from pathlib import Path

path = Path('..')/'test.txt'  # Text file with contents "Goodbye, world"
project_id = 'secret_ID'
file_path = 'data/' + path.name
f = gl.project_files.get(file_path=file_path, ref='master', project_id=project_id)   
f.content = base64.b64encode(path.open('rb').read())
f.save(branch_name='master', commit_message='Updated ' + file_path , encoding='base64')

TypeError                                 Traceback (most recent call last)
<ipython-input-17-7015cf3aaaad> in <module>()
     12 
     13 f.content = base64.b64encode(path.open('rb').read())
---> 14 f.save(branch_name='master', commit_message='Updated ' + file_path , encoding='base64')

/home/hidden_user/.virtualenvs/hidden_dir/lib/python3.5/site-packages/gitlab/objects.py in save(self, **kwargs)
    318     def save(self, **kwargs):
    319         if self._from_api:
--> 320             self._update(**kwargs)
    321         else:
    322             self._create(**kwargs)

/home/hidden_user/.virtualenvs/hidden_dir/lib/python3.5/site-packages/gitlab/objects.py in _update(self, **kwargs)
    313             raise NotImplementedError
    314 
--> 315         json = self.gitlab.update(self, **kwargs)
    316         self._set_from_dict(json)
    317 

/home/hidden_user/.virtualenvs/hidden_dir/lib/python3.5/site-packages/gitlab/__init__.py in update(self, obj, **kwargs)
    621 
    622         # build data that can really be sent to server
--> 623         data = obj._data_for_gitlab(extra_parameters=kwargs, update=True)
    624 
    625         r = self._raw_put(url, data=data, content_type='application/json')

/home/hidden_user/.virtualenvs/hidden_dir/lib/python3.5/site-packages/gitlab/objects.py in _data_for_gitlab(self, extra_parameters, update, as_json)
    227         data.update(extra_parameters)
    228 
--> 229         return json.dumps(data) if as_json else data
    230 
    231     @classmethod

/usr/lib/python3.5/json/__init__.py in dumps(obj, skipkeys, ensure_ascii, check_circular, allow_nan, cls, indent, separators, default, sort_keys, **kw)
    228         cls is None and indent is None and separators is None and
    229         default is None and not sort_keys and not kw):
--> 230         return _default_encoder.encode(obj)
    231     if cls is None:
    232         cls = JSONEncoder

/usr/lib/python3.5/json/encoder.py in encode(self, o)
    196         # exceptions aren't as detailed.  The list call should be roughly
    197         # equivalent to the PySequence_Fast that ''.join() would do.
--> 198         chunks = self.iterencode(o, _one_shot=True)
    199         if not isinstance(chunks, (list, tuple)):
    200             chunks = list(chunks)

/usr/lib/python3.5/json/encoder.py in iterencode(self, o, _one_shot)
    254                 self.key_separator, self.item_separator, self.sort_keys,
    255                 self.skipkeys, _one_shot)
--> 256         return _iterencode(o, 0)
    257 
    258 def _make_iterencode(markers, _default, _encoder, _indent, _floatstr,

/usr/lib/python3.5/json/encoder.py in default(self, o)
    177 
    178         """
--> 179         raise TypeError(repr(o) + " is not JSON serializable")
    180 
    181     def encode(self, o):

TypeError: b'R29vZGJ5ZSwgd29ybGQK' is not JSON serializable