Bug report

Bug summary

In my code, I need to check whether the matplotlib backend has already been set. If it has not been set, I need to then set it to 'WXAgg'. In matplotlib 2.*, I was able to do this, but in matplotlib 3.0.0 it fails.

Code for reproduction

import matplotlib
if not matplotlib.get_backend() == 'WXAgg':
    matplotlib.use('WXAgg')
print('backend:', matplotlib.get_backend())

Actual outcome

With matplotlib 3.0.0:

/Users/nebula/Python/PmagPy/programs/test.py:7: UserWarning: matplotlib.pyplot as already been /Users/nebula/Python/PmagPy/programs/test.py:7: UserWarning: matplotlib.pyplot as already been imported, this call will have no effect.
  matplotlib.use('WXAgg')
backend: MacOSX

Expected outcome

With matplotlib 2.2.3:

backend: WXAgg

Matplotlib version

• Operating system: OSX High Sierra 10.13.6
• Matplotlib version: 3.0.0
• Matplotlib backend (print(matplotlib.get_backend())):
• Python version: 3.6.6 (Anaconda, default channel)
• Jupyter version (if applicable):
• Other libraries: