I _think_ this may be the implementation: https://github.com/python/cpython/blob/60ac6ed5579f6666130fc264d3b748ee9575e3aa/Objects/odictobject.c https://github.c

I _think_ this may be the implementation: https://github.com/python/cpython/blob/60ac6ed5579f6666130fc264d3b748ee9575e3aa/Objects/odictobject.c https://github.com/python/cpython/blob/60ac6ed5579f6666130fc... There are copious notes in there about the implementation and it indicates linked lists are used. EDIT: Blargh, it's here: https://github.com/python/cpython/blob/60ac6ed5579f6666130fc264d3b748ee9575e3aa/Objects/odictobject.c https://github.com/python/cpython/blob/60ac6ed5579f6666130fc... . That file references this blog post which indicates they may be flagging and repacking as dilap commented: https://morepypy.blogspot.com/2015/01/faster-more-memory-efficient-and-more.html https://morepypy.blogspot.com/2015/01/faster-more-memory-eff...