trouble with real=True in dft

I am having some trouble using the real fft. I have a time series of temperature data:

da = surf_temp_1selection.copy()
months = np.arange(da.time.size)+1
da.time.values = months
da = da.chunk({'time':None})
da = da.mean(dim={'xt_ocean', 'yt_ocean'})
da

<xarray.DataArray 'surface_temp' (time: 2520)>
dask.array<mean_agg-aggregate, shape=(2520,), dtype=float32, chunksize=(2520,), chunktype=numpy.ndarray>
Coordinates:
  * time     (time) int64 1 2 3 4 5 6 7 8 ... 2514 2515 2516 2517 2518 2519 2520

When I use dft without the real=True flag all seems good:

dahat = xrft.dft(da, dim=['time'], window=True)
dahat

<xarray.DataArray 'rechunk-merge-43b3f58ebe71665aef5d349f10b36e8c' (freq_time: 2520)>
dask.array<rechunk-merge, shape=(2520,), dtype=complex128, chunksize=(2520,), chunktype=numpy.ndarray>
Coordinates:
  * freq_time          (freq_time) float64 -0.5 -0.4996 ... 0.4992 0.4996
    freq_time_spacing  float64 0.0003968

But when I give real=True I get:

dahat = xrft.dft(da, dim=['time'], window=True, real=True)
dahat


---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-89-2e819cfb9956> in <module>
      6 
      7 print(da)
----> 8 dahat = xrft.dft(da, dim=['time'], window=True, real=True)
      9 print(" ");print(" ");print(" ");
     10 print(dahat)

/g/data3/hh5/public/apps/miniconda3/envs/analysis3-19.10/lib/python3.6/site-packages/xrft/xrft.py in dft(da, spacing_tol, dim, real, shift, detrend, window, chunks_to_segments)
    250 
    251     rawdims = da.dims
--> 252     da, trans = _transpose(da, real)
    253     if dim is None:
    254         dim = list(da.dims)

/g/data3/hh5/public/apps/miniconda3/envs/analysis3-19.10/lib/python3.6/site-packages/xrft/xrft.py in _transpose(da, real, trans)
    194         transdim = list(da.dims)
    195         if real not in transdim:
--> 196             raise ValueError("The dimension along real FT is taken must "
    197                             "be one of the existing dimensions.")
    198         elif real != transdim[-1]:

ValueError: The dimension along real FT is taken must be one of the existing dimensions.

Am I doing something wrong?

[roxyboy]

real takes the dimension to take the real FFT over so in your case, dahat = xrft.dft(da, window=True, real='time') should do the trick.

[roxyboy]

Hmm, then I’m not sure why real DFT generated errors in Python 3.7 and not in 3.6. In any case, the tests for real DFT in Python 3.7 all pass in PR #88.