Signals and Slots in PySide

This page describes the use of signals and slots in PySide. 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.

PyQt’s new-style signals and slots were introduced in PyQt v4.5. The main goal of this new-style is to provide a more Pythonic syntax to Python programmers. PySide uses PSEP 100 [pyside.org] as its implementation guideline.

QtGui.QPushButton("Call someFunc")

QtGui.QPushButton("Call someFunc")

QtCore, QtGui

• 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.

1. #!/usr/bin/env python
2.  
3. import sys
4. from PySide import QtCore
5.  
6. # define a new slot that receives a string and has
7. # 'saySomeWords' as its name
8. @QtCore.Slot(str)
9. def saySomeWords(words):
10.     print words
11.  
12. class Communicate(QtCore.QObject):
13.     # create a new signal on the fly and name it 'speak'
14.     speak = QtCore.Signal(str)
15.  
16. someone = Communicate()
17. # connect signal and slot
18. someone.speak.connect(saySomeWords)
19. # emit 'speak' signal
20. someone.speak.emit("Hello everybody!")

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

1. #!/usr/bin/env python
2.  
3. import sys
4. from PySide import QtCore
5.  
6. # define a new slot that receives a C 'int' or a 'str'
7. # and has 'saySomething' as its name
8. @QtCore.Slot(int)
9. @QtCore.Slot(str)
10. def saySomething(stuff):
11.     print stuff
12.  
13. class Communicate(QtCore.QObject):
14.     # create two new signals on the fly: one will handle
15.     # int type, the other will handle strings
16.     speakNumber = QtCore.Signal(int)
17.     speakWord = QtCore.Signal(str)
18.  
19. someone = Communicate()
20. # connect signal and slot properly
21. someone.speakNumber.connect(saySomething)
22. someone.speakWord.connect(saySomething)
23. # emit each 'speak' signal
24. someone.speakNumber.emit(10)
25. someone.speakWord.emit("Hello everybody!")

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

1. #!/usr/bin/env python
2.  
3. import sys
4. from PySide import QtCore
5.  
6. # define a new slot that receives an C 'int' or a 'str'
7. # and has 'saySomething' as its name
8. @QtCore.Slot(int)
9. @QtCore.Slot(str)
10. def saySomething(stuff):
11.     print stuff
12.  
13. class Communicate(QtCore.QObject):
14.     # create two new signals on the fly: one will handle
15.     # int type, the other will handle strings
16.     speak = QtCore.Signal((int,), (str,))
17.  
18. someone = Communicate()
19. # connect signal and slot. As 'int' is the default
20. # we have to specify the str when connecting the
21. # second signal
22. someone.speak.connect(saySomething)
23. someone.speak[str].connect(saySomething)
24.  
25. # emit 'speak' signal with different arguments.
26. # we have to specify the str as int is the default
27. someone.speak.emit(10)
28. someone.speak[str].emit("Hello everybody!")

• An example of an object method emitting a signal:

1. #!/usr/bin/env python
2.  
3. import sys
4. from PySide import QtCore
5.  
6. class Communicate(QtCore.QObject):    # !!! Must inherit QObject for signals
7.    
8.    speak = QtCore.Signal()
9.  
10.    def __init__(self):
11.        # !!! Must init QObject else runtime error: PySide.QtCore.Signal object has no attribute ‘emit’
12.        super(Communicate, self).__init__()
13.  
14.    def speakingMethod():
15.        self.speak.emit()
16.  
17. someone = Communicate()
18. someone.speakingMethod()

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

1. Communicate.speak.connect(saySomething)   # Erroneous: refers to class Communicate, not an instance of the class
2.  
3. # raises exception: AttributeError: 'PySide.QtCore.Signal' object has no attribute 'connect'

QtCore import Signal as pyqtSignal

or

1. QtCore.pyqtSignal = QtCore.Signal
2. QtCore.pyqtSlot = QtCore.Slot

This way any call to pyqtSignal or pyqtSlot will be translated to a Signal or Slot call.

Categories:

• LanguageBindings
  • PySide