What is the difference between using ZMQ PUB with .connect() or .bind() methods?

Question

In Python ZMQ publisher/subscriber sample template, the publisher uses .bind() method and the subscriber uses .connect() method, that connected to the bind IP address.

But we can replace .bind() and .connect() each with the other.

My question is that what is the difference between two cases that determined below?
(two scripts in these cases work fine)

The first case, as default:

pub1.py:

import zmq
import time
from datetime import datetime

def create_pub_socket():
    context = zmq.Context()
    socket = context.socket(zmq.PUB)
    socket.bind("tcp://127.0.0.1:9002")  # notice
    return socket

def publish(pub_socket):
    message = {
        'data': 'hi my name is benyamin',
        'time': datetime.now().strftime('%Y-%m-%dT%H:%M:%S')
    }
    pub_socket.send_json(message, 0)
    return message

if __name__ == '__main__':
    socket = create_pub_socket()

    while True:
        print('\n')
        print('publisher: ', publish(socket))
        time.sleep(1)

sub1.py:

import zmq

if __name__ == '__main__':
    context = zmq.Context()
    socket = context.socket(zmq.SUB)
    socket.setsockopt(zmq.SUBSCRIBE, "")
    socket.connect("tcp://127.0.0.1:9002")  # notice

    while True:
        data = socket.recv_json()
        print('subscriber: ', data)
        print('\n')

And the second case, as the modified setup, that reversed the use of the .connect() and .bind() methods:

pub2.py:

import zmq
import time
from datetime import datetime

def create_pub_socket():
    context = zmq.Context()
    socket = context.socket(zmq.PUB)
    socket.connect("tcp://127.0.0.1:9002")  # notice
    return socket

def publish(pub_socket):
    message = {
        'data': 'hi my name is benyamin',
        'time': datetime.now().strftime('%Y-%m-%dT%H:%M:%S')
    }
    pub_socket.send_json(message, 0)
    return message

if __name__ == '__main__':
    socket = create_pub_socket()

    while True:
        print('\n')
        print('publisher: ', publish(socket))
        time.sleep(1)

sub2.py:

import zmq

if __name__ == '__main__':
    context = zmq.Context()
    socket = context.socket(zmq.SUB)
    socket.setsockopt(zmq.SUBSCRIBE, "")
    socket.bind("tcp://127.0.0.1:9002")  # notice

    while True:
        data = socket.recv_json()
        print('second subscriber: ', data)
        print('\n')

Answer 1

There is no difference here (single publisher and subscriber), but in other scenarios (multi publishers or subscribers), there is a difference depending on your policy:

1. i.e. suppose that there are two clients (Machine1, Machine2) and a Server.
   Each client must publish data using ZMQ, and the Server must subscribe that data from Machine1 and Machine2:

• Machine1 --> has a publisher (with .connect(Server IP) )

• Machine2 --> has a publisher (with .connect(Server IP) )

• Server --> has a subscriber (with .bind(Server IP/Self IP))

As you can see in the mentioned scenario we used the second case in the question.

2. Whereas, if we have two subscribers and a publisher, we must place the .bind() method in the publisher and place .connect() method in the subscribers (first case in the question).

---

[NOTE]:

• ^{.bind() method doesn't support the localhost as an IP, while .connect() method could be connected to each IP address that defined in .bind() method: Why doesn't zeromq work on localhost?}

• ^{Here is another example of changing .bind() and .connect() locate: This-Link}