Where is the PyQt/PySide event-loop running?

Question

If I open a Python interpreter and enter the following lines:

from PyQt4 import QtGui
app = QtGui.QApplication([])
w = QtGui.QLineEdit()
w.show()

then a text entry widget appears. I can interact with it while I interact with the interpreter. If I had attached any event handlers, they would be called too.

If I do import time; time.sleep(10), the Widget becomes unresponsive for 10 seconds.

If I run app.exec_(), that call blocks until I close the application.

My question is: what is the point of running app.exec_() if the widget is already responding to events? What is different? Is it just a way of preventing the application from closing while still servicing Qt's event loop (while True: pass would cause the event loop to block, as we understand from doing time.sleep(10))

I would also like to understand where Qt has hooked into CPython that allows the interpreter and the GUI to be active. I'm aware that Python 2 (and possibly 3?) has a so-called PyOS_InputHook mechanism, which is called roughly 10 times a second. IPython uses this to run a GUI (with lots of effort to handle GUI events faster than 10 times a second). Is the same thing happening when I instantiate a QApplication? If so, again, what is the point of calling app.exec_()

Answer 1

When you use PyQt/PySide in an interactive session, the event processing is set up automatically in the background in a way that allows you to interact directly with the objects. So for example you could create and show a window, and then add other widgets to it from within the python shell whilst the window is still visible. This behaviour is specific to interactive sessions, though - it's only there to allow you to easily experiment with things without the hassle of setting up the event processing yourself.

According to the PyQt Docs, PyOS_InputHook is used to process events whilst the interactive interpreter is waiting for user input (see: Using PyQt5 from the Python Shell) - and presumably a similar mechanism is also used by PySide.

For a normal PyQt/PySide application started from a script, you must explicitly call app.exec_() in order to start event handling. Otherwise, the script will just exit as soon as all the code has been executed (i.e. just like any other python script would).

(For a more in-depth look at Qt's event-processing, see: Threads, Events, QObjects).