Question: 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.

Answer: WORKAROUND: The way to avoid the warning is to use FixedLocator (that is part of matplotlib.ticker). Below I show a code to plot three charts. I format their axes in different ways. Note that the "set_ticks" silence the warning, but it changes the actual ticks locations/labels (it took me some time to figure out that FixedLocator uses the same info but keeps the ticks locations intact). You can play with the x/y's to see how each solution might affect the output. import matplotlib as mpl import matplotlib.pyplot as plt import numpy as np import matplotlib.ticker as mticker mpl.rcParams['font.size'] = 6.5 x = np.array(range(1000, 5000, 500)) y = 37*x fig, [ax1, ax2, ax3] = plt.subplots(1,3) ax1.plot(x,y, linewidth=5, color='green') ax2.plot(x,y, linewidth=5, color='red') ax3.plot(x,y, linewidth=5, color='blue') label_format = '{:,.0f}' # nothing done to ax1 as it is a "control chart." # fixing yticks with "set_yticks" ticks_loc = ax2.get_yticks().tolist() ax2.set_yticks(ax1.get_yticks().tolist()) ax2.set_yticklabels([label_format.format(x) for x in ticks_loc]) # fixing yticks with matplotlib.ticker "FixedLocator" ticks_loc = ax3.get_yticks().tolist() ax3.yaxis.set_major_locator(mticker.FixedLocator(ticks_loc)) ax3.set_yticklabels([label_format.format(x) for x in ticks_loc]) # fixing xticks with FixedLocator but also using MaxNLocator to avoid cramped x-labels ax3.xaxis.set_major_locator(mticker.MaxNLocator(3)) ticks_loc = ax3.get_xticks().tolist() ax3.xaxis.set_major_locator(mticker.FixedLocator(ticks_loc)) ax3.set_xticklabels([label_format.format(x) for x in ticks_loc]) fig.tight_layout() plt.show()  OUTPUT CHARTS:  Obviously, having a couple of idle lines of code like the one above (I'm basically getting the yticks or xticks and setting them again) only adds noise to my program. I would prefer that the warning was removed. However, look into some of the "bug reports" (from links on the comments above/below; the issue is not actually a bug: it is an update that is generating some issues), and the contributors that manage matplotlib have their reasons to keep the warning. OLDER VERSION OF MATPLOTLIB: If you use your Console to control critical outputs of your code (as I do), the warning messages might be problematic. Therefore, a way to delay having to deal with the issue is to downgrade matplotlib to version 3.2.2. I use Anaconda to manage my Python packages, and here is the command used to downgrade matplotlib: conda install matplotlib=3.2.2  Not all listed versions might be available. For instance, couldn't install matplotlib 3.3.0 although it is listed on matplotlib's releases page: https://github.com/matplotlib/matplotlib/releases