python 对其他文件内容进行修改操作

class ReadWriteFile(object):
    '''对文件进行内容更改操作'''
    def __init__(self,file=None,index=None,new_data=None):
        self.file=file
        self.index=index-1
        self.new_data=new_data

    def write_file(self):
        source_data=None
        with open(self.file,"r")as f:
            source_data=f.readlines()
            if "  " in source_data[self.index]:
                blank_count=source_data[self.index].rfind("  ")+2
                print(blank_count)
                # if self.index<=(len(source_data))
                try:
                    source_data[self.index] =str(blank_count*" ")+ self.new_data + '\n'
                except IndexError as error:
                    raise IndexError(error)
            else:
                try:
                    source_data[self.index] =  self.new_data + '\n'
                except IndexError as error:
                    raise IndexError(error)
        with open(self.file,'w')as f:
            for line in source_data:
                f.write(line)

a='appium'
b='webdriver'
da="from %s import %s"%(a,b)
r=ReadWriteFile("/Users/liming/Desktop/appium_dir/test/__init__.py",13,"source_data = ''").write_file()