Tag list vanishes after adding a tag

list.append() changes the list in place and returns None. Writing tags = tags.append(...) does add the tag, then immediately replaces your list with that None. The same trap exists for sort(), reverse(), extend(), insert() and remove(): every list method that modifies the list returns None, as a deliberate signal that it worked in place. If a method changes the object, call it on its own line; only assign the result of functions that build something new, like sorted() or a slice.

The pattern behind it: mutation and copying bugs.