python 写文件的疑问

#!/bin/pyton
#coding=gb18030


"""
写文件
"""
fn='c.txt'
fp=open(fn,'w+')
fp.write('i am William\n','a')
fp.close()


fa=open('a.txt','a')
fa.write('i love wangxinxin','a')

报错：

Traceback (most recent call last):
  File "D:\workspace\hellobao\src\bao\__init___06_file_1.py", line 9, in <module>
    fp.write('i am William\n','a')
TypeError: function takes exactly 1 argument (2 given)

把第一个追加fp.write('i am William\n','a')改为fp.write('i am William\n')就不报错