How important is metaprogramming to Python? Frankly, in my 16+ years as Python developer, I have never seen anything that can be remotely described as metaprogramming in Python, except in academic code and experimental GitHub projects.
The ABC module, collections, frozenset. Basically anywhere we add custom types or generate new classes to instantiate.
frozenset is just a built-in immutable set type with a nice hashing algorithm for the kinds of workloads you'd expect. It's defined in Objects/setobject.c and is a nice read.
I was just rambling and I've used frozenset in metaprogramming code when constructing classes via decorators to implement Lattice types with decorated monotone/morphism methods.
Sorry for the confusion!