issue with iris saver with delayed and dask==2025.4.0

[valeriupredoi]

Hi folks, apologies I had to renounce the issue template, this is more of a heads up that may need further investigation (it most probably is a Dask issue, as per usual): see ESMValGroup/ESMValCore#2716 and our failed test as of last night with the brand new dask==2025.4.0

/opt/conda/envs/esmvaltool/lib/python3.13/site-packages/iris/io/__init__.py:476: in save
    result = saver(source, target, **kwargs)
/opt/conda/envs/esmvaltool/lib/python3.13/site-packages/iris/fileformats/netcdf/saver.py:2746: in save
    with Saver(filename, netcdf_format, compute=compute) as sman:
/opt/conda/envs/esmvaltool/lib/python3.13/site-packages/iris/fileformats/netcdf/saver.py:437: in __exit__
    self.complete()

self = <iris.fileformats.netcdf.saver.Saver object at 0x7f7a5b668470>

    def complete(self) -> None:
        """Complete file by computing any delayed variable saves.
    
        This requires that the Saver has closed the dataset (exited its context).
    
        """
        if self._dataset.isopen():
            msg = (
                "Cannot call Saver.complete() until its dataset is closed, "
                "i.e. the saver's context has exited."
            )
            raise ValueError(msg)
    
        # Complete the saves now
>       self.delayed_completion().compute()
E       AttributeError: 'tuple' object has no attribute 'compute'

Cheers 🍺

[valeriupredoi]

just as a bit extra info:

• in dask=2025.3 def complete: self.delayed_completion() is a Delayed('store-f38f6909fea4fc14999e3af6962a7863') object
• in dask=2025.4 that same object becomes a tuple indeed (dask.array<store-map, shape=(), dtype=float64, chunksize=(), chunktype=numpy.ndarray>, dask.array<store-map, shape=(2,), dtype=int64, chunksize=(1,), chunktype=numpy.ndarray>, dask.array<store-map, shape=(), dtype=int64, chunksize=(), chunktype=numpy.ndarray>)

So a completely different API

[valeriupredoi]

Heads up thus continues to be an issue with the latest dask=2025.4.1 gah 😫🍺

[trexfeathers]

We are actively discussing when we can get time to investigate+fix this, I promise we haven't forgotten

[valeriupredoi]

Take your time, I'm at EGU myself, can't do much from here. But I can dig more and help next week 🍺