How to combine data taken from server between client and GUI?

[Larz60+]

If you want to keep client.py in a separate module, you need to provide some sort of interface to the data.
You can do this by wrapping your code in a function, and returning data, or by making client.py a class and exposing data
for outside use in the __init__ method, by defining data as self.data.

as a class:
Client.py

import socket

class Client:
    def __init__(self):
        self.HOST = '192.168.1.59'  # The server's hostname or IP address, NodeMCU IP Address
	    self.PORT = 80              # The port used by the server, PORT NUMBER in ARDUINO CODE

    def transfer_data(self):
        s.connect((HOST, PORT))
        s.sendall(b'Hello, world')
        self.data = s.recv(1024)

    
# To try run stand-alone
if __name__ == '__main__':
    cl = Client()
    cl.transfer_data()
    print(cl.data)

as a function:

import socket
HOST = '192.168.1.59'  # The server's hostname or IP address, NodeMCU IP Address
PORT = 80              # The port used by the server, PORT NUMBER in ARDUINO CODE 
 
with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
    s.connect((HOST, PORT))
    s.sendall(b'Hello, world')
    return s.recv(1024)

Now in the GUI class:
import Client

# and add to GUI.py

class Ui_MainWindow(object):
    def __init__(self):
        self.client = Client.Client()
        self.cdata = self.client.transfer_data()
# then when needed:
        data = self.client.data()

A better way would to have a main module that imports both GUI and Client
and interfaces externally.