bug in double subscript LaTeX?

The following test code doesn't render the _1 and _2 as double subscript.
Is this a bug?

import matplotlib.pyplot as plt
esno1dB = 10; esno2dB=10
plt.suptitle (r'${E_s/N_0}_1=%g, {E_s/N_0}_2=%g$'%(esno1dB,esno2dB))
plt.show()