Matplotlib make tick labels font size smaller

In a matplotlib figure, how can I make the font size for the tick labels using ax1.set_xticklabels() smaller?

Further, how can one rotate it from horizontal to vertical?

There is a simpler way actually. I just found:

import matplotlib.pyplot as plt
# We prepare the plot  
fig, ax = plt.subplots()

# We change the fontsize of minor ticks label 
ax.tick_params(axis='both', which='major', labelsize=10)
ax.tick_params(axis='both', which='minor', labelsize=8)

This only answers to the size of label part of your question though.

To specify both font size and rotation at the same time, try this:

plt.xticks(fontsize=14, rotation=90)

Please note that newer versions of MPL have a shortcut for this task. An example is shown in the other answer to this question: https://stackoverflow.com/a/11386056/42346

The code below is for illustrative purposes and may not necessarily be optimized.

import matplotlib.pyplot as plt
import numpy as np

def xticklabels_example():
    fig = plt.figure() 

    x = np.arange(20)
    y1 = np.cos(x)
    y2 = (x**2)
    y3 = (x**3)
    yn = (y1,y2,y3)
    COLORS = ('b','g','k')

    for i,y in enumerate(yn):
        ax = fig.add_subplot(len(yn),1,i+1)

        ax.plot(x, y, ls='solid', color=COLORS[i]) 

        if i != len(yn) - 1:
            # all but last 
            ax.set_xticklabels( () )
        else:
            for tick in ax.xaxis.get_major_ticks():
                tick.label.set_fontsize(14) 
                # specify integer or one of preset strings, e.g.
                #tick.label.set_fontsize('x-small') 
                tick.label.set_rotation('vertical')

    fig.suptitle('Matplotlib xticklabels Example')
    plt.show()

if __name__ == '__main__':
    xticklabels_example()

Alternatively, you can just do:

import matplotlib as mpl
label_size = 8
mpl.rcParams['xtick.labelsize'] = label_size 

Another alternative

I have two plots side by side and would like to adjust tick labels separately.

The above solutions were close however they were not working out for me. I found my solution from this matplotlib page.

ax.xaxis.set_tick_params(labelsize=20)

This did the trick and was straight to the point. For my use case, it was the plot on the right that needed to be adjusted. For the plot on the left since I was creating new tick labels I was able to adjust the font in the same process as seting the labels.

ie

ax1.set_xticklabels(ax1_x, fontsize=15)
ax1.set_yticklabels(ax1_y, fontsize=15)

thus I used for the right plot,

ax2.xaxis.set_tick_params(labelsize=24)
ax2.yaxis.set_tick_params(labelsize=24)

A minor subtlety... I know... but I hope this helps someone :)

Bonus points if anyone knows how to adjust the font size of the order of magnitude label.