Include filename or path in open_mfdataset

[jsignell]

When reading from multiple files, sometimes there is information encoded in the filename. For example in these grib files the time: ./ST4.2018092500.01h, ./ST4.2018092501.01h. It seems like a generally useful thing would be to allow the passing of a kwargs (such as path_as_coord or something) that would define a set of coords with one for the data from each file.

I think the code change would be small:

if path_as_coord:
    ds = ds.assign_coords(path=file_name)

In use it would be like:

>>>xr.open_mfdataset(['./ST4.2018092500.01h', './ST4.2018092501.01h'], engine='pynio', concat_dim='path')
<xarray.Dataset>
Dimensions:               (x: 881, y: 1121, time: 2)
Coordinates:
    lat             (x, y) float32 23.116999 ... 45.618984
    lon             (x, y) float32 -119.023 ... -59.954613
  * path            (path) <U20 './ST4.2018092500.01h' './ST4.2018092501.01h'
Dimensions without coordinates: x, y
Data variables:
    var_0  (time, x, y) float32 dask.array<shape=(2, 881, 1121), chunksize=(1, 881, 1121)>
    var_1   (time, x, y) float32 dask.array<shape=(2, 881, 1121), chunksize=(1, 881, 1121)>

For context I have implemented something similar in dask: dask/dask#3908

[dcherian]

There is a preprocess argument. You provide a function and it is run on every file.

[jsignell]

There is a preprocess argument. You provide a function and it is run on every file.

Yes but the input to that function is just the ds, I couldn't figure out a way to get the filename from within a preprocess function. This is what I was doing to poke around in there:

def func(ds):
    import pdb; pdb.set_trace()

xr.open_mfdataset(['./ST4.2018092500.01h', './ST4.2018092501.01h'], 
                   engine='pynio', concat_dim='path', preprocess=func)

[jhamman]

@jsignell - perhaps not a very pretty solution but we do save the source of each variable in the encoding dictionary.

ds['varname'].encoding['source']

Presumably, you could unpack this via a preprocess step.

[jsignell]

@jhamman that looks pretty good, but I'm not seeing the source in the encoding dict. Is this what you were expecting?

def func(ds):
    var = next(var for var in ds)
    return ds.assign(path=ds[var].encoding['source'])

xr.open_mfdataset(['./ST4.2018092500.01h', './ST4.2018092501.01h'], 
                   engine='pynio', concat_dim='path', preprocess=func)

---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-49-184da62ce353> in <module>()
----> 1 ds = xr.open_mfdataset(['./ST4.2018092500.01h', './ST4.2018092501.01h'], engine='pynio', concat_dim='path', preprocess=func)

/opt/conda/lib/python3.6/site-packages/xarray/backends/api.py in open_mfdataset(paths, chunks, concat_dim, compat, preprocess, engine, lock, data_vars, coords, autoclose, parallel, **kwargs)
    612     file_objs = [getattr_(ds, '_file_obj') for ds in datasets]
    613     if preprocess is not None:
--> 614         datasets = [preprocess(ds) for ds in datasets]
    615 
    616     if parallel:

/opt/conda/lib/python3.6/site-packages/xarray/backends/api.py in <listcomp>(.0)
    612     file_objs = [getattr_(ds, '_file_obj') for ds in datasets]
    613     if preprocess is not None:
--> 614         datasets = [preprocess(ds) for ds in datasets]
    615 
    616     if parallel:

<ipython-input-48-fd450fa1393a> in func(ds)
      1 def func(ds):
      2     var = next(var for var in ds)
----> 3     return ds.assign(path=ds[var].encoding['source'])

KeyError: 'source'

xarray version: '0.11.0+1.g575e97ae'

[shoyer]

Hmm. It really seems like the preprocess function should pass in the file-name along with the dataset.

[jhamman]

@shoyer and @jsignell - I'd also be happy to see this added to the preprocess function. Ideally the function signature would look like:

def preprocess(ds, filename=None):
    ...
    return ds

This would avoid a breaking change and allow us to add additional kwargs at a later date if need be.

[shoyer]

@jhamman The problem is that xarray needs way to figure out what arguments it can safely pass to preprocess, i.e., it needs to inspect the proprocess function and see if it can handle a filename argument. It's not obvious what the best way to do this in a backwards compatible way is...

[dcherian]

Hmm... Sorry @jsignell. I thought preprocess passed the filename too.

[jsignell]

Maybe we can inspect the preprocess function like this:

>>> preprocess = lambda a, b: print(a, b)
>>> preprocess .__code__.co_varnames
('a', 'b')

This response is ordered, so the first one can always be ds regardless of its name and then we can look for special names (like filename) in the rest.

From this answer: https://stackoverflow.com/a/4051447/4021797

[shoyer]

The danger with inspecting user provided functions is that it's pretty fragile, e.g., it fails if you use provide a signature like *args, **kwargs (which can happen pretty easily with decorators). Probably the best option is to come up with a new keyword argument to replace "preprocess" and to deprecate the current preprocess (if we can think of another good name). We could also do a deprecation cycle with FutureWarning, but that's pretty painful.

…

On Fri, Nov 9, 2018 at 12:29 PM Julia Signell ***@***.***> wrote: Maybe we can inspect the preprocess function like this: >>> preprocess = lambda a, b: print(a, b)>>> preprocess .__code__.co_varnames ('a', 'b') This response is ordered, so the first one can always be ds regardless of its name and then we can look for special names (like filename) in the rest. From this answer: https://stackoverflow.com/a/4051447/4021797 — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#2550 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/ABKS1kvSMx83hACWxXD3tOyIHGqyqum8ks5utbtigaJpZM4YVZ1J> .

[dcherian]

A dirty fix would be to add an attribute to each dataset.

[jsignell]

A dirty fix would be to add an attribute to each dataset.

I thought @jhamman was suggesting that already exists, but I couldn't find it: #2550 (comment)

[dcherian]

True, maybe we should track down why that isn't happening with your dataset

[jsignell]

Ah I don't think I understood that adding source to encoding was a new addition. In latest master ('0.11.0+3.g70e9eb8) this works fine:

def func(ds):
    var = next(var for var in ds)
    return ds.assign(path=ds[var].encoding['source'])

ds = xr.open_mfdataset(['./air_1.nc', './air_2.nc'], concat_dim='path', preprocess=func)

I do think it is misleading though that after you've concatenated the data, the encoding['source'] on a concatenated var seems to be the first path.

>>> ds['air'].encoding['source'] 
'~/air_1.nc'

I'll close this one though since there is a clear way to access the filename. Thanks for the tip @jhamman!