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PyQt5: Using decorators with methods connected to signals

I want to use a decorator to handle exceptions within a class. However, when using the decorator, it always gives me:
TypeError: f1() takes 1 positional argument but 2 were given
I am quite sure it occurs due to self, because it works perfectly outside the class. But I cannot manage to use it inside the class. Can someone help me?
Here is a MWE:
from PyQt5.QtWidgets import QPushButton, QVBoxLayout, QApplication, QWidget
import sys
def report_exceptions(f):
def wrapped_f(*args, **kwargs):
try:
f(*args, **kwargs)
except Exception as e:
print(f"caught: {e}")
return wrapped_f
class Window(QWidget):
def __init__(self):
super().__init__()
b1 = QPushButton('1')
b1.clicked.connect(self.f1)
layout = QVBoxLayout(self)
layout.addWidget(b1)
self.setLayout(layout)
#report_exceptions
def f1(self):
raise Exception("Error inside f1")
app = QApplication([])
window = Window()
window.show()
sys.exit(app.exec_())

The error occurs because the clicked signal sends a default checked parameter which the f1 method does not provide an argument for. There are several ways to work around this:
change the signature to allow for the extra argument:
def f1(self, checked=False):
use a lambda to consume the unwanted argument:
b1.clicked.connect(lambda: self.f1())
wrap the method as a pyqt slot:
#QtCore.pyqtSlot()
#report_exceptions
def f1(self):
The reason why this last one works is because signals with default arguments are treated as two separate overloads: one which which sends the parameter, and one which doesn't. The slot decorator therefore allows you to explicitly select which one is required (the other overload being #QtCore.pyqtSlot(bool)).
Another peculiarity specific to PyQt is that signals connected to undecorated slots will normally ignore unused parameters. The lambda solution above takes advantage of this mechanism, but your report_exceptions decorator effectively bypasses it, which is why you get the TypeError.

How to access self object within class from outside

I am writing an application that uses a Qt GUI and am now trying to multithread it (first time learner). A longer task will be contained within the worker thread eventually but output logs will be required to be written as it goes. The GUI class has a method to output these logs to a plain text widget. Up until now, everything has been running within the GUI class.
I currently have the following code (included the important bits for brevity):
class Worker(QRunnable):
#pyqtSlot()
def run(self):
Ui.logentry(self, "Test")
class Ui(QtWidgets.QMainWindow):
def __init__(self):
super(Ui, self).__init__()
uic.loadUi(resourcepath('./pycdra.ui'), self)
self.outputlog = self.findChild(QtWidgets.QPlainTextEdit, 'outputlog')
self.button = self.findChild(QtWidgets.QPushButton, 'button')
self.button.clicked.connect(self.Button)
self.threadpool = QThreadPool()
self.logentry("Available threads: %d" % self.threadpool.maxThreadCount())
def Button(self):
worker = Worker()
self.threadpool.start(worker)
def logentry(self, returntext):
self.outputlog.appendPlainText(self.timestamp() + " " + str(returntext))
self.outputlog.repaint()
def timestamp(self):
import datetime
ts = datetime.datetime.now(tz=None).strftime("%Y-%m-%d %H:%M:%S"))
return ts
def applic():
app = QApplication(sys.argv)
window = Ui()
window.show()
sys.exit(app.exec_())
applic()
When I try running this, the GUI loads perfectly, but upon pushing button the Ui.logentry part of Worker returns the error: AttributeError: 'Worker' object has no attribute 'outputlog'
I attempted making logentry() and timestamp() global so they can access and be accessed by both classes, but the closest I got with the line Ui.self.outputlog.appendPlainText(self.timestamp() + " " + str(returntext)) was that 'Ui' object has no attribute 'self'.
What am I missing? Am I right in making them global or are there other means to do this?

