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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?

Thanks for asking this question, I'm trying to solve this problem right now. I'm not strongly opinionated here, but looks like voters think the best answer is not the accepted one. What do you think? – Russia Must Remove Putin Jun 18, 2017 at 19:25 did I properly read that there are at least 5 different ways to change the fontsize in matplotlib? :D – famargar Feb 26, 2018 at 13:06 # 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.

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()
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.
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. 
                I think that using list comprehensions that have side effects is not the best practice. I'd suggest using a standard for loop instead.
– jjmontes
                Jul 31, 2019 at 17:59