pySerial write() works fine in Python interpreter, but not Python script

Recently, I am trying to make sort of "light control" on Arduino. I use Raspberry Pi to send the control message via serial port (USB cable).Here is the Arduino code :

int redled = 12;
int whiteled = 48;

void setup()
{
    Serial.begin(9600);
    pinMode(redled,OUTPUT);
    pinMode(whiteled,OUTPUT);
}

void loop()
{
    if(Serial.available())
    {
        char cmd = Serial.read();
        switch(cmd)
        {
            case'r':
            digitalWrite(redled,HIGH);
            delay(2000);
            digitalWrite(redled,LOW);
            break;

            case'w':
            digitalWrite(whiteled,HIGH);
            delay(2000);
            digitalWrite(whiteled,LOW);
            break;
        }
    }
    else
    {
        Serial.println("hello pi");
        delay(1000);
    }

}

After that, I used pySerial from Python interpreter to control the pins, and everything was working fine. Here is a piece of interpreter output:

Python 2.7.3 (default, Mar 18 2014, 05:13:23)
[GCC 4.6.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import serial
>>> ser = serial.Serial('/dev/ttyACM0',9600)
>>> x = ser.read(10)
>>> print 'x = ',x
x =  hellhello
>>> ser.write('w') #white led turn on and off
1
>>> ser.close()
>>>

Everything worked fine and led did turn on and off, so I decided to write a simple Python script to do the same:

import serial
import time
ser = serial.Serial('/dev/ttyACM0',9600)
x = ser.read(10)
print 'x = ',x

time.sleep(2)
ser.write('w')

ser.close()

The following is the execution command and result:

pi@raspberrypi ~ $ python serialtest.py
x =  helello pi

It only appeared the string from Arduino, but no led turn on at all. It looks like everything should be fine, so I don't know what the problem can be. I already search some articles and add "time.sleep(2)" before "ser.write()", but it still couldn't work.I would appreciate any help, many thanks in advance!

UPDATE : I made the controller send me back the data it was receiving and it looks like it isn't receiving anything when I am running the script, but receives everything when I send the data from the interpreter. The code of the arduino code now looks like this:

int redled = 12;
int whiteled = 48;

void setup()
{
    Serial.begin(9600);
    pinMode(redled,OUTPUT);
    pinMode(whiteled,OUTPUT);
}

void loop()
{
    if(Serial.available())
    {
        char cmd = Serial.read();
        switch(cmd)
        {
            case'r':
            digitalWrite(redled,HIGH);
            delay(2000);
            digitalWrite(redled,LOW);
            Serial.println("Cmd received");
            break;

            case'w':
            digitalWrite(whiteled,HIGH);
            delay(2000);
            digitalWrite(whiteled,LOW);
            Serial.println("Cmd received");
            break;
        }
    }   
}