There are various problems with your code. I'll try to address them in order of importance.
logentry is an instance method of Ui, which means that it requires an instance of Ui in order to be run; what you're trying to achieve would never work since that self refers to an instance of Worker, which clearly doesn't have any outputlog attribute (which is what logentry expects instead). Remember: the self argument is not there just for fun, it always refers to the object (the instance, for instance methods) the function is called from; self.logentry means "call the function logentry using self as first argument. Since, in your case, self refers to an instance of Ui, that explains your main issue, since that instance doesn't have that attribute.
None is exactly what its name says: nothing. The timestamp function will throw another AttributeError, since there's no strftime attribute in None. That is part of a datetime object, so you have to get a now object, then call its strftime function against it. In any case, the tz argument requires a tzinfo subclass.
pyqtSlot only works for Qt classes that inherit from QObject (such as QWidget). QRunnable isn't one of those classes. You can check the whole inheritance tree in the header of the documentation in each class: if there's a "Inherits:" field, go up until there's none. If you get a QObject inheritance at some point, then you can use slots and signals for that object, otherwise not. Consider that: QWidget also inherits from QObject; all Qt widgets inherit from QWidget; there are Qt classes that inherit from QObject but are not widgets.
Even ignoring all the above, access to UI elements is always forbidden from external threads (including QRunnable objects); while you can theoretically (but unreliably) get their properties, trying to set them will most likely cause a crash; in order to change something in a UI element, using signals is mandatory. Note that "access" also includes creation, and always results in a crash.
Calling repaint is a common (and wrong) attempt to solve the above issue; that's unnecessary, as properly setting widget properties (like using appendPlainText()) already results in a scheduled repaint on its own; there are very few cases for which repaint is actually necessary, and the rule of thumb is that if you're calling it you probably don't know what your doing or why your doing it. In any case, calling update() is always preferred, and it must always be called from the main UI thread anyway.
Using imports in a function is rarely required, as import statements should always be in the very beginning of the script; while there are cases for which imports can (or should) be done later or in a specific function, doing them in a function that is likely to be called often, makes using them there completely pointless. Also, datetime is part of the standard library, so importing it on demand will hardly affect performance (especially considering its "performance weight" against what a big library like Qt is compared to it).
When the ui is loaded from a .ui file (or a pyuic generated file), PyQt alread creates all widgets as instance attributes, so there's no need for findChild. Unfortunately there are a lot of tutorials that suggest that approach, and they are just completely and plain wrong. You can already access those widgets as self.outputlog and self.button right after uic.loadUi.
Function names (like variables and attributes) should always begin with a lower case letter, as only classes and constants should begin with upper cases (see the official Style Guide for Python Code). Also, object names should always explain what those object do (see "self-documenting code"): a function that does an "action" should have a verb; if it's named "Button" it doesn't tell me that it's going to do some processing, and that's not a very good thing.
A "main" function (like your applic) usually makes sense within the common if __name__ == '__main__': block, which ensures that that function doesn't get called in case the file gets imported instead of being directly run. (See this answer and the related question).
Since QRunnable doesn't inherit from QObject, we can create a QObject subclass that acts as a signal "proxy", and make it a member of the QRunnable instance. Then we must connect to that signal everytime we create a new Worker object.
Here is a revised version of your code, based on the above points.
import datetime
from PyQt5.QtCore import *
from PyQt5.QtWidgets import *
from PyQt5.uic import loadUi
class WorkerSignals(QObject):
mySignal = pyqtSignal(str)
class Worker(QRunnable):
def __init__(self):
super().__init__()
self.signalProxy = WorkerSignals()
self.mySignal = self.signalProxy.mySignal
def run(self):
self.mySignal.emit("Test")
class Ui(QMainWindow):
def __init__(self):
super(Ui, self).__init__()
loadUi('./pycdra.ui', self)
self.button.clicked.connect(self.startWorker)
self.threadpool = QThreadPool()
self.logentry("Available threads: %d" % self.threadpool.maxThreadCount())
def startWorker(self):
worker = Worker()
worker.mySignal.connect(self.logentry)
self.threadpool.start(worker)
def logentry(self, returntext):
self.outputlog.appendPlainText(self.timestamp() + " " + str(returntext))
def timestamp(self):
ts = datetime.datetime.now()
return ts.strftime("%Y-%m-%d %H:%M:%S")
def applic():
import sys
app = QApplication(sys.argv)
window = Ui()
window.show()
sys.exit(app.exec_())
if __name__ == "__main__":
applic()
I suggest you to carefully study the provided code and the differences with yours, and then do some careful, patient research both on the links given above and the following topics:
classes and instances, and the meaning of self;
python types (including None);
what is event driven programming and how it relates with graphical interfaces;
the most important Qt classes, QObject and QWidget, and all their properties and functions (yes, they are a lot);
general PyQt related topics
(most importantly, signals/slots and properties);
code styling and good practices;

