Why does Qt.AlignCenter onQLabel differ for image and text in PyQt5?

Question

I am new to python and currently doing some basic excerises. I'm trying to rewrite some application in which I used tkinter to do the same with PyQt5. Everything works apart from one problem - I have a QLabel containing image and I'm trying to align the image in the center of label but it doesn't want to, image stays aligned to the left. This was answered by @eyllanesc, who suggested that the QLabel is not centered with respect to the window and I should center the widget by changing to:

layout.addWidget(label_img, alignment=Qt.AlignCenter)

And that worked perfectly, however there are two more widgets (label_top, label_bottom) in the same layout, labels with text.And despite not beign aligned to center of the window, the text displayed centered. Why does label with text behave different to label with image?

Code below:

from PyQt5.QtWidgets import QApplication, QLabel, QWidget, QVBoxLayout, QPushButton, QFileDialog
from PyQt5.QtCore import Qt
from PyQt5.QtGui import QIcon, QPixmap

app=QApplication([])
window=QWidget()
window.setFixedSize(500,500)

layout=QVBoxLayout()

label_top=QLabel('PLEASE WAIT')
label_top.setAlignment(Qt.AlignCenter)
label_top.setStyleSheet("font: 20pt Bahnschrift; background-color: #ffd167; color: black")
layout.addWidget(label_top)

label_img=QLabel()
label_img.setFixedSize(300, 300)
label_img.setAlignment(Qt.AlignCenter)
image = QFileDialog.getOpenFileName(None,'Select file','D:\_Download', "Image files(*.png *.jpg *.jpeg *.gif)")
imagePath = image[0]
pixmap = QPixmap(imagePath)
pixmap.scaledToHeight(label_img.height(), Qt.SmoothTransformation)
label_img.setPixmap(pixmap)
#label_img.resize(pixmap.width(),pixmap.height())
layout.addWidget(label_img, alignment=Qt.AlignCenter)

label_bottom=QLabel('PLEASE WAIT')
label_bottom.setAlignment(Qt.AlignCenter)
label_bottom.setStyleSheet("font: 20pt Bahnschrift; background-color: #ffd167; color: black")
layout.addWidget(label_bottom)

window.setLayout(layout)
window.show()
app.setStyle('Fusion')
app.exec_()

Do not put a "relevant fragment" but place a [mre]. It would also be good to show what you get through an image. — eyllanesc, Jan 19 '20 at 16:36
Above code is reproducible example, it works with change suggested by you and with the additinal two labels added to the layout. Question is why does label_img require your change and label_top/label_bottom doesn't to be centered. Image is just random png or jpg, result is the same for any image I select with QFileDialog.getOpenFileName. — cyanidem, Jan 19 '20 at 16:44
I think you did not understand me, have you added a second code fragment that I don't know where to place? Well, I do not want to deduce it, but be explicit, so replace your second code fragment with a reproducible one. — eyllanesc, Jan 19 '20 at 16:46
Sorry, yes - I misunderstood. Hopefully now it's all clear and worth answering? — cyanidem, Jan 19 '20 at 16:54

eyllanesc · Accepted Answer · 2020-01-19T17:20:03.800

First, it must be understood that it means label.setAlignment(Qt.AlignCenter), that code indicates that the element that is displayed (QPixmap or text) in the QLabel will be in the center of the QLabel, not in the center of the window.

In the example the difference is given by the size of the labels since the first and third labels do not have fixed sizes unlike the second one that has a size of 300x500, so the behavior of a QLabel without fixed size is to occupy the entire space Possible as the following code shows:

from PyQt5.QtWidgets import QApplication, QLabel, QWidget, QVBoxLayout

if __name__ == '__main__':
    app=QApplication([])
    window=QWidget()
    window.setFixedSize(500,500)


    label_top=QLabel()
    label_top.setStyleSheet("background-color: red;")

    label_img=QLabel()
    label_img.setFixedSize(300, 300)
    label_img.setStyleSheet("background-color: green;")

    label_bottom=QLabel()
    label_bottom.setStyleSheet("background-color: blue;")

    layout=QVBoxLayout(window)
    layout.addWidget(label_top)
    layout.addWidget(label_img) 
    layout.addWidget(label_bottom)

    window.show()

    app.exec_()

So in QLabel that does not have a fixed size like the first and third, the alignment with respect to the window is irrelevant unlike the second because if you want the content of the QLabel (text or image) to be centered with respect to the window this It will have to be centered as well.

For example, other solutions are:

Do not set a fixed size to the width but only to the height.
establish that the width of the middle QLabel is 500.

Thank you, I got it! So, if I set label_img to fixed width of 500 (like whole window) its content (image) would appear in the center horizontally, right? — cyanidem, Jan 19 '20 at 17:18

Why does Qt.AlignCenter onQLabel differ for image and text in PyQt5?

1 Answers1

Linked

Related