On a related note, after inspecting the UnicodeEror C code, the exception object keeps explicit references to 'encoding', 'object', 'start', 'end' and 'reason'. That means that if those properties are set (the C code does have setters) then the properties stored in UnicodeError go out of sync with the args tuple in BaseException. And so pickling and unpickling will restore the original values, and not those that were set after the exception being created, unless args is too modified.

My suggestion would be to just fetch all 5 properties from the args tuple inside the getters and setters, and in the setters, recreate the tuple with a modified value. The constructor could do argument validation but would not set any properties, because that would be delegated to BaseException.