Reading data from the serial port hangs my GUI

In this answer I will show 2 solutions:

import sys

import serial
import threading

from PyQt5 import uic, QtCore, QtWidgets

class Helper(QtCore.QObject):
    textSignal = QtCore.pyqtSignal(str)

def leer_puerto(port, baudrate, helper):
    arduino = serial.Serial(port, baudrate)
    while arduino.isOpen():
        rawString = arduino.readline()
        helper.textSignal.emit(str(rawString)[7:-5])


class Ventana(QtWidgets.QDialog):
    def __init__ (self, parent=None):
        super(Ventana, self).__init__(parent)
        uic.loadUi('fichajes.ui', self)
        self.timer = QtCore.QTimer(self)
        self.timer.timeout.connect(self.tick)
        self.timer.start(1000)
        self.btnSalir.clicked.connect(self.close)
        port = "COM4"
        self.helper = Helper()
        self.helper.textSignal.connect(self.txtUID.setText)
        threading.Thread(target=leer_puerto, args=(port, 9600, self.helper), daemon=True).start()

    @QtCore.pyqtSlot()
    def tick(self):
        self.lblTime.setText(QtCore.QTime.currentTime().toString("hh:mm:ss"))
        self.lblDate.setText(QtCore.QDate.currentDate().toString("dd-MM-yyyy"))


if __name__ == '__main__':
    app = QtWidgets.QApplication(sys.argv)
    ventana = Ventana()
    ventana.show()
    sys.exit(app.exec_())

import sys

from PyQt5 import uic, QtCore, QtWidgets, QtSerialPort


class Ventana(QtWidgets.QDialog):
    def __init__ (self, parent=None):
        super(Ventana, self).__init__(parent)
        uic.loadUi('fichajes.ui', self)
        self.timer = QtCore.QTimer(self)
        self.timer.timeout.connect(self.tick)
        self.timer.start(1000)
        self.btnSalir.clicked.connect(self.close)
        port = "COM4"
        self.arduino = QtSerialPort.QSerialPort(port, self)
        self.arduino.setBaudRate(QtSerialPort.QSerialPort.Baud9600)
        self.arduino.readyRead.connect(self.onReadyRead)

        if not self.arduino.open(QtSerialPort.QSerialPort.ReadWrite):
            # http://doc.qt.io/qt-5/qserialport.html#SerialPortError-enum
            print("Problema con el puerto", self.arduino.error())

    @QtCore.pyqtSlot()
    def tick(self):
        self.lblTime.setText(QtCore.QTime.currentTime().toString("hh:mm:ss"))
        self.lblDate.setText(QtCore.QDate.currentDate().toString("dd-MM-yyyy"))

    @QtCore.pyqtSlot()
    def onReadyRead(self):
        while self.arduino.canReadLine():
            rawString = self.arduino.readLine()
            self.txtUID.setText(str(rawString)[7:-5])

    def closeEvent(self, event):
        if self.arduino.isOpen():
            self.arduino.close()
        super(Ventana, self).closeEvent(event)


if __name__ == '__main__':
    app = QtWidgets.QApplication(sys.argv)
    ventana = Ventana()
    ventana.show()
    sys.exit(app.exec_())

1) The easiest way to use a thread is to instantiate an object of the Thread class with a target function and make a call to its start () method.

import threading
def worker():
    """funcion que realiza el trabajo en el thread"""
    print 'Estoy trabajando para Genbeta Dev'
    return
threads = list()
for i in range(3):
    t = threading.Thread(target=worker)
    threads.append(t)
    t.start()

2) See that the parameters match any type of object can be passed as a parameter to a thread.

import threading
def worker(count):
    """funcion que realiza el trabajo en el thread"""
    print "Este es el %s trabajo que hago hoy para Genbeta Dev" % count
    return
threads = list()
for i in range(3):
    t = threading.Thread(target=worker, args=(i,))
    threads.append(t)
    t.start()

3) The output of the previous example would be like the one that follows would come out the following acceptance code:

Este es el 0 trabajo que hago hoy para Genbeta Dev
Este es el 1 trabajo que hago hoy para Genbeta Dev
Este es el 2 trabajo que hago hoy para Genbeta Dev

4) I forgot, by the way, that arguments can be used to name the threads that we create, although it is not necessary. Each instance of the Thread class has an assigned name by default.

Naming threads can be useful, for example, when clarifying our code.

import threading
import time
def worker():
    print threading.currentThread().getName(), 'Lanzado'
    time.sleep(2)
    print threading.currentThread().getName(), 'Deteniendo'
def servicio():
    print threading.currentThread().getName(), 'Lanzado'
    print threading.currentThread().getName(), 'Deteniendo'
t = threading.Thread(target=servicio, name='Servicio')
w = threading.Thread(target=worker, name='Worker')
z = threading.Thread(target=worker)
w.start()
z.start()
t.start()

5) For the following we show The exit of the previous code would be:

Worker Lanzado
Thread-1 Lanzado
Servicio Lanzado
Worker Deteniendo
Thread-1 Deteniendo
Servicio Deteniendo

6) If we are going to debug or log something related to threads, it is best that we use the logging module for this, we place the following commands:

import threading
import logging
import time
logging.basicConfig( level=logging.DEBUG,
    format='[%(levelname)s] - %(threadName)-10s : %(message)s')
def worker():
    logging.debug('Lanzado')
    time.sleep(2)
    logging.debug('Deteniendo')
w = threading.Thread(target=worker, name='Worker')
w.start()

7) The logging module supports the inclusion of the name of the thread natively, the output of one of the previous messages would be similar to this:

[DEBUG] - Worker : Lanzado

8) To raise a thread as a daemon we just have to invoke its setDaemon () method by passing it the True argument.

import threading
import logging
import time
logging.basicConfig( level=logging.DEBUG,
    format='[%(levelname)s] - %(threadName)-10s : %(message)s')
def daemon():
    logging.debug('Lanzado')
    time.sleep(2)
    logging.debug('Deteniendo')
d = threading.Thread(target=daemon, name='Daemon')
d.setDaemon(True)
d.start()

conclusion:

Today we have made a first contact with the threading module of Python and we have learned to use the logging object to use debug messages with our threads.