serial communication raspberrypi2

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Python

i have raspberrypi 2 modlde. i need to make serial data from the pi module. this is my code

#!/usr/bin/env python


           import time
           import serial


           ser = serial.Serial(

               port='/dev/ttyAMA0',
               baudrate = 9600,
               parity=serial.PARITY_NONE,
               stopbits=serial.STOPBITS_ONE,
               bytesize=serial.EIGHTBITS,
               timeout=1
           )
           counter=0


           while 1:
               ser.write('Write counter: %d \n'%(counter))
               time.sleep(1)
               counter += 1

this is the error msg

Traceback (most recent call last):
  File "/home/pi/Desktop/serial.py", line 3, in <module>
    import serial
  File "/home/pi/Desktop/serial.py", line 6, in <module>
    ser=serial.Serial(
AttributeError: 'module' object has no attribute 'Serial'
>>>

what is the error.pls help me...
thank you