dft fails with real option after xarray update

[tomchor]

I have recently updated xarray to the latest version and noticed that I can't perform a dft if one of the dimensions is set to have a real transform.

Example:

In [12]: xrft.dft(du.u, dim="x", shift=False)                                         
Out[12]: 
<xarray.DataArray 'fftn-e982410c8538317e81efde78e78ed80c' (itime: 30, freq_x: 160, y: 160, z: 144)>
dask.array<shape=(30, 160, 160, 144), dtype=complex128, chunksize=(2, 160, 160, 144)>
Coordinates:
  * itime           (itime) int64 167750 168000 168250 ... 174500 174750 175000
  * freq_x          (freq_x) float64 0.0 0.0025 0.005 ... -0.0075 -0.005 -0.0025
  * y               (y) float64 1.25 3.75 6.25 8.75 ... 391.2 393.8 396.2 398.8
  * z               (z) float64 -89.69 -89.06 -88.44 ... -1.562 -0.9375 -0.3125
    freq_x_spacing  float64 0.0025

Now if I use the the real option:

In [15]: xrft.dft(du.u, dim="x", shift=False, real="x")                               
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-15-f1b65d8824b4> in <module>
----> 1 xrft.dft(du.u, dim="x", shift=False, real="x")

~/miniconda3/envs/py36/lib/python3.6/site-packages/xrft/xrft.py in dft(da, spacing_tol, dim, real, shift, detrend, window, chunks_to_segments)
    350     if trans:
    351         enddims = [prefix + d for d in rawdims if d in dim]
--> 352         return daft.transpose(*enddims)
    353     else:
    354         return daft

~/miniconda3/envs/py36/lib/python3.6/site-packages/xarray/core/dataarray.py in transpose(self, transpose_coords, *dims)
   1642                 raise ValueError('arguments to transpose (%s) must be '
   1643                                  'permuted array dimensions (%s)'
-> 1644                                  % (dims, tuple(self.dims)))
   1645 
   1646         variable = self.variable.transpose(*dims)

ValueError: arguments to transpose (('freq_x',)) must be permuted array dimensions (('itime', 'y', 'z', 'freq_x'))

Thanks

[TomNicholas]

It looks like this now breaks because an extra check on the 'dims' argument to da.transpose() was imposed in xarray PR #2556. To fix this then we would just need to supply all the dims in the correct order to daft.transpose(), rather than just the "enddims".

[roxyboy]

So... basically we should update the _transpose function in xrft @TomNicholas ?

[TomNicholas]

Sorry for the slow response.

Your PR #88 does exactly this:

we would just need to supply all the dims in the correct order

so I think you should merge it :)

[roxyboy]

Ok, I'm gonna merge this @rabernat