`WholeFileCacheFileSystem`: `_cat_file` exists on instance but not on class

[hadim]

WholeFileCacheFileSystem delegates _cat_file to the wrapped filesystem at the instance level, but the method is not defined on the class. This means hasattr(type(fs), '_cat_file') returns False while hasattr(fs, '_cat_file') returns True.

In practice this causes an AttributeError in libraries that check filesystem capabilities on the class (e.g. universal_pathlib, zarr). I hit this when reading a Zarr store through UPath("filecache::s3://...").

Minimal reproducer (fsspec only)

import fsspec

fs = fsspec.filesystem("filecache", target_protocol="s3", cache_storage="/tmp/test")

print(type(fs).__name__)                        # WholeFileCacheFileSystem
print(hasattr(fs, "_cat_file"))                  # True
print(hasattr(type(fs), "_cat_file"))            # False

type(fs)._cat_file                               # AttributeError

Versions

Package	Version
python	3.13.13
fsspec	2026.2.0
s3fs	2026.2.0
universal_pathlib	0.3.10
zarr	3.1.5

Question

Is this instance-only delegation intentional by design? Or should _cat_file (and similar methods) be defined on the class as well? If this is by design, I'll report the issue downstream to zarr and/or universal_pathlib instead.