JobTask.dump raises TypeError when replacing bytes to string

[marinelay]

In the file luigi.contrib.hadoop.py, the JobTask.dump method shows a weird behavior.

luigi/luigi/contrib/hadoop.py

Lines 965 to 978 in 64d6c48

	def dump(self, directory=''):
	"""
	Dump instance to file.
	"""
	with self.no_unpicklable_properties():
	file_name = os.path.join(directory, 'job-instance.pickle')
	if self.__module__ == '__main__':
	d = pickle.dumps(self)
	module_name = os.path.basename(sys.argv[0]).rsplit('.', 1)[0]
	d = d.replace(b'(c__main__', "(c" + module_name)
	open(file_name, "wb").write(d)
	else:
	pickle.dump(self, open(file_name, "wb"))

I believe the variable d is bytes type by pickle.dumps (https://docs.python.org/3/library/pickle.html#pickle.dumps),
and the variable module_name should be string type because sys.argv[0] is string type (https://docs.python.org/3/library/sys.html#sys.argv).

Then, d.replace(b'(c__main__', "(c" + module_name) always return TypeError: a bytes-like object is required, not 'str' because its replacement should be any bytes-like object (https://docs.python.org/3/library/stdtypes.html#bytes.replace).
It is simplified code of this situation:

from luigi.contrib.hadoop import JobTask

class My(JobTask):
    pass

a = My()
a.dump('my')

Output :

Traceback (most recent call last):
  File "/home/wonseok/current/luigi/my_test.py", line 8, in <module>
    a.dump('my')
  File "/home/wonseok/current/luigi/luigi/contrib/hadoop.py", line 974, in dump
    d = d.replace(b'(c__main__', "(c" + module_name)
TypeError: a bytes-like object is required, not 'str'

It is related issue #2402
If I'm mistaken, I'd appreciate it if you could let me know.
Thank you!