<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:

def ls(self, src, extra_args=[]):
        '''List files in a directory'''
        src = [self._full_hdfs_path(x) for x in src]
        output = self._getStdOutCmd([self._hadoop_cmd, 'fs', '-ls'] + extra_args + src, True)
        return self._transform_ls_output(output, self.hdfs_url)