Python 3 struct.pack(): char format requires a bytes object of length 1

Question

I'm trying to send push notifications using the APNS from Python (I know there are lots of libraries that do just that, but this has pedagogic intentions).

I started using this script (source):

def send_push(token, payload):
 # Your certificate file
 cert = 'ck.pem'
 # APNS development server
 apns_address = ('gateway.sandbox.push.apple.com', 2195)
 # Use a socket to connect to APNS over SSL
 s = socket.socket()
 sock = ssl.wrap_socket(s, ssl_version=ssl.PROTOCOL_SSLv3, certfile=cert)
 sock.connect(apns_address)
 # Generate a notification packet
 token = binascii.unhexlify(token)
 fmt = '!cH32sH{0:d}s'.format(len(payload))
 cmd = '\x00'
 message = struct.pack(fmt, cmd, len(token), token, len(payload), payload)
 sock.write(message)
 sock.close()

Which works, but Python 2.x only supports TSL until version 1. So I tried to run it using Python 3, and I get this error:

Traceback (most recent call last):
 File "push_notificator.py", line 52, in <module>
 send_notification(TOKEN, json.dumps(TEST_PAYLOAD))
 File "push_notificator.py", line 46, in send_push
 payload
struct.error: char format requires a bytes object of length 1

So it seems I must convert the payload to binary, but I'm really lost. This is the first time that I work with binary data on Python.

Answer 1

@cdonts's answer ultimately helped me out, but i thought it might be cleaner in a separate answer, instead of a comment...

@cdonts's answer: https://stackoverflow.com/a/31551978/2298002

I had to encode both cmd as well as payload, before packing. here is my code that solved it...

cmd = bytes(cmd, "utf-8")
payload = bytes(payload, "utf-8")

here is a longer code snippet to demonstrate in context...

token = "<string apns token from iOS client side>"
try:
 token = binascii.unhexlify(token)
 payload = json.dumps(payload)
 fmt = "!cH32sH{0:d}s".format(len(payload))
 cmd = '\x00'
 #python3 requirement
 cmd = bytes(cmd, "utf-8")
 payload = bytes(payload, "utf-8")
 msg = struct.pack(fmt, cmd, len(token), token, len(payload), payload)
except Exception as e: # ref:
 print(e) 

@cdonts THANK YOU!! (https://stackoverflow.com/a/31551978/2298002)

Answer 2

In Python 3.x use:

bytes(payload, "utf-8")

Replace utf-8 with the necessary encoding.

Hope it helps.

• Tae: Thanks for your answer @cdonts. Unfortunately it doesn't work, I get the same error: struct.error: char format requires a bytes object of length 1.
• cdonts: @Tae Maybe the object length is greater than 1 or the problem is with token instead of payload.
• Tae: If that were the case then the error would occur running the code under Python 2 too. Finally I tried another format: !BH32sH%ds, but I'm still puzzled about why the other one works with Python 2 but not with Python 3. As far as I know the syntax is the same.
• TonyTony: what is the fully executable code? Could you paste that out? Thanks!
• greenhouse: @cdonts's answer helped solved my problem (with an additional step), and without having to change formats, thank you! i posted a new answer that provides more detail: stackoverflow.com/a/61992926/2298002