PySide is a nice Python wrapper for Qt which hides some internal aspects of it. For instance, thread safety: what's safe when it comes to signals and slots?

1 Signals and Slots in Plain C++ Qt

Are they safe? The Qt documentation on Signals and Slots Across Threads suggests the right connection will be automatically picked – that'll be a queued connection in the case of multithreading to keep it all safe. They offer the Mandelbrot Example in C++, which we have for Python too:

• The signal is emitted from the auxiliary thread.
• The slot is run from the main Qt thread.
• The connection is made from the main Qt thread.

2 Signals and Slots in PySide

The PySide Mandelbrot Example uses the traditional signals and slots syntax. Using the new syntax would have:

• the signal be defined in the QThread class definition:

  class CliThread(QThread):
      actionRequested = Signal(str, str)

  Note how you can specify the arguments that the slot would have to take. It would be emitted from an instance of this class;

• the slot still be defined from the main Qt thread, not necessarily in a class. The documentation suggests annotations but I haven't found any need for them to make it all work:

  def act(self, action, arg):
      getattr(self, action)(arg)

  The slot is a class attribute here, but again, there's no need for that, of course.

• the connection still be made from the main Qt thread:

  clithread.actionRequested.connect(pool.act)

3 References

• Signals and Slots Across Threads
• Mandelbrot Example
• The new syntax for Signals and Slots in PySide
• Writing Web Scrapers in Python with Qt and WebKit