matplotlib show() freezes the python shell

I use python, pylab and matplotlib for plottings. One thing disturbing is : when I call ipython and then run some plot code, eg;

x=[1,2,3]
plot(x)
show()

The figure is shown on the screen, but the python shell is freezed that I can not type anything on the python command window. I looked for raisons and found it a common GUI issues with the threads.

So I add this in my bashrc script:
alias ipython='ipython -pylab'

The problem is fixed !