[Python] UserWarning: FixedFormatter should only be used together with FixedLocator

I have used for a long time small subroutines to format axes of charts I'm plotting. A couple of examples:

def format_y_label_thousands(): # format y-axis tick labels formats
ax = plt.gca()
label_format = '{:,.0f}'
ax.set_yticklabels([label_format.format(x) for x in ax.get_yticks().tolist()])

def format_y_label_percent(): # format y-axis tick labels formats
ax = plt.gca()
label_format = '{:.1%}'
ax.set_yticklabels([label_format.format(x) for x in ax.get_yticks().tolist()])


However, after an update to matplotlib yesterday, I get the following warning when calling any of these two functions:

UserWarning: FixedFormatter should only be used together with FixedLocator
ax.set_yticklabels([label_format.format(x) for x in ax.get_yticks().tolist()])


What is the reason for such a warning? I couldn't figure it out looking into matplotlib's documentation.

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