Here's an example:
class DemoFrame(wx.Frame):
def __init__(self, parent):
wx.Frame.__init__(self, parent)
self.panel = wx.Panel(self, -1)
...
initialize other elements
...
self.DoStuff()
def DoStuff(self):
self.panel.SetBackGroundColour(wx.Colour(240, 240, 240))
...
do something
...
Now as you know this is definitely not a good example of initializing your GUI since do something would most probably freeze the GUI while it's running, so I tweaked it to this:
import threading
class DemoFrame(wx.Frame):
def __init__(self, parent):
wx.Frame.__init__(self, parent)
self.panel = wx.Panel(self, -1)
...
initialize other elements
...
DoStuffThead = threading.Thread(target = self.DoStuff, ())
DoStuffThead.start()
def DoStuff(self):
wx.CallAfter(self.ChangeBG, )
...
do something
...
def ChangeBG(self):
self.panel.SetBackGroundColour(wx.Colour(240, 240, 240))
Above code should work exactly the same as the first one does when do something is blank, but to my surprise I noticed there's little background drawing glitches when running the latter codes.
What part went wrong? Isn't this the right way to update GUI in threads?
It's a bad approach to update GUI from worker thread, not event saying it's not thread safe.
You have to communicate with main thread to update GUI.
The best way to achieve desired result is to user wx.PostEvent method. You can create custom events for your needs, inheriting from wx.PyEvent, and you better inherit threading.Thread to keep window you want to communicate within that thread class as an instance variable.
The best illustration of how to update GUI having long-running task can be found in wxPython wiki (first example).
After searching and playing with wxpython for a while, I finally found a solution for this, and it's actually quite simple, just refresh the panel and everything will be all right(add this line into ChangeBG method): self.panel.refresh(). I've no idea why the glitch exists though.
As to Rostyslav's answer, thanks a lot mate!
"It's a bad approach to update GUI from worker thread:", I think you mean it's rude to directly insert GUI codes into worker thread(which is exactly what I did in the first example) in terms of thread-safety concern, basically those GUI codes should be wrapped into thread-safe method(which is exactly what I was trying to do in my second example) and then queued into GUI main thread.
I found that there are basically three thread-safe methods as to GUI updating in worker thread: wx.PostEvent, wx.CallAfter and wx.CallLater, but I never liked wx.PostEvent, it's kind of cumbersome and you have to come up with your own event too, that's why wx.CallAfter is a better choice for me, it's more pythonic and easy to use, and actually wx.CallAfter is like a high level wrapper for wx.PostEvent if you check out the source code in _core.py:
def CallAfter(callable, *args, **kw):
"""
Call the specified function after the current and pending event
handlers have been completed. This is also good for making GUI
method calls from non-GUI threads. Any extra positional or
keyword args are passed on to the callable when it is called.
:see: `wx.CallLater`
"""
app = wx.GetApp()
assert app is not None, 'No wx.App created yet'
if not hasattr(app, "_CallAfterId"):
app._CallAfterId = wx.NewEventType()
app.Connect(-1, -1, app._CallAfterId,
lambda event: event.callable(*event.args, **event.kw) )
evt = wx.PyEvent()
evt.SetEventType(app._CallAfterId)
evt.callable = callable
evt.args = args
evt.kw = kw
wx.PostEvent(app, evt)
Well, I never tried wx.PostEvent implementation in my app, but I'm sure it would work as well.
Oh also I found this article very helpful: wxPython and Threads
Related
My question concerns PyQT5.
I try to have a dialog window with a button that when clicked
updates some text of a QTextEdit field
calls a function (which needs much time to terminate)
Something like this:
class StartDialog(QtWidgets.QWidget, start_dialog_ui.Ui_Dialog):
def __init__(self, parent):
super(self.__class__, self).__init__()
self.setupUi(self)
self.OKButton.clicked.connect(self.start)
def start(self):
self.startDialogTextEdit.append("simulation running ...")
run_lengthy_function(self)
However, when I run my GUI I notice that the text is updated only after the lengthy function has terminated, although the QTextEdit.append is called before the lengthy function. How can I enforce that the text is updated in advance?
What I tried so far (but didn't work) was to let Python wait some time before triggering the lengthy function call, i.e.
from time import sleep
class StartDialog(QtWidgets.QWidget, start_dialog_ui.Ui_Dialog):
def __init__(self, parent):
super(self.__class__, self).__init__()
self.setupUi(self)
self.OKButton.clicked.connect(self.start)
def start(self):
self.startDialogTextEdit.append("simulation running ...")
sleep(5)
run_lengthy_function(self)
The repaint is called in event loop so sleep the whole thread does not change anything.
