Let's say I've got a function that requests input. How can I pause the function that calls this function while I am waiting for user input in an entry widget. I tried it with a while loop and time.sleep(sec). Furthermore I executed the function within another thread (so the main thread should not be interrupted), but the problem that always occurs is that the whole program freezes (typing in entry impossible)!
Because I do not have that much experience with Python I am truly stuck.
PS: I am coding on a mac.
The code I used:
import time
_input = []
def get_input()
try:
return _input[0]
except IndexError:
return None
def req():
while get_input() == None:
time.sleep(1)
return get_input()
The function req() is always called within a function which is called via 'getattr()' in a function which parses the input in the entry widget. The variable '_input' automatically gets the user input from the entry. The input I then successfully got from the '_input' variable is then discarded.
Maybe the problem is that the function is running and that is why another function cannot be executed... but shouldn't that be irrelevant if I was using a distinct thread? Why didn't that work...?
Here's a function that creates a simple Tkinter GUI that allows the user to input data into an Entry widget. When the user hits Enter the function gets the current value from the Entry, closes the GUI and returns the value to the calling code. The calling code will block until tkinter_input returns. If the user closes the GUI window with the close button the contents of the Entry are ignored, and None is returned.
import tkinter as tk
def tkinter_input(prompt=""):
root = tk.Tk()
tk.Label(root, text=prompt).pack()
entry = tk.Entry(root)
entry.pack()
result = None
def callback(event):
nonlocal result
result = entry.get()
root.destroy()
entry.bind("<Return>", callback)
root.mainloop()
return result
result = tkinter_input("Enter data")
print(result)
The way to wait for user input is to open up a dialog. A modal dialog will force the user to dismiss the dialog, a non-modal will allow the user to continue to use the main application.
In your case, you can create a dialog using a Toplevel and fill it with any widgets that you want, then use the wait_window function to wait for that window to be destroyed. To make it modal you can create a "grab" on the toplevel. To keep this simple, I have not done that in the following example.
Here is a basic example. The key is the call to wait_window which will not return until the dialog is destroyed.
import tkinter as tk
class CustomDialog(object):
def __init__(self, parent, prompt="", default=""):
self.popup = tk.Toplevel(parent)
self.popup.title(prompt)
self.popup.transient(parent)
self.var = tk.StringVar(value=default)
label = tk.Label(self.popup, text=prompt)
entry = tk.Entry(self.popup, textvariable=self.var)
buttons = tk.Frame(self.popup)
buttons.pack(side="bottom", fill="x")
label.pack(side="top", fill="x", padx=20, pady=10)
entry.pack(side="top", fill="x", padx=20, pady=10)
ok = tk.Button(buttons, text="Ok", command=self.popup.destroy)
ok.pack(side="top")
self.entry = entry
def show(self):
self.entry.focus_force()
root.wait_window(self.popup)
return self.var.get()
To use it, call the show method:
dialog = CustomDialog(root, prompt="Enter your name:")
result = dialog.show()
With the above, result will have the string that you entered.
For more information about creating dialogs, see Dialog Windows on the effbot site,
GUI programming is quite different from normal python scripts.
When you see the GUI pop up, it is already running in the mainloop. That means that your code is only invoked from the mainloop as a callback attached to some event or as a timeout function. Your code actually interrupts the flow of events in the mainloop.
So to keep the GUI responsive, callbacks and timeouts have to finish quickly (say in 0.1 second max). This is why you should not run long loops in a callback; the GUI will freeze.
So the canonical way to do a long calculation in a GUI program is to split it up into small pieces. Instead of e.g. looping over a long list of items in a for loop, you create a global variable that holds the current position in the list. You then create a timeout function (scheduled for running by the after method) that takes e.g. the next 10 items from the list, processes them, updates the current position and reschedules itself using after.
The proper way to get input for a function is to get the necessary input before starting the function. Alternatively, you could use a messagebox in the function to get the input. But in general it is considered good design to keep the "guts" of your program separate from the GUI. (Consider that you might want to switch from Tkinter to the GTK+ or QT toolkits in the future.)
