Qt for Python Signals and Slots

This page describes the use of signals and slots in Qt for Python. The emphasis is on illustrating the use of so-called new-style signals and slots, although the traditional syntax is also given as a reference.

The main goal of this new-style is to provide a more Pythonic syntax to Python programmers.

Traditional syntax: SIGNAL () and SLOT()

QtCore.SIGNAL() and QtCore.SLOT() macros allow Python to interface with Qt signal and slot delivery mechanisms. This is the old way of using signals and slots.

The example below uses the well known clicked signal from a QPushButton. The connect method has a non python-friendly syntax. It is necessary to inform the object, its signal (via macro) and a slot to be connected to.

import sys                                                                                          
from PySide2.QtWidgets import QApplication, QPushButton                                             
from PySide2.QtCore import SIGNAL, QObject                                                          
                                                                                                    
def func():                                                                                         
    print("func has been called!")                                                                     
                                                                                                    
app = QApplication(sys.argv)                                                                        
button = QPushButton("Call func")                                                                   
QObject.connect(button, SIGNAL ('clicked()'), func)                                                 
button.show()                                                                                                                                                                                                                                                                                                            
sys.exit(app.exec_())

New syntax: Signal() and Slot()

The new-style uses a different syntax to create and to connect signals and slots. The previous example could be rewritten as:

import sys                                                                                          
from PySide2.QtWidgets import QApplication, QPushButton                                             
                                                                                                    
def func():                                                                                         
 print("func has been called!")                                                                     
                                                                                                    
app = QApplication(sys.argv)                                                                        
button = QPushButton("Call func")                                                                   
button.clicked.connect(func)                                                                           
button.show()                                                                                          
sys.exit(app.exec_())

Using QtCore.Signal()

Signals can be defined using the QtCore.Signal() class. Python types and C types can be passed as parameters to it. If you need to overload it just pass the types as tuples or lists.

In addition to that, it can receive also a named argument name that defines the signal name. If nothing is passed as name then the new signal will have the same name as the variable that it is being assigned to.

The Examples section below has a collection of examples on the use of QtCore.Signal().

Note: Signals should be defined only within classes inheriting from QObject. This way the signal information is added to the class QMetaObject structure.

Using QtCore.Slot()

Slots are assigned and overloaded using the decorator QtCore.Slot(). Again, to define a signature just pass the types like the QtCore.Signal() class. Unlike the Signal() class, to overload a function, you don't pass every variation as tuple or list. Instead, you have to define a new decorator for every different signature. The examples section below will make it clearer.

Another difference is about its keywords. Slot() accepts a name and a result. The result keyword defines the type that will be returned and can be a C or Python type. name behaves the same way as in Signal(). If nothing is passed as name then the new slot will have the same name as the function that is being decorated.

Examples

The examples below illustrate how to define and connect signals and slots in PySide2. Both basic connections and more complex examples are given.

• Hello World example: the basic example, showing how to connect a signal to a slot without any parameters.

import sys
from PySide2 import QtCore, QtGui

# define a function that will be used as a slot
def sayHello():
 print 'Hello world!'

app = QtGui.QApplication(sys.argv)

button = QtGui.QPushButton('Say hello!')

# connect the clicked signal to the sayHello slot
button.clicked.connect(sayHello)
button.show()

sys.exit(app.exec_())

• Next, some arguments are added. This is a modified Hello World version. Some arguments are added to the slot and a new signal is created.

import sys                                                                  
from PySide2.QtWidgets import QApplication, QPushButton                     
from PySide2.QtCore import QObject, Signal, Slot                            
                                                                            
app = QApplication(sys.argv)                                                
                                                                            
# define a new slot that receives a string and has                          
# 'saySomeWords' as its name                                                
@Slot(str)                                                                  
def say_some_words(words):                                                  
    print(words)                                                               
                                                                            
class Communicate(QObject):                                                 
 # create a new signal on the fly and name it 'speak'                       
 speak = Signal(str)                                                        
                                                                            
someone = Communicate()                                                     
# connect signal and slot                                                   
someone.speak.connect(say_some_words)                                         
# emit 'speak' signal                                                         
someone.speak.emit("Hello everybody!")

• Add some overloads. A small modification of the previous example, now with overloaded decorators.

import sys                                                                  
from PySide2.QtWidgets import QApplication, QPushButton                     
from PySide2.QtCore import QObject, Signal, Slot                            
                                                                            
app = QApplication(sys.argv)                                                
                                                                            
# define a new slot that receives a C 'int' or a 'str'                      
# and has 'saySomething' as its name                                        
@Slot(int)                                                                  
@Slot(str)                                                                  
def say_something(stuff):                                                   
    print(stuff)                                                            
                                                                            
class Communicate(QObject):                                                 
    # create two new signals on the fly: one will handle                    
    # int type, the other will handle strings                               
    speak_number = Signal(int)                                              
    speak_word = Signal(str)                                                  
                                                                            
someone = Communicate()                                                     
# connect signal and slot properly                                          
someone.speak_number.connect(say_something)                                 
someone.speak_word.connect(say_something)                                   
# emit each 'speak' signal                                                  
someone.speak_number.emit(10)                                               
someone.speak_word.emit("Hello everybody!")

• An example with slot overloads and more complicated signal connections and emissions:

import sys
from PySide2.QtWidgets import QApplication, QPushButton
from PySide2.QtCore import QObject, Signal, Slot

app = QApplication(sys.argv)

# define a new slot that receives a C 'int' or a 'str'
# and has 'saySomething' as its name
@Slot(int)
@Slot(str)
def say_something(stuff):
    print(stuff)

class Communicate(QObject):
    # create two new signals on the fly: one will handle
    # int type, the other will handle strings
    speak = Signal((int,), (str,))

someone = Communicate()
# connect signal and slot. As 'int' is the default
# we have to specify the str when connecting the
# second signal
someone.speak.connect(say_something)
someone.speak[str].connect(say_something)

# emit 'speak' signal with different arguments.
# we have to specify the str as int is the default
someone.speak.emit(10)
someone.speak[str].emit("Hello everybody!")

• An example of an object method emitting a signal:

import sys                                                                  
from PySide2.QtCore import QObject, Signal                                  
                                                                            
# Must inherit QObject for signals                                          
class Communicate(QObject):                                                 
    speak = Signal()                                                        
                                                                            
    # Must init QObject else runtime error:                                 
    #   PySide2.QtCore.Signal object has no attribute ‘emit’                
    def __init__(self):                                                     
        super(Communicate, self).__init__()                                 
                                                                            
    def speaking_method(self):                                              
        self.speak.emit()                                                   
                                                                            
someone = Communicate()                                                     
someone.speaking_method()

• Signals are runtime objects owned by instances, they are not class attributes:

# Erroneous: refers to class Communicate, not an instance of the class
Communicate.speak.connect(say_something)
# raises exception: AttributeError: 'PySide2.QtCore.Signal' object has no attribute 'connect'