basic_luigi_cli_task.py

class ATask(luigi.Task):

    def requires(self):
        # Defines what is the upstream task to this one.
        # Is later used by the input() function.
        return APreviousTask()

    def output(self):
        return luigi.LocalTarget(self.input().path + ".new_extension")

    def run(self):
        # Loop over the input file, and just write each line to the output file.
        with self.input().open() as infile, self.output().open() as outfile:
            for line in infile:
                outfile.writeline(line)