Now onto threads. You might think that using threads can make long-running tasks easier. But that is not necessarily the case. For one thing, the standard Python implementation (we shall call it CPython) has a Global Interpreter Lock that ensures that only one thread at a time can be running Python bytecode. So every time your long-running calculation thread is running, the other thread containing the mainloop is halted. In Python 3 the thread scheduling is improved w.r.t. Python 2 as to try and not starve threads. But there is no guarantee that the GUI thread gets enough runtime when the other thread is doing a ton of work.
Another restriction is that the Tkinter GUI toolkit is not thread safe. So the second thread should not use Tkinter calls. It will have to communicate with the GUI thread by e.g. setting variables or using semaphores. Furthermore, data structures that are used by both threads might have to be protected by Locks, especially if both threads try to modify them.
In short, using threads is not as simple as it seems. Also multithreaded programs are notoriously difficult to debug.
Related
I recently started to use tkwait casually and noticed that some functionality only works under special conditions. For example:
import tkinter as tk
def w(seconds):
dummy = tk.Toplevel(root)
dummy.title(seconds)
dummy.after(seconds*1000, lambda x=dummy: x.destroy())
dummy.wait_window(dummy)
print(seconds)
root = tk.Tk()
for i in [5,2,10]:
w(i)
root.mainloop()
The code above works just fine and as expected:
The for loop calls the function
The function runs and blocks the code for x seconds
The window gets destroyed and the for loop continues
But in a more event driven environment these tkwait calls gets tricky. The documentation states quote:
If an event handler invokes tkwait again, the nested call to tkwait
must complete before the outer call can complete.
Instead of an output of >>5 >>2 >>10 you will get >>10 >>2 >>5 because the nested call blocks the inner and the outer will block the inner. I suspect a nested event loop or an equivalent of the mainloop processes events in the normal fashion while waiting.
Am I doing something wrong by using this feature? Because if you think about it, nearly all tkinter dialog windows are using this feature and I've never read about this behavior before.
An event driven example might be:
import tkinter as tk
def w(seconds):
dummy = tk.Toplevel(root)
dummy.title(seconds)
dummy.after(seconds*1000, lambda x=dummy: x.destroy())
dummy.wait_window(dummy)
print(seconds)
root = tk.Tk()
btn1 = tk.Button(
root, command=lambda : w(5), text = '5 seconds')
btn2 = tk.Button(
root, command=lambda : w(2), text = '2 seconds')
btn3 = tk.Button(
root, command=lambda : w(10), text = '10 seconds')
btn1.pack()
btn2.pack()
btn3.pack()
root.mainloop()
As an additional problem that raises with wait_something is that it will prevent your process to finish if the wait_something never was released.
Basically, you need great care if you're using an inner event loop because:
Conditions that would terminate the outer event loop aren't checked for until the inner event loop(s) are finished.
It's really quite easy to end up recursively entering an inner event loop by accident.
The recursive entry problem is usually most easily handled by disabling the path that enters the event loop while the inner event loop runs. There's often an obvious way to do this, such as disabling the button that you'd click.
The condition handling is rather more difficult. In Tcl, you'd handle it by restructuring things slightly using a coroutine so that the thing that looks like an inner event loop isn't, but rather is just parking things until the condition is satisfied. That option is... rather more difficult to do in Python as the language implementation isn't fully non-recursive (and I'm not sure that Tkinter is set up to handle the mess of async function coloring). Fortunately, provided you're careful, it's not too difficult.
It helps if you know that wait_window is waiting for a <Destroy> event where the target window is the toplevel (and not one of the inner components) and that destroying the main window will trigger it as all the other windows are also destroyed when you do that. In short, as long as you avoid reentrancy you'll be fine with it. You just need to arrange for the button that was clicked to be disabled while the wait is ongoing; that's good from a UX perspective too (the user can't do it, so don't provide the visual hint that they can).
def w(seconds, button):
dummy = tk.Toplevel(root)
dummy.title(seconds)
dummy.after(seconds*1000, lambda x=dummy: x.destroy())
button["state"] = "disabled" # <<< This, before the wait
dummy.wait_window(dummy)
button["state"] = "normal" # <<< This, after the wait
print(seconds)
btn1 = tk.Button(root, text = '5 seconds')
# Have to set the command after creation to bind the button handle to the callback
btn1["command"] = (lambda : w(5, btn1))
This all omits little things like error handling.