You can call repaint manually by:
self.startDialogTextEdit.repaint()
or call static method:
QCoreApplication.processEvents()
which also call repaint internally
The solution for the case that the text is displayed in the QTextEdit is to call qApp.processEvents(), this force to the GUI update:
def start(self):
self.startDialogTextEdit.append("simulation running ...")
QtWidgets.qApp.processEvents()
[...]
On the other hand if the task is heavy it may be blocking the GUI, so maybe one solution is to run it on another thread, I can not give a proper recommendation since I do not know your function
I have noticed that there are a lot of users, myself included, who don't quite grasp the concept of signals and slots in Qt. I was hoping to get some clarification on the following:
#I have a function that runs as soon as the GUI is built, this takes the information from
#a list and puts it into a string which is then uploaded to a texbox. At the bottom of this
#loop, I want it to call a function in the parent thread via signals and slots, as
#recommended by other users.
class MainWindow(QtGui.QMainWindow):
#all the code needed to build the GUI
thread_mythread = threading.Thread(target = self.updateText, args = ())
thread_mythread.start()
def clearText(self):
self.TextEdit.clear()
def updateText(self):
self.trigger.connect(self.clearText)
while True:
self.trigger.emit()
NewString = list.pop(0)
#I think I may have to use append, as setText() is not safe outside of the parent thread
self.TextEdit.append(NewString)
Although probably terribly incorrect, I attempt to use signals. Is this the proper way to do it? I also get an error that says that the Main Window object has no attribute "trigger",why is this?
thank you.
The reason you get that error is exactly the reason described by the error message - the signal trigger has not been defined anywhere in your class. You need to define it before you can emit it.
Signals and slots are used to communicate between different objects. In your example you are trying to do everything from within your MainWindow class and there is no interaction with other objects. You also only need to make the call to connect() once. You would typically call this either in the class constructor or from your main function after instantiating the objects you want to connect together.
Take a look at http://pyqt.sourceforge.net/Docs/PyQt4/new_style_signals_slots.html for some examples of how to use signals and slots properly in PyQt.
For threading, use QThread rather than threading.Thread as it is better integrated with the Qt framework. This post shows some simple examples of how to use QThread in PyQt. The second method (using moveToThread()) is considered to be the most correct way to create new threads.
The basic idea for your kind of problem is:
handle GUI operations from the main thread
handle blocking operations (in your case the while loop) in a separate thread
emit signals from the worker thread to call functions (slots) in the main thread and vice versa
Also note that:
You cannot call any methods of QWidget its descendents from a secondary thread
Signals can also send data if you need to pass it between threads
To add to #user3419537 good answer. A very quick threading example:
from PyQt4.QtCore import QObject, pyqtSlot, pyqtSignal, QThread, \
Q_ARG, Qt, QMetaObject
class MyWorker(QObject):
# define signal
clear = pyqtSignal()
update_text_signal = pyqtSignal(str) # passes a string back
finished = pyqtSignal()
def __init__(self, parent=None):
super(MyWorker, self).__init__(parent)
# Add functions etc.
#pyqtSlot(list)
def update_text(self, string_list):
#Intensive operation
self.clear.emit() # moved outside of while
while(True):
#This is infinite loop so thread runs forever
new_string = self.string_list.pop(0)
self.update_text_signal.emit(new_string) # Fixed this line
#Finished
self.finished.emit()
Then in your MainWindow class
self.my_thread = QThread()
self.handler = MyWorker()
self.handler.moveToThread(self.my_thread)
self.handler.clear.connect(self.clearText)
self.handler.update_text_signal.connect(self.update_line_edit)
self.handler.finished.connect(self.my_thread.quit)
# Start Thread
self.my_thread.start()
#pyqtSlot(str)
def update_line_edit(self, text):
self.TextEdit.append(text)
QMetaObject.invokeMethod(self.handler, 'update_text',
Qt.QueuedConnection,
Q_ARG(list, string_list))
You will need to call self.my_thread.quit() before your application closes to stop the thread and avoid the error: QThread: Destroyed while thread is still running
Please read docs for QMetaObject.invokeMethod.
I am developing my pet project. It is a small app which downloads my inbox folder from Facebook. I want it to be available in both CLI and GUI (in PyQt) mode. My idea was that I write a class for the communication, and then the front-ends. I know that the downloading process is blocking which is not problem in CLI mode but it is in GUI.
I know that there is QNetworkAccessManager but I would have to re-implement the class that I have already written and then maintain two classes the same time.
