PyQt5 - The "setContentsMargins" method doesn't work properly in a Tab widget (there is a default 9px margin)

Question

I'm writing a GUI with PyQt5 and I realized that the setContentsMargins method doesn't work properly in a Tab widget. Inside of it there is always a 9px margin for each side (see the attached screenshot):

Here is an example (inside the Tab1 I put a TreeView widget with a using the QVBoxLayout):

import sys
from PyQt5.QtWidgets import QMainWindow, QApplication, QWidget, QTabWidget, QVBoxLayout, QTreeView

class App(QMainWindow):

    def __init__(self):
        super().__init__()
        self.title = 'PyQt5 tabs'
        self.left = 0
        self.top = 0
        self.width = 300
        self.height = 200
        self.setWindowTitle(self.title)
        self.setGeometry(self.left, self.top, self.width, self.height)
        
        self.table_widget = MyTableWidget(self)
        self.setCentralWidget(self.table_widget)
        
        self.show()
    
class MyTableWidget(QWidget):
    
    def __init__(self, parent):
        super(QWidget, self).__init__(parent)
        self.layout = QVBoxLayout(self)

        self.tabs = QTabWidget()
        self.tab1 = QWidget()
        self.tab2 = QWidget()
        self.tabs.resize(300,200)

        self.tabs.addTab(self.tab1,"Tab 1")
        self.tabs.addTab(self.tab2,"Tab 2")

        self.tab1.layout = QVBoxLayout(self)
        self.TreeView=QTreeView()
        self.tab1.layout.addWidget(self.TreeView)
        self.tab1.setLayout(self.tab1.layout)
        
        self.tab1.setStyleSheet("background-color: red")
        self.tab1.setContentsMargins(-4, -4, -4, -4) # this instruction doesn't work! there is always a default margin (9px for each side)!

        self.layout.addWidget(self.tabs)
        self.setLayout(self.layout)

if __name__ == '__main__':
    app = QApplication(sys.argv)
    ex = App()
    sys.exit(app.exec_())

As you can see, if I use the instruction self.tab1.setContentsMargins(10, 10, 10, 10) all the margins, will be 19px (9+10), but what about if a want 5px? The instruction self.tab1.setContentsMargins(-4, -4, -4, -4) doesn't work.

Answer 1

The additional margin is from the widget's layout. Set it to zero or any other value:

self.tab1.layout.setContentsMargins(0, 0, 0, 0)