I have a python code that includes tkinter window and other running tasks.
I've been trying to bind "WM_DELETE_WINDOW" event to a function that exits my python code when I close the window but can't achieve that.
This is what I try:
def on_exit():
root.destroy()
sys.exit()
root.protocol('WM_DELETE_WINDOW', on_exit)
The window is destroyed successfully but the python code doesn't exit. Any possible reason for sys.exit() not to work?
What am I doing wrong? any alternative approach should I try?
Doing some testing I figured out what can be the problem.
Here's a small code that summarizes my code which is much bigger.
import tkinter as tk
import sys
root = tk.Tk()
submitted = tk.IntVar()
def on_exit():
root.destroy()
sys.exit()
root.protocol('WM_DELETE_WINDOW', on_exit)
def submit():
submitted.set(1)
print("submitted")
button= tk.Button(root, text="Submit",command=submit)
button.pack()
button.wait_variable(submitted)
root.mainloop()
I believe now that wait_variable is the source of the problem.
And the code actually exits when I added submitted.set(1) to on_exit() ( or if I clicked the button first before closing the window ) but if I tried closing the window without pressing the button, the code won't exit.
So does this mean that wait_variable not only makes tkinter app wait, but also prevents python code exiting?!
I tried os._exit(1) and it worked, but I think it's not clean.
As your updated question points out the problem is wait_variable(). Going off the documentation for this method wait_variable() enters a local event loop that wont interrupt the mainloop however it appears that until that local event loop is terminated (the variable is updated in some way) it will prevent the python instance from terminating as there is still an active loop. So in order to prevent this you have also correctly pointed out you need to update this variable right before you terminate the tk instance.
This might seam odd but it is the behavior I would expect. It is my understanding that an active loop needs to be terminated before a python instance can exit.
As Bryan has pointed out in the comments the wait_variable() method is "a function which calls the vwait command inside the embedded tcl interpreter. This tcl interpreter knows nothing about python exceptions which is likely why it doesn't recognize the python exception raised by sys.exit()"
Link to relevant documentation:
wait_variable()
Relevant text from link:
wait_variable(name)
Waits for the given Tkinter variable to
change. This method enters a local event loop, so other parts of the
application will still be responsive. The local event loop is
terminated when the variable is updated (setting it to it’s current
value also counts).
You can also set the variable to whatever it is currently set as to terminate this event loop.
This line should work for you:
submitted.set(submitted.get())
That said you do not actually need sys.exit(). You can simply use root.destroy().
You new function should look like this:
def on_exit():
submitted.set(submitted.get())
root.destroy()
The python instance will automatically close if there is no more code after the mainloop.
I'm trying to learn how to use the thread module. I followed along with the instructions here: http://effbot.org/zone/tkinter-threads.htm
My hope is the test script will:
Print out the "count" every two seconds
Show a pop-up dialog window (also every 2 seconds)
The pop-ups should be allowed to accumulate (if I don't click "OK" for a while, there should be
multiple pop-ups)
However, when I run this script it will freeze the main window and after a while crash. I think I'm not implementing the thread module correctly.
Could someone please have a look and point out what I'm doing wrong?
Here is what I've tried so far:
from Tkinter import *
import thread
import Queue
import time
class TestApp:
def __init__(self, parent):
self.super_Parent = parent
self.main_container = Frame(parent)
self.main_container.pack()
self.top_frame = Frame(self.main_container)
self.top_frame.pack(side=TOP)
self.bottom_frame = Frame(self.main_container)
self.bottom_frame.pack(side=TOP)
self.text_box = Text(self.top_frame)
self.text_box.config(height=20, width=20)
self.text_box.pack()
self.queue = Queue.Queue()
self.update_me()
def show_popup(self):
self.my_popup = Toplevel(self.main_container)
self.my_popup.geometry('100x100')
self.popup_label = Label(self.my_popup, text="Hello!")
self.popup_label.pack(side=TOP)
self.pop_button = Button(self.my_popup, text="OK", command=self.my_popup.destroy)
self.pop_button.pack(side=TOP)
def write(self, line):
self.queue.put(line)
def update_me(self):
try:
while 1:
line = self.queue.get_nowait()
if line is None:
self.text_box.delete(1.0, END)
else:
self.text_box.insert(END, str(line))
self.text_box.see(END)
self.text_box.update_idletasks()
except Queue.Empty:
pass
self.text_box.after(100, self.update_me)
def pipeToWidget(input, widget):
widget.write(input)
def start_thread():
thread.start_new(start_test, (widget,))
def start_test(widget):
count = 0
while True:
pipeToWidget(str(count) + "\n", widget)
count += 1
time.sleep(2)
widget.show_popup()
root = Tk()
widget = TestApp(root)
start_button = Button(widget.bottom_frame, command=start_thread)
start_button.configure(text="Start Test")
start_button.pack(side=LEFT)
root.title("Testing Thread Module")
root.mainloop()
I can't reproduce your problem, but I can see why it would happen.
You're using the queue to pass messages from the background thread to the main thread for updating text_box, which is correct. But you're also calling widget.show_popup() from the background thread, which means it creates and displays a new Toplevel in the background thread. That's not correct.
All UI code must run in the same thread—not all UI code for each top-level window, all UI code period. On some platforms, you may get away with running each window in its own thread (or even free-threading everything), but that isn't supposed to work, and definitely will crash or do improper things on some platforms. (Also, that single UI thread has to be the initial thread on some platforms, but that isn't relevant here.)
So, to fix this, you need to do the same dance for creating the popups that you do for updating the textbox.
The obvious way to do that is to move the widget.show_popup() to the loop in update_me(). If you want it to happen 2 seconds after the textbox updates, just add self.top_frame.after(2000, self.show_popup) to the method.
But I'm guessing you're trying to teach yourself how to have multiple independent updating mechanisms, so telling you "just use a single update queue for everything" may not be a good answer. In that case, just create two queues, and a separate update method servicing each queue. Then, do your pipeToWidget, sleep 2 seconds, then pipeToPopup.
Another way around this is to use mtTkinter. It basically does exactly what you're doing, but makes it automatic, pushing each actual Tk GUI call onto a queue to be run later by the main loop. Of course your objects themselves have to be thread-safe, and this also means that you have to deal with the GUI calls from one thread getting interleaved with calls from another thread. But as long as neither of those is a problem (and they don't seem to be in your case), it's like magic.
If you want to know why this is freezing and/or crashing for you on Win7 and not for me on OS X 10.8… well, you really need to look into a mess of Tcl, C, and Python code, and also at how each thing is built. And, unless it's something simple (like your Tk build isn't free-threaded), it wouldn't tell you much anyway. The code isn't supposed to work, and if it seems to work for me… that probably just means it would work every time until the most important demo of my career, at which point it would fail.
I have an already large Tkinter program, so that I have an init file, where the root = Tk() window is defined (containing basically a Text widget and a few other things), some more code, and last the call to mainloop() function.
Everything works, until I needed to call a procedure before the mainloop, and I wanted to raise a wait window at the begin, to be destroyed at procedure's end.
I wrote something like:
msg = Message(root, text='wait a few seconds...')
msg.pack()
But it doesn't and cannot work, since mainloop() has not been called yet!
If I instead do:
msg = Message(root, text='wait a few seconds...')
msg.pack()
mainloop()
The program stops at this first mainloop, doesn't finish the procedure call.
mainloop() should be used as your last program line, after which the Tkinter program works by a logic driven by user clicks and interactions, etc.
