Why do I suddenly have a bouncing Python rocket?

Question

Recently, I made a simple change to some matplotlib code I run on OS X (10.10; Python 2.7.6), commenting out a single line that set the backend I use

#matplotlib.use('agg')

Now, as long as my code is running, I get a bouncing Python icon in my dock, that presents no UI and only offers Force Quit... as an command:

Generally dock icons that continue bouncing without any UI are a "bad thing". Why am I getting this and is it something I should be concerned about?

Answer 1

Not a "bad thing". The icon appears in the dock when Python interacts with the GUI. Agg is a non-interactive backend. With agg you did not see the icon since there was no Python UI interaction.