from mojo.UI import exportPreferences, importPreferences

pref = exportPreferences()
pref['glyphViewQuickPreviewKey'] = ' '
importPreferences(pref)

But although the below code also works (it's just an example), it only takes effect after restarting the app. Is there any way to make the changes to menuShortCuts instantly reflected in the UI?

from mojo.UI import exportPreferences, importPreferences

pref = exportPreferences()
keys = {"File > Save": [1048576, 'B']}
pref['menuShortCuts'] = keys
importPreferences(pref)

I haven’t yet found out how to make the main menu update. just a comment about the preferences API for now:

you can read/write a single preference setting using getDefault and setDefault, and send a notification with preferencesChanged:

from mojo.UI import setDefault, getDefault, preferencesChanged

print('old settings:')
print(getDefault("glyphViewGridx"), getDefault("glyphViewGridy"))

gridsize = 70
setDefault("glyphViewGridx", gridsize)
setDefault("glyphViewGridy", gridsize)
preferencesChanged()

print('new settings:')
print(getDefault("glyphViewGridx"), getDefault("glyphViewGridy"))

use this API to modify the menuShortCuts preference AND update the application menu.

from mojo.UI import getMenuShortCuts, setMenuShortCuts

# get menu shortcuts
print(getMenuShortCuts().keys())
print()

# set menu shortcuts
shortcuts = {
    ('File', 'Save') : (1048576, 'b'),
    ('RoboFont', 'Check for Updates') : (1048576, "u"),
    ('RoboFont', 'License') : (1048576, "l"),
}
setMenuShortCuts(shortcuts)
print(getMenuShortCuts().keys())

removeMenuShortCut(('RoboFont > Check for Updates'))

…but it is not removing the shortcut from the menu. (I’ve filed a bug report)

in the meantime, you can clear a shortcut by setting it to an empty string.