calls antlr4 on grammar file

def call_antlr4(arg):
    "calls antlr4 on grammar file"
    # pylint: disable=unused-argument, unused-variable
    antlr_path = os.path.join(ROOT_DIR, "java", "antlr-4.7-complete.jar")
    classpath = os.pathsep.join([".", "{:s}".format(antlr_path), "$CLASSPATH"])
    generated = os.path.join(ROOT_DIR, 'src', 'pymoca', 'generated')
    cmd = "java -Xmx500M -cp \"{classpath:s}\" org.antlr.v4.Tool {arg:s}" \
          " -o {generated:s} -visitor -Dlanguage=Python3".format(**locals())
    print(cmd)
    proc = subprocess.Popen(cmd.split(), cwd=os.path.join(ROOT_DIR, 'src', 'pymoca'))
    proc.communicate()
    with open(os.path.join(generated, '__init__.py'), 'w') as fid:
        fid.write('')