Here, I need a sequence of raise window > do stuff > destroy window > mainloop
You are correct that mainloop needs to be called once, after your program has initialized. This is necessary to start the event loop, which is necessary for windows to draw themselves, respond to events, and so on.
What you can do is break your initialization into two parts. The first -- creating the wait window -- happens prior to starting the event loop. The second -- doing the rest of the initialization -- happens once the event loop has started. You can do this by scheduling the second phase via the after method.
Here's a simple example:
import Tkinter as tk
import time
class SampleApp(tk.Tk):
def __init__(self, *args, **kwargs):
# initialize Tkinter
tk.Tk.__init__(self, *args, **kwargs)
# hide main window
self.wm_withdraw()
# show "please wait..." window
self.wait = tk.Toplevel(self)
label = tk.Label(self.wait, text="Please wait...")
label.pack()
# schedule the rest of the initialization to happen
# after the event loop has started
self.after(100, self.init_phase_2)
def init_phase_2(self):
# simulate doing something...
time.sleep(10)
# we're done. Close the wait window, show the main window
self.wait.destroy()
self.wm_deiconify()
app = SampleApp()
app.mainloop()
You should use Tkinter's method to run asyncore's loop function, but you should use asyncore.poll(0) instead of asyncore.loop(). If you call function asyncore.poll(0) every x ms, it has no longer an effect on Tkinter's main loop.
I'm in a bind, since this is being written on a classified machine I am unable to copy+paste here. Being somewhat a novice, my approach is probably unorthodox.
I have a GUI written in Tkinter with several buttons. Each button is linked to a class that, in effect, runs a short script. When the button is clicked, I inititalize a class log_window which is simply a Tkinter text widget. I then create a global variable linking log to the log_window I just created, and as the script runs I pipe sys.stdout/stderr to log (I created a write method specifically for this). Everything is kosher, except that the log_window text widget doesn't update with my piped stdout until after the class calling it is finished. However, if I simply print within the class, it will print in the order it is called.
Example
import Tkinter
from Tkinter import *
import time
class log_window:
def __init__(self,master):
self.textframe = Tkinter.Frame(master)
self.text = Text(self.textframe)
self.text.pack()
self.textframe.pack()
def write(self,text):
self.text.insert(END,text)
class some_func1: # This effectively waits 5 seconds then prints both lines at once
def __init__(self,master):
log.write("some text")
time.sleep(5)
log.write("some text")
class some_func2: # This prints the first object, waits 5 seconds, then prints the second
def __init__(self,master):
print "some text"
time.sleep(5)
print "some text"
if __name__ == '__main__':
global log
root = Tk()
log = log_window(root)
root.after(100,some_func1, root)
root.after(100,some_func2, root)
root.mainloop()
Sorry if my example is a little bit muffed, but I think it makes the point. The piping I do is through Popen and some system calls, but they aren't part of the issue, so I only highlighted what, I presume, is the LCD of the issue.
I don't know the details of Tkinter's concurrency, but fiddling around reveals that if you put
master.update_idletasks()
after each call to log.write, it updates on cue. You could give log a .flush() method to do that (like file handles have), or you could just make log.write call it after writing.
When you call sleep it causes your whole GUI to freeze. You must remember that your GUI runs an event loop, which is an infinite loop that wraps all your code. The event loop is responsible for causing widgets to redraw when they are changed. When a binding is fired it calls your code from within that loop, so as long as your code is running, the event loop can't loop.
You have a couple of choices. One is to call update_idletasks after adding text to the widget. This lets the event loop service "on idle" events -- things that are schedule to run when the program isn't doing anything else. Redrawing the screen is one such event, and there are others as well.
The other option is to run your functions in a thread or separate process. Because Tkinter isn't thread safe, these other threads or processes can't directly communicate with the GUI. What they must do is push a message onto a queue, and then your main (GUI) thread must poll the queue and pull messages off. It would be easy to build this code into your log class, and polling the queue can be done using the event loop -- just write a method that pulls messages off the queue and inserts them into the widget, the calls itself using after a few hundred milliseconds later.
You have to update your widget content by adding self.text.update() after self.text.insert(END,text)