issues: 789653499

Is your feature request related to a problem? Please describe. I am retrieving files from AWS: https://registry.opendata.aws/wrf-se-alaska-snap/. An example: ``` import s3fs import xarray as xr

s3 = s3fs.S3FileSystem(anon=True) s3path = 's3://wrf-se-ak-ar5/gfdl/hist/daily/1980/WRFDS_1980-01-0[12].nc' remote_files = s3.glob(s3path) fileset = [s3.open(file) for file in remote_files]

ds = xr.open_mfdataset(fileset, concat_dim='Time', decode_cf=False) ds ``` Data files for 1980 are missing time coordinate, so the above code fails. The time could be obtained by parsing file name, however in the current implementation the source attribute is available only when the fileset consists of strings or Paths.

Describe the solution you'd like I would suggest to return to the original suggestion in #2550 - pass filename_or_object as an argument to preprocess function, but with necessary inspection. Here is my attempt (code in open_mfdataset): ``` open_kwargs = dict( engine=engine, chunks=chunks or {}, lock=lock, autoclose=autoclose, **kwargs )

if preprocess is not None:
    # Get number of free arguments
    from inspect import signature
    parms = signature(preprocess).parameters
    num_preprocess_args = len([p for p in parms.values() if p.default == p.empty])
    if num_preprocess_args not in (1, 2):
        raise ValueError('preprocess accepts only 1 or 2 arguments')

if parallel:
    import dask

    # wrap the open_dataset, getattr, and preprocess with delayed
    open_ = dask.delayed(open_dataset)
    getattr_ = dask.delayed(getattr)
    if preprocess is not None:
        preprocess = dask.delayed(preprocess)
else:
    open_ = open_dataset
    getattr_ = getattr

datasets = [open_(p, **open_kwargs) for p in paths]
file_objs = [getattr_(ds, "_file_obj") for ds in datasets]
if preprocess is not None:
    if num_preprocess_args == 1:
        datasets = [preprocess(ds) for ds in datasets]
    else:
        datasets = [preprocess(ds, p) for (ds, p) in zip(datasets, paths)]

With this, I can define function *fix* as follows: def fix(ds, source): vtime = datetime.strptime(os.path.basename(source.path), 'WRFDS_%Y-%m-%d.nc') return ds.assign_coords(Time=[vtime])

ds = xr.open_mfdataset(fileset, preprocess=fix, concat_dim='Time', decode_cf=False) This is backward compatible, *preprocess* can accept any number of arguments: from functools import partial import xarray as xr

def fix1(ds): print('fix1') return ds

def fix2(ds, file): print('fix2:', file.as_uri()) return ds

def fix3(ds, file, arg): print('fix3:', file.as_uri(), arg) return ds

fileset = [Path('/home/george/Downloads/WRFDS_1988-04-23.nc'), Path('/home/george/Downloads/WRFDS_1988-04-24.nc') ] ds = xr.open_mfdataset(fileset, preprocess=fix1, concat_dim='Time', parallel=True) ds = xr.open_mfdataset(fileset, preprocess=fix2, concat_dim='Time') ds = xr.open_mfdataset(fileset, preprocess=partial(fix3, arg='additional argument'), concat_dim='Time') fix1 fix1 fix2: file:///home/george/Downloads/WRFDS_1988-04-23.nc fix2: file:///home/george/Downloads/WRFDS_1988-04-24.nc fix3: file:///home/george/Downloads/WRFDS_1988-04-23.nc additional argument fix3: file:///home/george/Downloads/WRFDS_1988-04-24.nc additional argument ``` Describe alternatives you've considered The simple solution would be to make xarray s3fs aware. IMHO this is not particularly elegant. Either a check for an attribute, or an import within a try/except block would be needed.