Use the class' initialised form
class foo:pass
selfuotsideclass=foo()
Use variable selfoutsideclass as self

Drag and drop with pyqt5 (SIGNAL)

I am trying to get drag and drop (with images or files) working for my listwidget in pyqt5. I can find a lot of examples with pyqt4, but there is one part that does not work in the newer version:
In the "dropevent":
self.emit(QtCore.SIGNAL("dropped"), links)
and in the MainForm:
self.connect(self.view, QtCore.SIGNAL("dropped"), self.pictureDropped)
I read a lot of posts but can't seem to find an answer how this should be written down with pyqt5. Could anyone please help me with this?
link to the entire example:
PyQT4: Drag and drop files into QListWidget

Simple.
The signal must come first
The slot come as parameter.
You must connect only after the signal was created
You must emit only after the connection was made.
Here we go with a small example:
self.signalOwner.mySignal.connect(self.slotFunction)
In your example let's say and consider that the view owns the signal and that pictureDropped is your slot function, so:
self.view.dropped.connect(self.pictureDropped)
Remember, your signal must emit certain type(s) or nothing at all andddd your #pyqtSlot function must receive same type(s) with the function receiving the same amount of parameter that your signal emits.
I have made a post some short time ago about drag and drop with images, had many difficulties to figure it out how to accept events and how the classes behave, it's a bit deeper than only signals so, if you need here is the POST :D

Your linked example uses the old-style signal and slot syntax, whereas you need to use the new-style syntax. In PyQt4 (versions 4.5 or later), you could use either style, but PyQt5 only supports the new style.
The old-style syntax allowed custom signals to be emitted on-the-fly, but the new-style syntax demands that the signal is declared beforehand on the class:
class ListWidget(QtWidgets.QListWidget):
dropped = QtCore.pyqtSignal(list)
def __init__(self, parent=None):
...
def dropEvent(self, event):
...
self.dropped.emit(list_of_files)
The signal connection is then made like this:
class MainWindow(QtWidgets.QMainWindow):
def __init__(self, parent=None):
...
self.listWidget = ListWidget()
self.listWidget.dropped.connect(self.handleDropped)
def handleDropped(self, list_of_files):
print('dropped:', list_of_files)

Passing an argument when starting new QThread() in PyQt

I have a multi-threaded application written in Python in which one thread "takes care" of the GUI, and the other is the worker thread. However, the worker thread has two main functions (or so to say two main jobs), and I need to tell the run function which job exactly to do.
So what I had in mind was to create a run function in the worker thread which will take one parameter (save for "self). The parameter will either be "create" or upload. Without further ado, here's the somewhat-code that I have so far:
GUI.py
class GUI(QMainWindow):
def __init__(self, parent=None):
super, etc
self.worker = worker.Worker()
def create(self):
self.worker.start()
def upload(self):
self.worker.start()
Worker.py
class Worker(QThread):
def __init__(self, parent=None):
super, etc
def run(self):
self.create_data() # OR self.upload_data(), depends
So the question is, how can I tell worker.start() which function I want it to perform? I realize one could directly use worker.run() method, but I was told by the "Rapid GUI development with PyQT" never to call worker.run() directly, and always to use worker.start().

