Ref: http://doc.qt.io/qt-5/qscreen.html#grabWindow
You say you require a screenshot of a window, therefore
screenshot = screen.grabWindow(0, 0, 0, 100, 100)
is not the appropriate call here, since it captures the entire screen, cropped according to the final 4 parameters. (the 100 parameters are width and height).
screenshot = screen.grabWindow( widget.winId() )
captures the widget window. However, the reason you don't perhaps get what you expected on this call is that you don't create a solid widget and/or the widget hasn't been shown. Try the following example, making sure the app is on your primary display before clicking the button.
from PyQt5 import QtWidgets
import sys
app = QtWidgets.QApplication(sys.argv)
w = QtWidgets.QWidget()
grab_btn=QtWidgets.QPushButton('Grab Screen')
def click_handler():
screen = QtWidgets.QApplication.primaryScreen()
screenshot = screen.grabWindow( w.winId() )
screenshot.save('shot.jpg', 'jpg')
w.close()
grab_btn.clicked.connect(click_handler)
layout = QtWidgets.QVBoxLayout()
layout.addWidget(grab_btn)
w.setLayout(layout)
w.show()
sys.exit(app.exec_())
I've tested this on Windows.
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