In Python 3.x you need to convert your str object to a bytes object for base64 to be able to encode them. You can do that using the str.encode method:

>>> import json
>>> import base64
>>> d = {"alg": "ES256"} 
>>> s = json.dumps(d)  # Turns your json dict into a str
>>> print(s)
{"alg": "ES256"}
>>> type(s)
<class 'str'>
>>> base64.b64encode(s)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.2/base64.py", line 56, in b64encode
    raise TypeError("expected bytes, not %s" % s.__class__.__name__)
TypeError: expected bytes, not str
>>> base64.b64encode(s.encode('utf-8'))
b'eyJhbGciOiAiRVMyNTYifQ=='

If you pass the output of your_str_object.encode('utf-8') to the base64 module, you should be able to encode it fine.

You must be careful about the datatypes.

If you read a binary image, you get bytes. If you encode these bytes in base64, you get ... bytes again! (see documentation on b64encode)

json can't handle raw bytes, that's why you get the error.

I have just written some example, with comments, I hope it helps:

from base64 import b64encode
from json import dumps

ENCODING = 'utf-8'
IMAGE_NAME = 'spam.jpg'
JSON_NAME = 'output.json'

# first: reading the binary stuff
# note the 'rb' flag
# result: bytes
with open(IMAGE_NAME, 'rb') as open_file:
    byte_content = open_file.read()

# second: base64 encode read data
# result: bytes (again)
base64_bytes = b64encode(byte_content)

# third: decode these bytes to text
# result: string (in utf-8)
base64_string = base64_bytes.decode(ENCODING)

# optional: doing stuff with the data
# result here: some dict
raw_data = {IMAGE_NAME: base64_string}

# now: encoding the data to json
# result: string
json_data = dumps(raw_data, indent=2)

# finally: writing the json string to disk
# note the 'w' flag, no 'b' needed as we deal with text here
with open(JSON_NAME, 'w') as another_open_file:
    another_open_file.write(json_data)

The problem was with the encoding. json returns utf-8 encoded text which the base64 module cannot handle (it wants ascii). The correction is therefore to encode('ascii') the json decoded string before passing it to base64:

In [8]: base64.urlsafe_b64decode(
            json.loads(
                json.dumps(base64.urlsafe_b64encode(content))
            )
            .encode('ascii')
        ) == content
Out[7]: True

according to your code:

>>> compressed_json = data['base64(zip(o))']
>>> decompressed_json = zlib.decompress(b64decode(compressed))
>>> decompressed_json
b'"C:\\\\Users\\\\Beeri\\\\Google Drive\\\\Finance\\\\BPicks\\\\Stock Info\\\\daily data copy - Copy\\\\A.json"'
>>> json.loads(decompressed_json)
'C:\\Users\\Beeri\\Google Drive\\Finance\\BPicks\\Stock Info\\daily data copy - Copy\\A.json'

As mentioned by Karl, you basically compressed the file path not the actual file: try this

import zlib, json
from base64 import b64encode, b64decode

path = r"filename.json"

def compress_json(data):
    return {'base64(zip(o))': b64encode(zlib.compress(json.dumps(data).encode('utf-8'))).decode()}

def decompress_json(data):
    compressed_json =  data['base64(zip(o))']
    return json.loads(zlib.decompress(b64decode(compressed_json)))

with open(path, "r") as f:
    data = f.read()
    compressed_json = compress_json(data)
    print(compressed_json)
    """
    {'base64(zip(o))': 'eJxTio7JUwCCaggFAjFKKYklqTFKVkCWkYGRga6Bqa6hhYKhmZW
    BARDFKOkgq80vSM0DqbUw1jO3RJHJyS+HSpiZo0hkZKZnYNeSnJNfnIpdU1l+TmkuWM7QwtT
    U0sQMIlmrQ7T7Ta1MLfG638LQFIcHzHF6wMIYpwfQ/IbsASMjEzNLcjxggdcDuMLf3AKX83EH
    P3pgIDvfwBgYBVDnx+TFKgEASGiHCQ=='}
    """
    decompressed_json = decompress_json(compressed_json)
    print(decompressed_json)
    """
[
    {
        "date": "2020-05-18 16:00:00",
        "open": 83.79,
        "low": 83.67,
        "high": 83.79,
        "close": 83.67,
        "volume": 1855946
    },
    {
        "date": "2020-05-18 15:59:00",
        "open": 83.815,
        "low": 83.77,
        "high": 83.83,
        "close": 83.79,
        "volume": 1822469
    },
    {
        "date": "2020-05-18 15:58:00",
        "open": 83.9,
        "low": 83.78,
        "high": 83.9,
        "close": 83.815,
        "volume": 1803855
    }
]
"""