window_correction seems sensitive to formatting of dim

[cspencerjones]

I haven't been able to reproduce this with synthetic data (sorry!)

But for my real data, xrft.power_spectrum(u_unfiltered.isel(x0=1000,y0=1000).load(),dim='time', window='hann',window_correction=True) gives the error:

---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
~/.conda/envs/pangeo2/lib/python3.7/site-packages/xarray/core/dataarray.py in _getitem_coord(self, key)
    692         try:
--> 693             var = self._coords[key]
    694         except KeyError:

KeyError: 't'

During handling of the above exception, another exception occurred:

KeyError                                  Traceback (most recent call last)
<ipython-input-40-09826e26c18a> in <module>
----> 1 xrft.power_spectrum(u_unfiltered.isel(x0=1000,y0=1000).load(),dim='time', window='hann',window_correction=True)

~/.conda/envs/pangeo2/lib/python3.7/site-packages/xrft/xrft.py in power_spectrum(da, dim, real_dim, scaling, window_correction, **kwargs)
    712                 )
    713             else:
--> 714                 windows, _ = _apply_window(da, dim, window_type=kwargs.get("window"))
    715                 ps = ps / (windows ** 2).mean()
    716         fs = np.prod([float(ps[d].spacing) for d in updated_dims])

~/.conda/envs/pangeo2/lib/python3.7/site-packages/xrft/xrft.py in _apply_window(da, dims, window_type)
     97             win_func(len(da[d]), sym=False), dims=da[d].dims, coords=da[d].coords
     98         )
---> 99         for d in dims
    100     ]
    101 

~/.conda/envs/pangeo2/lib/python3.7/site-packages/xrft/xrft.py in <listcomp>(.0)
     97             win_func(len(da[d]), sym=False), dims=da[d].dims, coords=da[d].coords
     98         )
---> 99         for d in dims
    100     ]
    101 

~/.conda/envs/pangeo2/lib/python3.7/site-packages/xarray/core/dataarray.py in __getitem__(self, key)
    702     def __getitem__(self, key: Any) -> "DataArray":
    703         if isinstance(key, str):
--> 704             return self._getitem_coord(key)
    705         else:
    706             # xarray-style array indexing

~/.conda/envs/pangeo2/lib/python3.7/site-packages/xarray/core/dataarray.py in _getitem_coord(self, key)
    695             dim_sizes = dict(zip(self.dims, self.shape))
    696             _, key, var = _get_virtual_variable(
--> 697                 self._coords, key, self._level_coords, dim_sizes
    698             )
    699 

~/.conda/envs/pangeo2/lib/python3.7/site-packages/xarray/core/dataset.py in _get_virtual_variable(variables, key, level_vars, dim_sizes)
    169         ref_var = dim_var.to_index_variable().get_level_variable(ref_name)
    170     else:
--> 171         ref_var = variables[ref_name]
    172 
    173     if var_name is None:

KeyError: 't'

Whereas xrft.power_spectrum(u_unfiltered.isel(x0=1000,y0=1000).load(),dim=['time'], window='hann',window_correction=True) is totally fine.

Any idea what is causing this? Is all testing performed on data with single character dimension names? Perhaps that is the problem?

[roxyboy]

Do you mean that if you generate a synthetic data with the same dimensions and coordinates as your u_unfiltered, you don't get an error whether dim is a string or list?
And one naive question: why are you loading the data before applying xrft...?

[cspencerjones]

And one naive question: why are you loading the data before applying xrft...?

Oh sorry I think that was left in from debugging!

Do you mean that if you generate a synthetic data with the same dimensions and coordinates as your u_unfiltered, you don't get an error whether dim is a string or list?

No this is not what I mean. I just tried with the example from the docs in my setup and I didn't get the error. But the example has dimension name 'x', and I haven't tried tweaking it.

[dougiesquire]

It looks like _apply_window does not support dim being a string (only a list of string). We must've missed this, but it should be an easy fix.

[dougiesquire]

The issue is that there is a for d in dims inside _apply_window, so yes, things will work for single character dimension names

[roxyboy]

I think in dft, if dim is given as a string, it is converted to a list...?

xrft/xrft/xrft.py

Line 341 in 2bb85b3

if isinstance(dim, str):

So the dim that _apply_window takes is a list no matter what, I think.

[dougiesquire]

I think in dft, if dim is given as a string, it is converted to a list...?

xrft/xrft/xrft.py

Line 341 in 2bb85b3

if isinstance(dim, str):

So the dim that _apply_window takes is a list no matter what, I think.

Hmm weird... I'll try and get to the bottom of this this morning

[rabernat]

Thanks for reporting this @cspencerjones! We appreciate it.

[paigem]

I think I've just run into this same problem when using xrft's detrend() function. The code I tried to run is included below, but it looks like the same fix that @dougiesquire made in _apply_window() should be added to detrend() as well. I can probably make the fix by the end of today.

I'm using xrft version 0.3.0 on Pangeo Cloud.

#Create dummy data
import numpy as np
x = np.arange(-5,5,0.01)
a = np.sin(2*x) + x + 3

# Convert to xarray
import xarray as xr
ax = xr.DataArray(a,dims=['time'])

# No error with constant detrend
import xrft
xrft.detrend(ax,'time',detrend_type='constant')

# Error with linear detrend
xrft.detrend(ax,'time',detrend_type='linear')

ValueError

--------------------------------------------------------------------------- ValueError Traceback (most recent call last) /srv/conda/envs/notebook/lib/python3.8/site-packages/xarray/core/common.py in _get_axis_num(self, dim) 171 try: --> 172 return self.dims.index(dim) 173 except ValueError:

ValueError: tuple.index(x): x not in tuple

During handling of the above exception, another exception occurred:

ValueError Traceback (most recent call last)
in
----> 1 ax_detrend_linear = xrft.detrend(ax2,'time',detrend_type='linear')

/srv/conda/envs/notebook/lib/python3.8/site-packages/xrft/detrend.py in detrend(da, dim, detrend_type)
49 elif detrend_type == "linear":
50 data = da.data
---> 51 axis_num = [da.get_axis_num(d) for d in dim]
52 chunks = getattr(data, "chunks", None)
53 if chunks:

/srv/conda/envs/notebook/lib/python3.8/site-packages/xrft/detrend.py in (.0)
49 elif detrend_type == "linear":
50 data = da.data
---> 51 axis_num = [da.get_axis_num(d) for d in dim]
52 chunks = getattr(data, "chunks", None)
53 if chunks:

/srv/conda/envs/notebook/lib/python3.8/site-packages/xarray/core/common.py in get_axis_num(self, dim)
166 return tuple(self._get_axis_num(d) for d in dim)
167 else:
--> 168 return self._get_axis_num(dim)
169
170 def _get_axis_num(self: Any, dim: Hashable) -> int:

/srv/conda/envs/notebook/lib/python3.8/site-packages/xarray/core/common.py in _get_axis_num(self, dim)
172 return self.dims.index(dim)
173 except ValueError:
--> 174 raise ValueError(f"{dim!r} not found in array dimensions {self.dims!r}")
175
176 @Property

ValueError: 't' not found in array dimensions ('time',)

[rabernat]

One possible workaround for this issue is to just put the dim inside a list, e.g.

xrft.detrend(ax, ['time'], detrend_type='constant')

Does this work?

[paigem]

@rabernat yep I did that so I could run my current computation, but I still think it probably makes sense to update detrend() to allow for the dim to just be a string.

[dougiesquire]

Thanks for bringing this up @paigem. I'll also update detrend() in the current PR (#143).

[paigem]

That would be great, thanks @dougiesquire!