How do I write text in subscript in the axis labels and the legend?

I have the following axis labels and legend.

plt.ylabel("ratio_2")
plt.xlabel("n_1")
plt.legend(('alpha_1','alpha_2' ), loc = '开发者_运维知识库best',shadow = True)   

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Put dollar signs around the formula: plt.xlabel("$n_1$")

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The easiest way I know is to enable TeX mode for matplotlib,

from http://www.scipy.org/Cookbook/Matplotlib/UsingTex:

from matplotlib import rc
rc('font',**{'family':'sans-serif','sans-serif':['Helvetica']})
rc('text', usetex=True)