---
author: ''
category: ''
date: 2012/01/19 18:14
description: ''
link: ''
priority: ''
slug: BB974
tags: pyqt, python
title: 'PyQt Quickie: QTimer'
type: text
updated: 2012/01/19 18:14
url_type: ''
---
QTimer is a fairly simple class: you use it when you want something to happen "in a while" or "every once in a while".

The first case is something like this:

.. code-block:: python

   # call f() in 3 seconds
   QTimer.singleShot(3000, f)

The second is this:

.. code-block:: python

   # Create a QTimer
   timer = QTimer()
   # Connect it to f
   timer.timeout.connect(f)
   # Call f() every 5 seconds
   timer.start(5000)

Simple, right? Well, yes, but it has some tricks.

1) You have to keep a reference to``timer``

   If you don't, it willget garbage-collected, and f() will never be called.

2) It may not call ``f()`` in 5 seconds.

   It will call ``f()`` more or less 5 seconds *after you enter the event loop*. That may not be quickly after you start the timer at all!

3) You may get overlapping calls.

   If ``f()`` takes long to finish and re-enters the event loop (for example, by calling ``processEvents``) maybe the timer will timeout and call it again before it's finished. That's almost never a good thing.

So, you can do this:

.. code-block:: python

   def f():
       try:
           # Do things
       finally:
           QTimer.singleShot(5000, f)

   f()

What that snippet does, is, calls ``f()`` only once. But ``f`` itself schedules itself to run in 5 seconds. Since it does it in a ``finally``, it will do so even if things break.

That means no overlapping calls. It also means it won't be called every 5 seconds, but 5 seconds plus whatever ``f`` takes to run. Also, no need to keep any reference to a QTimer.

Final tip: You can also use QTimer to do something "as soon as you are in the event loop"

.. code-block:: python

   QTimer.singleShot(0, f)

Hope it was useful!