Sorry for not responding on this earlier. It seems to work if I explicitly set the WX/WXAgg backend before importing pylab

import wx
import matplotlib
matplotlib.use('WXAgg')
import pylab
app = wx.App(False)
app.frame = wx.Frame(None)
app.frame.Centre()
app.frame.Show()

It's likely that this is related to the work in #3421 that enabled support for WXPython phoenix

If you do

from __future__ import print_function
import matplotlib.pyplot as plt
import matplotlib
print(matplotlib.get_backend())

what do you get?

My suspicion here is that you are importing wx, it may or may not be installing the input hook, and then pylab imports some other gui framework which install's it's input hook. Your wx app then sets up waits for it's event loop to be run (which will never happen as python is letting the other event loop run).

I get 'Qt4Agg' as the default backend.

If I set the backend as 'WXAgg' before importing pylab as jenshnielsen suggested, the error happens as before.

Can you try 1.5.1rc1? I can reproduce the hanging with 1.5.0 but not with 1.5.1rc1

My original code still fails with 1.5.1. However, if I add matplotlib.use('WXAgg'), then all works as expected.

In your original example you will import both WX and Qt4 since matplotlib defaults to the Qt4Agg backend and you import WX. Importing more than one GUI framework is unsupported and will in general lead to wrong behaviour. If you want to use matplotlib within a WX app you should make sure that it runs with the WX backend.

I think this can close, it looks like this is only triggered when more than one gui backend is involved is an unsupported way