I was searching for a while and I came up with one solutions where I subclass QObject and my FB class, implement the signals, than create a QThread object and use moveToThread() method which would be OK, but I must take care of creating and stopping the tread.
Is it possible to wrap my Python class to something to make it behave like QNetworkAccessManager? So the methods would return immediately and the object will emit signals when the data is ready.
Update
Thank you the comments. I have 2 main classes. The first is called SimpleGraph which just hides urllib2.urlopen(). It prepares the query and returns the decoded json got from Facebook. The actual work is happening in FBMDown class:
class FBMDown(object):
def __init__(self, token):
self.graph = SimpleGraph(token)
self.last_msg_count = 0
def _message_count(self, thread_id):
#TODO: Sanitize thread_id
p = {'q': 'SELECT message_count FROM thread WHERE thread_id = {0} LIMIT 1'.format(thread_id)}
self.last_msg_count = int(self.graph.call(params=p, path='fql')['data'][0]['message_count'])
return self.last_msg_count
So here when _message_count is called it returns the number of messages of the given thread id. This method works great in CLI mode, blocking is not a problem there.
I want to wrap this class into a class (if it is possible) which works asynchronous like QNetworkAccessManager, so it would not block the GUI but it would emit a signal when the data is ready.
The only technique I know now is to subclass QObject. It looks like this:
class QFBMDown(QtCore.QObject):
msg_count_signal = QtCore.pyqtSignal(int)
def __init__(self, parent=None):
QtCore.QObject.__init__(self, parent)
def get_msg_count(self):
#Here happens the IO
time.sleep(2)
self.msg_count_signal.emit(1)
And this is my Windows class:
class Window(QtGui.QMainWindow):
def __init__(self):
super(Window, self).__init__()
self.centralwidget = QtGui.QWidget(self)
self.button_getmsgcount = QtGui.QPushButton(self.centralwidget)
self.setCentralWidget(self.centralwidget)
self.i = 0
self.timer = QtCore.QTimer()
self.timer.timeout.connect(self.block_indicator)
self.timer.start(20)
#self.worker = QtCore.QThread()
self.fbobj = QFBMDown()
#self.fbobj.moveToThread(self.worker)
#self.worker.start()
self.button_getmsgcount.clicked.connect(self.fbobj.get_msg_count)
self.fbobj.msg_count_signal.connect(self.show_msg_count)
def block_indicator(self):
self.button_getmsgcount.setText(str(self.i))
self.i += 1
def show_msg_count(self, data):
print 'Got {0} msgs in the thread'.format(data)
Now when I press the button the GUI blocks. If I de-comment line 13, 15, 16 (where I create a thread, move fbobj into this thread then start it) it does not block, it works nearly as I want but I have to do everything by hand every time and I have to take care of shutting down the thread (now I implement QMainWindow's closeEvent method where I shut down the thread before quit but I'm sure that this is not the right way to do).
Is it even possible what I want to do? I would not want to implement a new class and then maintain two classes which are doing the same thing.
The reason I am so insisted to do it like this is to make my app work without Qt but give a nice gui for those who don't want to type command line arguments.
I'm trying to create a thread, and end it when the frame of a wxPython app is closed. Here's my code:
#! /usr/bin/env python
import time, wx
from threading import Thread
class UpdateThread(Thread):
def __init__(self):
self.stopped = False
Thread.__init__(self)
def run(self):
while not self.stopped:
self.updateExchange()
time.sleep(1)
def updateExchange(self):
print("Updated...")
class tradeWindow(wx.Frame):
def __init__(self, parent, id):
wx.Frame.__init__(self, parent, id, "Exchange", size = (500, 190))
panel = wx.Panel(self)
def OnExit(self):
tickThread.stopped # I've also tried: tickThread.stopped = True
tickThread = UpdateThread()
tickThread.start()
if __name__ == "__main__":
app = wx.PySimpleApp()
frame = tradeWindow(parent = None, id = -1)
frame.Show()
app.MainLoop()
But when I close the frame, it keeps printing.
There is no magic Frame.OnExit method. You're mixing up frames and apps. Frames, like other windows, close. Apps exit.
So, you could put your code in your app class's OnExit method. But that's not what you want here.
Look at this simple tutorial for the OnExit method. Again, it's not what you want here, but you should know how it works (and which object it gets called on).
You can always bind EVT_CLOSE to call anything you want in your window. But you have to do this explicitly.
Normally, you'd call the method OnClose or OnCloseWindow. Calling it OnExit is just going to lead to major confusion (as it has).
The event handler method that you bind to has to actually be an event handler, meaning it takes an event parameter (as well as the self).
