_cat_file implementation

[ivirshup]

Hi, I have a feature request. Could the sshfs.SSHFileSystem get an implementation for _cat_file?

I'm trying to use sshfs with zarr, but hit a NotImplementedError when I try to construct a group.

Roughly what I've run:

import sshfs, zarr

fs = sshfs.SSHFileSystem(host)
store = zarr.storage.FSStore("/path/to/data.zarr", fs=fs, mode="r")

g = zarr.open(store)

File /usr/local/Cellar/python@3.9/3.9.12/Frameworks/Python.framework/Versions/3.9/lib/python3.9/asyncio/tasks.py:442, in wait_for(fut, timeout, loop)
    437     warnings.warn("The loop argument is deprecated since Python 3.8, "
    438                   "and scheduled for removal in Python 3.10.",
    439                   DeprecationWarning, stacklevel=2)
    441 if timeout is None:
--> 442     return await fut
    444 if timeout <= 0:
    445     fut = ensure_future(fut, loop=loop)

File /usr/local/lib/python3.9/site-packages/fsspec/asyn.py:395, in AsyncFileSystem._cat_file(self, path, start, end, **kwargs)
    394 async def _cat_file(self, path, start=None, end=None, **kwargs):
--> 395     raise NotImplementedError

Full Traceback

---------------------------------------------------------------------------
NotImplementedError                       Traceback (most recent call last)
Cell In[69], line 1
----> 1 g = zarr.open(store, mode="r")

File /usr/local/lib/python3.9/site-packages/zarr/convenience.py:120, in open(store, mode, zarr_version, path, **kwargs)
    118     return open_array(_store, mode=mode, **kwargs)
    119 elif contains_group(_store, path):
--> 120     return open_group(_store, mode=mode, **kwargs)
    121 else:
    122     raise PathNotFoundError(path)

File /usr/local/lib/python3.9/site-packages/zarr/hierarchy.py:1465, in open_group(store, mode, cache_attrs, synchronizer, path, chunk_store, storage_options, zarr_version, meta_array)
   1462 # determine read only status
   1463 read_only = mode == 'r'
-> 1465 return Group(store, read_only=read_only, cache_attrs=cache_attrs,
   1466              synchronizer=synchronizer, path=path, chunk_store=chunk_store,
   1467              zarr_version=zarr_version, meta_array=meta_array)

File /usr/local/lib/python3.9/site-packages/zarr/hierarchy.py:164, in Group.__init__(self, store, path, read_only, chunk_store, cache_attrs, synchronizer, zarr_version, meta_array)
    162     mkey = _prefix_to_group_key(self._store, self._key_prefix)
    163     assert not mkey.endswith("root/.group")
--> 164     meta_bytes = store[mkey]
    165 except KeyError:
    166     if self._version == 2:

File /usr/local/lib/python3.9/site-packages/zarr/storage.py:1393, in FSStore.__getitem__(self, key)
   1391 key = self._normalize_key(key)
   1392 try:
-> 1393     return self.map[key]
   1394 except self.exceptions as e:
   1395     raise KeyError(key) from e

File /usr/local/lib/python3.9/site-packages/fsspec/mapping.py:143, in FSMap.__getitem__(self, key, default)
    141 k = self._key_to_str(key)
    142 try:
--> 143     result = self.fs.cat(k)
    144 except self.missing_exceptions:
    145     if default is not None:

File /usr/local/lib/python3.9/site-packages/fsspec/asyn.py:114, in sync_wrapper.<locals>.wrapper(*args, **kwargs)
    111 @functools.wraps(func)
    112 def wrapper(*args, **kwargs):
    113     self = obj or args[0]
--> 114     return sync(self.loop, func, *args, **kwargs)

File /usr/local/lib/python3.9/site-packages/fsspec/asyn.py:99, in sync(loop, func, timeout, *args, **kwargs)
     97     raise FSTimeoutError from return_result
     98 elif isinstance(return_result, BaseException):
---> 99     raise return_result
    100 else:
    101     return return_result

File /usr/local/lib/python3.9/site-packages/fsspec/asyn.py:54, in _runner(event, coro, result, timeout)
     52     coro = asyncio.wait_for(coro, timeout=timeout)
     53 try:
---> 54     result[0] = await coro
     55 except Exception as ex:
     56     result[0] = ex

File /usr/local/lib/python3.9/site-packages/fsspec/asyn.py:409, in AsyncFileSystem._cat(self, path, recursive, on_error, batch_size, **kwargs)
    407     ex = next(filter(is_exception, out), False)
    408     if ex:
--> 409         raise ex
    410 if (
    411     len(paths) > 1
    412     or isinstance(path, list)
    413     or paths[0] != self._strip_protocol(path)
    414 ):
    415     return {
    416         k: v
    417         for k, v in zip(paths, out)
    418         if on_error != "omit" or not is_exception(v)
    419     }

File /usr/local/Cellar/python@3.9/3.9.12/Frameworks/Python.framework/Versions/3.9/lib/python3.9/asyncio/tasks.py:442, in wait_for(fut, timeout, loop)
    437     warnings.warn("The loop argument is deprecated since Python 3.8, "
    438                   "and scheduled for removal in Python 3.10.",
    439                   DeprecationWarning, stacklevel=2)
    441 if timeout is None:
--> 442     return await fut
    444 if timeout <= 0:
    445     fut = ensure_future(fut, loop=loop)

File /usr/local/lib/python3.9/site-packages/fsspec/asyn.py:395, in AsyncFileSystem._cat_file(self, path, start, end, **kwargs)
    394 async def _cat_file(self, path, start=None, end=None, **kwargs):
--> 395     raise NotImplementedError

NotImplementedError: