tkinter callback update GUI

Tkinter is a python standard GUI package, it is easy to use if you want to do some simple GUI tasks. To control the widget action, we need to add callback method. while the GUI won't update until the callback method finished. For example, if I want to display a image in a window, and click a button to change to photo to another one.

class ClientGUI(Frame):
    def __init__(self, parent):
        Frame.__init__(self, parent)
        self.parent = parent
        self.initUI()
    def initUI(self):
        self.parent.title("demo")
        self.pack(fill=BOTH, expand=1)
        self.changeButton = Button(self, text="changePic", width = 10, height = 1, command=self.onClick)
        self.changeButton.place(x=10,y=100)
        self.initialImage = Image.open('pic1.jpg')
        tkimage = ImageTk.PhotoImage(self.initialImage)
        self.imgLabel = Label(self,image=tkimage)
        self.imgLabel.image = tkimage
        self.imgLabel.place(x=10,y=150)
       
    def onClick(self):
        imagedata = Image.open("pic2.jpg")
        img = ImageTk.PhotoImage(imagedata)
        print img,"\n"
        self.imgLabel.configure(image = img)
        self.imgLabel.image = img

This code works fine to display the picture and change the picture when button clicked. But if we add some code in the onClick method, like this:

for i in range(0,10):
    time.sleep(1)

Then after clicking the button, it will take 10 seconds before the image change. Same thing happens when there is a loop in the callback method, unless the loop is finished, this method won't callback. So what if we want to update the picture immediately we click the button, it means update the GUI, actually we only need to add one single line to solve this problem, add

self.update()

list update

If you need to modify the sequence you are iterating over while inside the loop (for example to duplicate selected items), it is recommended that you first make a copy. Iterating over a sequence does not implicitly make a copy. The slice notation makes this especially convenient:

for w in words[:]:  # Loop over a slice copy of the entire list.
    if len(w) > 6:
        words.insert(0, w)

#words: ['defenestrate', 'cat', 'window', 'defenestrate']

Dimension Unit Measurements Supported in Android

Pixels px
Actual screen pixels

Inches in
Physical measurement

Millimeters mm
Physical measurement

Points pt
Common font measurement unit

Screen density independent pixels dp
Pixels relative to 160dpi screen(Preferable for dimension screen compatibility)

Scale-independent pixels sp
Best for scalable font display