Node de-listifies output lists with a single element

[effigies]

Summary / Actual behavior

A Select interface can return a single-element list, when that is the element selected. However, if placed in a Node, this will be unwrapped and the output will be the element itself.

Expected behavior

Node should not modify the outputs of an interface.

How to replicate the behavior

from nipype.pipeline import engine as pe
from nipype.interfaces import utility as niu

select_if = niu.Select(inlist=[[1, 2, 3], [4]], index=1)
select_nd = pe.Node(niu.Select(inlist=[[1, 2, 3], [4]], index=1), name='select_nd')

ifres = select_if.run()
ndres = select_nd.run()

assert ifres.outputs.out == [4]
assert ndres.outputs.out == 4

Platform details:

{'pkg_path': '/anaconda3/lib/python3.6/site-packages/nipype',
 'commit_source': 'archive substitution',
 'commit_hash': '%h',
 'nipype_version': '1.1.0',
 'sys_version': '3.6.5 |Anaconda, Inc.| (default, Apr 26 2018, 08:42:37) \n[GCC 4.2.1 Compatible Clang 4.0.1 (tags/RELEASE_401/final)]',
 'sys_executable': '/anaconda3/bin/python',
 'sys_platform': 'darwin',
 'numpy_version': '1.14.3',
 'scipy_version': '1.1.0',
 'networkx_version': '2.1',
 'nibabel_version': '2.3.0',
 'traits_version': '4.6.0'}
1.1.0

Execution environment

Choose one

• My python environment outside container

[effigies]

I think the issue is that when the Node.result.outputs is populated from Interface.result.outputs, and the trait is an OutputMultiObject, each sees a single-element list to squash. That is, the interface sees [[4]] and returns [4], and the node sees [4] an returns 4.

I'll try to resolve this by promoting OutputMultiObject values of x to [x] during this process.

[effigies]

Actually, it's because we save and reload the results file. So I'm going to officially blame @oesteban. :-)

Fix should be the same, but it should occur in save_resultsfile.

[oesteban]

Sorry about that