CPython's PyCapsule Implementation in C: A void* Wrapper for Extension Modules
A PyCapsule is a typed, name-checked void* wrapper: the name stops you reading the wrong capsule, and the destructor gets one shot to clean up before the memory goes.
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CPython's range Implementation in C: Lazy Sequences That Never Materialize
range holds four PyObject pointers and nothing else — the numbers are computed on demand, so range(10**18) costs the same as range(3).
CPython's Sentinel Implementation in C: Unique Marker Objects for Defaults
Each sentinel() call allocates a distinct, GC-tracked object that infers its module from the calling frame — which is exactly what makes it picklable.
CPython's set and frozenset Implementation in C: One Hash Table, Two Flavors
CPython’s set is an open-addressing hash table with three slot states — unused, dummy, active — and dummies are what keep deleted keys from breaking probe chains.
CPython's SimpleNamespace Implementation in C
SimpleNamespace is a PyObject_HEAD plus a single dict pointer — tp_dictoffset points right at it, so attribute access is just dict lookup.