There were multiple things which stood out to me, as I was trying to setup a simple calendar popup for my textclock:
1.: The functions in https://awesomewm.org/doc/api/classes/awful.widget.calendar_popup.html#Functions were not named correctly (calendar vs calendar_popup) which (naturally) produced an error when used as awful.widget.calendar.month() .
2.: The example for calendar_popup.month() was obviously missing a line (where does the month_calendar come from?) . Resolved by copying the line from the attach-example
3.: The same examples also only refer to the function as calendar.month() which should be changed to awful.widget.calendar_popup.month()
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