
                return v1 - v2
            if op == "*":
                return v1 * v2
            if op == "/":
                return v1 / v2
            return 0
        if ctx.getChildCount() == 2:
            opc = ctx.getChild(0).getText()
            if opc == "+":
                return self.visit(ctx.getChild(1))
            if opc == "-":
                return - self.visit(ctx.getChild(1))
            return 0
        if ctx.getChildCount() == 1:
            return self.visit(ctx.getChild(0))
        return 0

    def visitStart_(self, ctx:ExprParser.Start_Context):
        for i in range(0, ctx.getChildCount(), 2):
            print(self.visit(ctx.getChild(i)))
        return 0

Listeners

Antlr listeners perform an LR tree traversal. enter and exit methods are called during the tranversal. A parse tree node is visited twice, first for the enter method, then the exit method after all children have been walked.

To implement a listener, add the -listener option to the antlr4 command. Add a class that inherits from the generated listener with code that implements the analysis.

The following example implements an expression evaluator using a listener.

Driver.py:

python

import sys
from antlr4 import *
from ExprLexer import ExprLexer
from ExprParser import ExprParser
from ListenerInterp import ListenerInterp

def main(argv):
    input_stream = FileStream(argv[1])
    lexer = ExprLexer(input_stream)
    stream = CommonTokenStream(lexer)
    parser = ExprParser(stream)
    tree = parser.start_()
    if parser.getNumberOfSyntaxErrors() > 0:
        print("syntax errors")
    else:
        linterp = ListenerInterp()
        walker = ParseTreeWalker()
        walker.walk(linterp, tree)

if __name__ == '__main__':
    main(sys.argv)

ListenerInterp.py:

python

import sys
from antlr4 import *
from ExprParser import ExprParser
from ExprListener import ExprListener

class ListenerInterp(ExprListener):
    def __init__(self):
        self.result = {}

    def exitAtom(self, ctx:ExprParser.AtomContext):
        self.result[ctx] = int(ctx.getText())

    def exitExpr(self, ctx:ExprParser.ExprContext):
        if ctx.getChildCount() == 3:
            if ctx.getChild(0).getText() == "(":
                self.result[ctx] = self.result[ctx.getChild(1)]
            else:
                opc = ctx.getChild(1).getText()
                v1 = self.result[ctx.getChild(0)]
                v2 = self.result[ctx.getChild(2)]
                if opc == "+":
                    self.result[ctx] = v1 + v2
                elif opc == "-":
                    self.result[ctx] = v1 - v2
                elif opc == "*":
                    self.result[ctx] = v1 * v2
                elif opc == "/":
                    self.result[ctx] = v1 / v2
                else:
                    ctx.result[ctx] = 0
        elif ctx.getChildCount() == 2:
            opc = ctx.getChild(0).getText()
            if opc == "+":
                v = self.result[ctx.getChild(1)]
                self.result[ctx] = v
            elif opc == "-":
                v = self.result[ctx.getChild(1)]
                self.result[ctx] = - v
        elif ctx.getChildCount() == 1:
            self.result[ctx] = self.result[ctx.getChild(0)]

    def exitStart_(self, ctx:ExprParser.Start_Context):
        for i in range(0, ctx.getChildCount(), 2):
            print(self.result[ctx.getChild(i)])

Further information can be found from the ANTLR 4 definitive guide.

Examples

The examples from the ANTLR 4 book converted to Python are here.

There are many examples of grammars that target the Python3 target in the grammars-v4 Github repository.