For anyone interested in "trying it out": if you're not against cloning and compiling CPython yourself, here is a PEP 584 C implementation I have PR'd against master right now. I'm keeping it in sync with the draft PEP as it changes, so subtraction performance is not overly optimized yet, but it will show you the *exact* behavior outlined in the PEP on the dict builtin and its subclasses. The relevant branch is called "addiction". You can clone it from: