AbstractFileSystem is not abstract

I'm trying to learn more about how fsspec works (so I can better understand the IO part of the Pangeo stack), but I'm confused by the structure of fsspec/filesystem_spec.

Despite the name, it seems AbstractFileSystem is not an abstract base class (nor is AbstractBufferedFile) - I'm able to import and instantiate it directly. It contains lots of actual code that does things.

Neither is there a typing.Protocol that subclasses could conform to that could play the same role. (In fact there is no typing at all, which seems like an issue for a project whose purpose is to ensure interface compatibility. Wouldn't it help solve issues like https://github.com/fsspec/filesystem_spec/issues/1411?)

Finally I can't find a written spec document either, at least not one that conforms to RFC 2119 like the Zarr Spec does for example. The fsspec documentation just points me to fsspec/spec.py.

As a new user / potential developer using fsspec, how am I supposed to know which parts of AbstractFileSystem are implementation details that can be overridden, and which are actual requirements? Is there some reason why this lack of a distinct specification doesn't cause problems in practice? Apologies if I have missed something here, I am new to this whole area.

Start with fsspec/spec.py and the documentation section on the package structure to compare the described AbstractFileSystem and AbstractBufferedFile roles. Review the linked typing issue #625 and compatibility issue #1411 for related context. Done would require a clear decision about the interface contract and documentation of which members are requirements versus implementation details.