ftrace in python

#!/usr/bin/env python

import argparse
import os

t = "/sys/kernel/debug/tracing"

def write_to(base, name, data):
    with open(os.path.join(base, name), "w") as f:
        f.write(data)

def enable_ftrace(args):
    write_to(t, "current_tracer", "function_graph")
    write_to(t, "set_graph_function", "vfs_fstat")
    write_to(t, "tracing_on", "1")

def disable_ftrace(args):
    write_to(t, "tracing_on", "0")

if __name__=="__main__":


    cmds = { "enable": enable_ftrace,
             "disable": disable_ftrace,
    }

    def not_a_command(args):
        print("unkown command")

    parser = argparse.ArgumentParser()
    parser.add_argument("cmd")
    args = parser.parse_args()
    c = cmds.get(args.cmd, not_a_command)
    c(args)

while executing this "python ftrace_in_python.py ls", I am getting unknown command. I am willing to ftrace the "ls" command.

Little help will be appreciated, thanks in advance.