np.datetime64('NAT') fill value

[jvail]

I do not understand why this (xarray 0.17.0)

da = xr.DataArray([np.datetime64('NAT')])
da.to_netcdf('da')

is OK and the following gives an error

da = xr.DataArray([np.datetime64('NAT')], attrs={'_FillValue': np.datetime64('NAT')})
da.to_netcdf('da')

TypeError: Invalid value for attr '_FillValue': numpy.datetime64('NaT') must be a number, a string, an ndarray or a list/tuple of numbers/strings for serialization to netCDF files

What is a "proper" fill value for datedime64? Thank you, Jan

[max-sixty]

IIUC, _FillValue is just a string there — it raises an error because the values of attrs must be one of those types.

Is there a specific reason _FillValue needs to be in the attrs? (I'm not a big netcdf user so there may be)

[max-sixty]

Could you link to docs / code on where it stores _FillValue as at attrs?

[dcherian]

ValueError: could not convert string to float: b'NAT'

Times are stored as numbers representing "hours since xxxx" (for e.g.). So you cannot assign a string for _FillValue. Why are you changing it?

[jvail]

ValueError: could not convert string to float: b'NAT'

Times are stored as numbers representing "hours since xxxx" (for e.g.). So you cannot assign a string for _FillValue. Why are you changing it?

I just tested a str because if I use a datetime64 xarray complains and if I use a str xarray is happy but netcdf fails. The confusing part is: If I can use a value like datetime64('NAT') inside an array why can I not use it as a fill value?

[dcherian]

datetime64('NAT') inside an array why can I not use it as a fill value?

Because netCDF does not accept it. it has no understanding of datetime types so we "encode" it to a number. The in-memory NaT will be represented by _FillValue on disk (and it must be a number).

Why are you trying to set _FillValue? Is the automatic choice not good enough?

[jvail]

Why are you trying to set _FillValue? Is the automatic choice not good enough?

It is actually set by xarray-simlab. I just tried to make a little test case to figure out why saving my xarray dataset (the ouput of xarray-simlab) fails.

Because netCDF does not accept it. it has no understanding of datetime types so we "encode" it to a number. The in-memory NaT will be represented by _FillValue on disk (and it must be a number).

Then it might be an xarray issue - this is why I ask the question here :). Why does xarray accept 'NAT' as a value but not as a fill value when I want to save it as netcdf? I don't get it - sorry, maybe I am too slow. I would expect that anything that can be a value can be a fill value.