tkinter Label updating but not correctly

问题

Using Python 3.5. I have a tkinter form. The user clicks a button to import many files into a Listbox. Another button loops thru the files and reads and extracts data from them.

I have a Label on the form that indicates the status of the loop. The status, for the most part, works as expected except that extra characters are added on the end. I'm not sure where the characters come from. I also print() the same content as the Label back to the screen and the print() displays exactly what it should.

My question is why is my Label not displaying the correct string?

My code, greatly shortened:

class tk_new_db:
    def __init__(self, master):
        self.master = master  # sets 'root' to the instance variable 'master'
        self.var_text_2 = StringVar()
        self.var_text_2.set('STATUS: Active')
        self.label_6 = Label(master, textvariable=self.var_text_2,
                             font=self.font_10)
        self.label_6.grid(row=15, sticky=W, padx=15)

    def execute_main(self):  # extract data from files
        file_num = 0
        nf = len(self.listbox_1.get(0, END))
        for fr in li:
            file_num += 1
            print('STATUS: Extracting Loads from File '
                  '{} in {}'.format(file_num, nf))
            self.var_text_2.set('STATUS: Extracting Loads from File '
                                '{} in {}'.format(file_num, nf))
            self.master.update_idletasks()

The print() is writing the following: STATUS: Extracting Loads from File 1 in 5

The Label is writing the following: STATUS: Extracting Loads from File 1 in 5 nce...

It always adds ' nce...' on the Form.

EDIT:

I do use self.var_text_2 earlier in the program. The ' nce...' looks like it is a fragment of the previous string. I've since tried resetting the variable in two different ways, but I'm still getting the same result.

    self.var_text_2.set('STATUS: Checking .F06 Files for Convergence...')
    self.master.update_idletasks()
    self.var_text_2.__del__()

    self.var_text_2.set('STATUS: Checking .F06 Files for Convergence...')
    self.master.update_idletasks()
    self.var_text_2.set('')

How do you properly delete the StringVar() for reuse?

回答1:

The only explanation I can think of is that you're stacking labels on top of each other in the same row and column, so when you add a long string that ends in "nce..." and later update the screen by adding a shorter label in the same grid cell, the trailing text of the longer label underneath is showing through.

The reasons I draw this conclusion are:

this is a common mistake I've seen several times
there are no bugs in tkinter that would cause this
there is nothing in the code that you've shown that could possibly add "nce..." to the end of the string
the fact that you use sticky="w" rather than sticky="ew", which will cause a shorter label to not completely overlay a longer label if placed in the same row and column

回答2:

I think I can explain. Root.mainloop repeatedly calls root.update. Root.update executes tasks on the 'main' event queue. In brief, it only executes idletasks when the event queue is empty after checking for file events (on non-Windows), window-gui events, and timer events. Idletasks, which are not well documented but seem to primarily be screen update tasks, are never added to the main event queue. It is therefore possible that idletasks will remain undone indefinitely. Root.update_idletasks exists to force execution of idletasks anyway.

When mainloop is running, calling update within a task run by update is usually unnecessary and possibly worse. It is something only for adventurous experts. Hence the warnings you have read (which assume that you are running mainloop).

When mainloop is not running and update is not being called repeatedly, either because you never called mainloop or because you block it with a long running loop, then you likely must call update yourself. What you seem to have discovered is that part of completely handling a StringVar update is a main event and not just an idletask.

I sympathize with your desire to do repeated tasks with a for loop, but doing so currently means taking responsibility for event handling yourself. I hope in the future that it will be possible to have 'async for' (new in 3.5) interoperate with Tk.mainloop, but it is not as easy as I hoped.

In the meanwhile, you could put the body of the for loop into a callback and loop with root.after.

来源：https://stackoverflow.com/questions/38985226/tkinter-label-updating-but-not-correctly

标签

python

tkinter

label