Next, if you add an EVT_CLOSE handler, you're overriding the default one, meaning that Destroy never gets called unless you do it yourself.
Here's a tutorial on binding EVT_CLOSE, which shows all of the above steps.
Finally, as DavidRobinson explained, just doing tickThread.stopped won't do anything; you have to set it to True.
Putting it all together:
class tradeWindow(wx.Frame):
def __init__(self, parent, id):
wx.Frame.__init__(self, parent, id, "Exchange", size = (500, 190))
panel = wx.Panel(self)
self.Bind(wx.EVT_CLOSE, self.OnClose)
def OnClose(self, event):
tickThread.stopped = True
self.Destroy()
One more note:
In any serious threaded program, if you want to share a value between threads, you generally need to synchronize it with some kind of sync object. If you're waiting for a signal from another thread, the typical way to handle that is with a Condition. If, on the other hand, you just want to share the value, you can use a Lock.
If you really know what you're doing, you can often get away with letting the Global Interpreter Lock handle synchronization for you. But in general, this is a bad idea. For example, your main thread could be running on core 0, and your background thread on core 1, and there is nothing in the Python language definition that guarantees that the computer will ever copy the updated value from core 0's cache to core 1's. So, your background thread could be spinning forever, watching the old value and never getting the new value to look at. As it turns out, with CPython 2.0-3.3 on an x86, this can't possibly happen with your code—but on less you can prove that (or at least identify the cases that are safe), don't count on it.
Finally, you asked whether a daemon thread is an appropriate solution. From the docs:
The significance of this flag is that the entire Python program exits when only daemon threads are left.
In other words, your program can exit without having to stop your daemon threads. But…
Note Daemon threads are abruptly stopped at shutdown. Their resources (such as open files, database transactions, etc.) may not be released properly. If you want your threads to stop gracefully, make them non-daemonic and use a suitable signalling mechanism such as an Event.
Try setting tickThread.setDaemon(True) before tickThread.start() - Daemon threads should exit with their parent threads.
I have a relatively large application written in Python and using PyQT as a GUI frontend. The entire application is in one class, in one file.
Here's an example code:
class Application(QMainWindow):
def __init__(self):
super(etc...)
self.connect(self.mainBtn, SIGNAL("clicked()"), self.do_stuff)
def do_stuff(self):
<checking some parameters>
else:
do_some_other_long_stuff()
def do_some_other_long_stuff(self):
500 lines of code of stuff doing
However, this is the problem: when I click the mainBtn, everything goes fine, except the GUI kind of freezes - I can't do anything else until the function is performed (and it's a web scraper so it takes quite a bit of time). When the function do_some_other_long_stuff ends, everything goes back to normal. This is really irritating.
Is there a way to somehow "background" the do_some_other_stuff process? I looked into QThreads and it seems it does just that, however that would require me to rewrite basically all of code, put half of my program in a different class, and therefore have to change all the variable names (when getting a variable from GUI class and putting it in working class)
Duplicate of Handling gui with different threads,
How to keep track of thread progress in Python without freezing the PyQt GUI?, etc.
Your do_stuff() function needs to start up the computing thread and then return. Multi-threading is the name given to running multiple activities in a single process - by definition if something is going on "in the background", it's running on a separate thread. But you don't need to split functions into a different classes to use threads, just be sure that the computing functions don't do anything with the GUI and the main thread doesn't call any of the functions used by the computing thread.
EDIT 10/23: Here's a silly example of running threads in a single class - nothing in the language or the threading library requires a different class for each thread. The examples probably use a separate class for processing to illustrate good modular programming.
from tkinter import *
import threading
class MyApp:
def __init__(self, root):
self.root = root
self.timer_evt = threading.Event()
cf = Frame(root, borderwidth=1, relief="raised")
cf.pack()
Button(cf, text="Run", command=self.Run).pack(fill=X)
Button(cf, text="Pause", command=self.Pause).pack(fill=X)
Button(cf, text="Kill", command=self.Kill).pack(fill=X)
def process_stuff(self): # processing threads
while self.go:
print("Spam... ")
self.timer_evt.wait()
self.timer_evt.clear()
def Run(self): # start another thread
self.go = 1
threading.Thread(target=self.process_stuff, name="_proc").start()
self.root.after(0, self.tick)
def Pause(self):
self.go = 0
def Kill(self): # wake threads up so they can die
self.go = 0
self.timer_evt.set()
def tick(self):
if self.go:
self.timer_evt.set() # unblock processing threads
self.root.after(1000, self.tick)
def main():
root = Tk()
root.title("ProcessingThread")
app = MyApp(root)
root.mainloop()
main()