doctest with block_reduce has ImportError

[pllim]

This has been reported upstream at scikit-image/scikit-image#3551 . Not sure how best to handle this failure in the meantime.

https://travis-ci.org/astropy/astropy/jobs/455917279

_______________________ [doctest] docs/nddata/index.rst ________________________
323 
324 The functions `~astropy.nddata.block_reduce` and
325 `~astropy.nddata.block_replicate` resize images. The example below reduces the
326 size of the image by a factor of 4. Note that the result is a `numpy.ndarray`;
327 the mask, metadata, etc are discarded:
328 
329 .. doctest-requires:: skimage
330 
331     >>> from astropy.nddata import block_reduce, block_replicate
332     >>> smaller = block_reduce(ccd, 4)
UNEXPECTED EXCEPTION: ImportError("cannot import name '_validate_lengths'",)
Traceback (most recent call last):
  File "/home/travis/miniconda/envs/test/lib/python3.6/doctest.py", line 1330, in __run
    compileflags, 1), test.globs)
  File "<doctest index.rst[61]>", line 1, in <module>
  File "/tmp/astropy-test-rp8r1_8c/lib/python3.6/site-packages/astropy/utils/decorators.py", line 827, in block_reduce
    func = make_function_with_signature(func, name=name, **wrapped_args)
  File "/tmp/astropy-test-rp8r1_8c/lib/python3.6/site-packages/astropy/nddata/decorators.py", line 245, in wrapper
    result = func(data, *args, **kwargs)
  File "/tmp/astropy-test-rp8r1_8c/lib/python3.6/site-packages/astropy/nddata/utils.py", line 370, in block_reduce
    from skimage.measure import block_reduce
  File "/home/travis/miniconda/envs/test/lib/python3.6/site-packages/skimage/measure/__init__.py", line 3, in <module>
    from ._marching_cubes_classic import (marching_cubes_classic,
  File "/home/travis/miniconda/envs/test/lib/python3.6/site-packages/skimage/measure/_marching_cubes_classic.py", line 3, in <module>
    from .._shared.utils import warn
  File "/home/travis/miniconda/envs/test/lib/python3.6/site-packages/skimage/_shared/utils.py", line 10, in <module>
    from ..util import img_as_float
  File "/home/travis/miniconda/envs/test/lib/python3.6/site-packages/skimage/util/__init__.py", line 8, in <module>
    from .arraycrop import crop
  File "/home/travis/miniconda/envs/test/lib/python3.6/site-packages/skimage/util/arraycrop.py", line 8, in <module>
    from numpy.lib.arraypad import _validate_lengths
ImportError: cannot import name '_validate_lengths'
/tmp/astropy-test-rp8r1_8c/lib/python3.6/site-packages/docs/nddata/index.rst:332: UnexpectedException

[mwcraig]

Trying out a fix for this at astropy/ccdproc#651 by xfailing some tests...

[pllim]

Huh, I don't see this in numpy-dev job anymore, e.g., https://travis-ci.org/astropy/astropy/jobs/470351360

[pllim]

BTW this is still causing numpy-dev job to fail in cron, but not push event. I couldn't find any obvious reason why. But since this has been fixed upstream, I don't think it is worth pursuing...

[astrojuanlu]

This is still making #8313 fail, see https://circleci.com/gh/astropy/astropy/21267

[astrojuanlu]

A fix will be released "in the next couple of weeks" scikit-image/scikit-image#3556 (comment)

[pllim]

It is now resurfacing in Sphinx job too.

[pllim]

Attempting to hack around it at #8357

[bsipocz]

scikit-image 0.14.2 is available now both on conda and pip, so I'm closing this issue.