do not use the empty list as default argument value
do not use the empty list as default argument value

Since default arguments are only evaluated once, this could cause
unexpected behaviour when the default list is modified in place. This
doesn't happen in the current code, but using None instead of [] as the
default value should prevent any future problems.

foreni-packages/urwid,harlowja/urwid,urwid/urwid,tonycpsu/urwid,tonycpsu/urwid,drestebon/urwid,ivanov/urwid,drestebon/urwid,douglas-larocca/urwid,harlowja/urwid,urwid/urwid,rndusr/urwid,drestebon/urwid,inducer/urwid,inducer/urwid,harlowja/urwid,foreni-packages/urwid,inducer/urwid,ivanov/urwid,rndusr/urwid,rndusr/urwid,wardi/urwid,ivanov/urwid,wardi/urwid,douglas-larocca/urwid,tonycpsu/urwid,douglas-larocca/urwid,wardi/urwid,foreni-packages/urwid,urwid/urwid,hkoof/urwid,hkoof/urwid,hkoof/urwid