Enhance flexibiliy of SelectFiles output list coercion

Author: mwaskom

nipype/interfaces/io.py

@@ -562,8 +562,12 @@ class SelectFilesInputSpec(DynamicTraitedSpec, BaseInterfaceInputSpec):
         desc="When matching mutliple files, return them in sorted order.")
     raise_on_empty = traits.Bool(True, usedefault=True,
         desc="Raise an exception if a template pattern matches no files.")
-    force_lists = traits.Bool(False, usedefault=True,
-        desc="Return all values as lists even when matching a single file.")
+    force_lists = traits.Either(traits.Bool(), traits.List(traits.Str()),
+        default=False, usedefault=True,
+        desc=("Whether to return outputs as a list even when only one file "
+              "matches the template. Either a boolean that applies to all "
+              "output fields or a list of output field names to coerce to "
+              " a list"))
 
 
 class SelectFiles(IOBase):
@@ -646,6 +650,10 @@ def _list_outputs(self):
         info = dict([(k, v) for k, v in self.inputs.__dict__.items()
                      if k in self._infields])
 
+        force_lists = self.inputs.force_lists
+        if isinstance(force_lists, bool):
+            force_lists = self._outfields if force_lists else []
+
         for field, template in self._templates.iteritems():
 
             # Build the full template path
@@ -673,7 +681,7 @@ def _list_outputs(self):
                 filelist.sort()
 
             # Handle whether this must be a list or not
-            if not self.inputs.force_lists:
+            if field not in force_lists:
                 filelist = list_to_filename(filelist)
 
             outputs[field] = filelist

nipype/interfaces/tests/test_io.py

@@ -43,6 +43,16 @@ def test_selectfiles():
                      "interfaces/spm/preprocess.py")
     yield assert_equal, res.outputs.preprocess, [wanted]
 
+    dg.inputs.package = "fsl"
+    dg.inputs.force_lists = ["model"]
+    res = dg.run()
+    preproc = op.join(op.dirname(nipype.__file__),
+                      "interfaces/fsl/preprocess.py")
+    model = [op.join(op.dirname(nipype.__file__),
+                     "interfaces/fsl/model.py")]
+    yield assert_equal, res.outputs.preprocess, preproc
+    yield assert_equal, res.outputs.model, model
+
     templates = {"converter": "interfaces/dcm{to!s}nii.py"}
     dg = nio.SelectFiles(templates, base_directory=base_dir)
     dg.inputs.to = 2