The start method of QThread doesn't accept arguments. However, you've inherited QThread so you're free to customize it at will. So, to implement what you want, just pass arguments into the constructor of Worker.
Here's your code sample slightly modified to show this in action:
class Worker(QThread):
def __init__(self, do_create_data=True, parent=None):
super(QThread, self).__init__()
self.do_create_data = create_data
def run(self):
if self.create_data:
self.create_data()
else:
self.upload_data(), depends

Eli Bendersky's answer is correct, however the order of arguments appears wrong.
If you call the Worker class like this:
The argument order that worked for me:
def __init__(self, parent=None, do_create_data=True):
The order shown in Eli Bendersky's answer produced this error message for me:
TypeError: QThread(QObject parent=None): argument 1 has unexpected type 'str'
Not sure why, but I'm sure someone can help explain.

PyQt4.QtCore.pyqtSignal object has no attribute 'connect'

I'm having issues with a custom signal in a class I made.
Relevant code:
self.parse_triggered = QtCore.pyqtSignal()
def parseFile(self):
self.emit(self.parse_triggered)
Both of those belong to the class: RefreshWidget.
In its parent class I have:
self.refreshWidget.parse_triggered.connect(self.tabWidget.giveTabsData())
When I try to run the program, I get the error:
AttributeError: 'PyQt4.QtCore.pyqtSignal' object has no attribute 'connect'
Help?
Thanks in advance.

I had the same exact problem as you.
Try moving
self.parse_triggered = QtCore.pyqtSignal()
out of your constructor but inside your class declaration. So instead of it looking like this:
class Worker(QtCore.QThread):
def __init__(self, parent = None):
super(Worker, self).__init__(parent)
self.parse_triggered = QtCore.pyqtSignal()
It should look like this:
class Worker(QtCore.QThread):
parse_triggered = QtCore.pyqtSignal()
def __init__(self, parent = None):
super(Worker, self).__init__(parent)
This might not be at all what you are looking for, but it worked for me. I switched back to old-style signals anyways because I haven't found a way in new-style signals to have an undefined number or type of parameters.

You also get that error message if you fail to call super() or QObject.__init__() in your custom class.
A checklist for defining custom signals in a class in Qt in Python:
your class derives from QObject (directly or indirectly)
your class __init__ calls super() (or calls QObject.__init__() directly.)
your signal is defined as a class variable, not an instance variable
the signature (formal arguments) of your signal matches the signature of any slot that you will connect to the signal e.g. () or (int) or (str) or ((int,), (str,))

I have recently started working with PySide (Nokia's own version of PyQt), and saw the exact same behaviour (and solution) with custom new-style signals. My biggest concern with the solution was that using a class variable to hold the signal would mess things up when I have multiple instances of that class (QThreads in my case).
From what I could see, QtCore.QObject.__init__(self) finds the Signal variable in the class and creates a copy of that Signal for the instance. I have no idea what QObject.__init__() does, but the resulting Signal does proper connect(), disconnect() and emit() methods (and also a __getitem__() method), whereas the class Signal or standalone Signal variables created outside of a QObject-derived class do not have these methods and can't be used properly.

To use the signal/slot system you need to have a QObject inherited class.
Here is a simple example:
from PySide import QtCore
class LivingBeing(QtCore.QObject):
bornSignal = QtCore.Signal() # initialise our signal
def __init__(self,name):
QtCore.QObject.__init__(self) # initialisation required for object inheritance
self.bornSignal.connect(self.helloWorld) # connect the born signal to the helloworld function
self.name = name #
self.alive = False
def summonFromClay(self):
self.alive = True
self.bornSignal.emit() # emit the signal
def helloWorld(self):
print "Hello World !, my name is %s, this place is so great !" % self.name
# now try the little piece of code
if __name__ == '__main__':
firstHuman = LivingBeing('Adam')
firstHuman.summonFromClay()

I had the same problem.
I forgot that if a class uses Signals, then it must inherit from QObject. I was doing some re-factoring and did not pay attention to this.

Why do you connect directly to the signal, while you can do
self.connect(widget, SIGNAL('parse_triggered()'), listener.listening_method)?
where self is, for example, the form itself, and may be the same as listener