Python qt: qapplication

QWidgets make extensive use of an eventloop to communicate between each other, and handle widget construction/deletion.

Any code making use of signals/slots requies that a QCoreApplication is running. Any visible GUI code requires that a QApplication (a subclass) is running.

GUI example

from Qt import QtWidgets
import sys

qapp = QtWidgets.QApplication(sys.argv)
# your UI code
sys.exit(qapp.exec_())

Console Example

from Qt import QtCore
import sys

qapp = QtCore.QCoreApplication(sys.argv)
# your console code
sys.exit(qapp.exec_())