I have to draw different plots, so I have combined several different settings of a design and use them like this:

rcp_single_ticks = {'xtick.direction' : 'in', 'ytick.direction' : 'in', 'xtick.top' : True, 'ytick.right' : True}
plt.rcParams.update(rcp_single_ticks)

This works well even if I combine multiple designs, with one exception. Sometimes multiple designs access the same key, in particular text.latex.preamble. In this case, the settings for this key should be appended and not replaced. Is there a way to append to a setting instead of replacing it?