$ cat mwe.py 
#!/usr/bin/env python

import time
import progressbar

bar = progressbar.ProgressBar(max_value=1, widgets=[progressbar.Bar("=", "[", "]"), " ", progressbar.Percentage(), ' (', progressbar.ETA(), ') '])
bar.start()
bar.update(0)
time.sleep(0.5)
bar.update(0.5)
time.sleep(0.5)
bar.update(1)
bar.finish()
$ ./mwe.py                                                                                                                                                                                                                                                                                                                                    
Traceback (most recent call last):
  File "./mwe.py", line 7, in <module>
    bar.start()
  File "/dev/shm/gerrit/venv/stable-3.5/lib/python3.5/site-packages/progressbar/bar.py", line 606, in start
    self.update(self.min_value, force=True)
  File "/dev/shm/gerrit/venv/stable-3.5/lib/python3.5/site-packages/progressbar/bar.py", line 562, in update
    StdRedirectMixin.update(self, value=value)
  File "/dev/shm/gerrit/venv/stable-3.5/lib/python3.5/site-packages/progressbar/bar.py", line 150, in update
    DefaultFdMixin.update(self, value=value)
  File "/dev/shm/gerrit/venv/stable-3.5/lib/python3.5/site-packages/progressbar/bar.py", line 55, in update
    self.fd.write(line)
TypeError: write() argument must be str, not bytes