I have tried

plt.rcParams['image.aspect'] = 'equal'

to set the default for axis to equal, but it has now effect in matplotlib 2.0.0.
To what I inferred from the documentation shouldn't this be the command?

If not, what is it?

Other people have the same question at

https://stackoverflow.com/questions/35558958/matplotlib-set-default-value-for-plt-axis-to-equal