skimage.util.apply_parallel doesn't work with skimage.transform.downscale_local_mean when applied on a dask array about scikit-image HOT 3 CLOSED

Ah! The error is that extra_arguments is used for positional arguments. You should be using extra_keywords for keyword arguments. Using this line:

downscaled = skimage.util.apply_parallel(
        skimage.transform.downscale_local_mean,
        rand_array,
        extra_keywords={"factors": (10, 10)},  # <-- here
        dtype=rand_array.dtype,
        )

fixes the problem.

Potentially, there is a discussion to be had about validating the type of extra_arguments. But maybe that should be the job of the type checker, not a runtime check...

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As a side note, for this particular function, dask array has a built-in function called dask.array.coarsen.

from scikit-image.

Agreed. This is potentially a something we could address with a nicer error or warning message but doesn't seem like a bug. 👍

@rcremese, if you are interested in adding a nicer error message for cases like this, please feel very welcome to make a PR and request a review from me.

I'll close this as resolved for now, but don't hesitate to re-open or add something. Thanks for reaching out.

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