Python sockets error TypeError: a bytes-like object is required, not 'str' with send function

Question

I am trying to create a program that will open a port on the local machine and let others connect into it via netcat. My current code is.

s = socket.socket()
host = '127.0.0.1'
port = 12345
s.bind((host, port))

s.listen(5)
while True:
    c, addr = s.accept()
    print('Got connection from', addr)
    c.send('Thank you for connecting')
    c.close()

I am new to Python and sockets. But when I run this code it will allow me to send a netcat connection with the command:

nc 127.0.0.1 12345

But then on my Python script I get the error for the c.send:

TypeError: a bytes-like object is required, not 'str'

I am basically just trying to open a port, allow netcat to connect and have a full shell on that machine.

Carousel

Have you tried encoding yet? – Monde

No. I can comment out the c.send and it will run. Do you think that string needs to be encoded? – Carousel

Answer 1

The reason for this error is that in Python 3, strings are Unicode, but when transmitting on the network, the data needs to be bytes instead. So... a couple of suggestions:

1. Suggest using c.sendall() instead of c.send() to prevent possible issues where you may not have sent the entire msg with one call (see docs).
2. For literals, add a 'b' for bytes string: c.sendall(b'Thank you for connecting')
3. For variables, you need to encode Unicode strings to byte strings (see below)

Best solution (should work w/both 2.x & 3.x):

output = 'Thank you for connecting'
c.sendall(output.encode('utf-8'))

Epilogue/background: this isn't an issue in Python 2 because strings are bytes strings already -- your OP code would work perfectly in that environment. Unicode strings were added to Python in releases 1.6 & 2.0 but took a back seat until 3.0 when they became the default string type. Also see this similar question as well as this one.

Selfaggrandizement

Thank you for this. This is very helpful and also answers my question. – Carousel

Answer 2

You can decode it to str with receive.decode('utf_8').

Orle

Answer 3

You can change the send line to this:

c.send(b'Thank you for connecting')

The b makes it bytes instead.

Horseleech

Answer 4

An alternative solution is to introduce a method to the file instance that would do the explicit conversion.

import types

def _write_str(self, ascii_str):
    self.write(ascii_str.encode('ascii'))

source_file = open("myfile.bin", "wb")
source_file.write_str = types.MethodType(_write_str, source_file)

And then you can use it as source_file.write_str("Hello World").

Joachima