task: metadata: install: pip install -e .[dev]
patch: diff --git a/nipype/interfaces/utility/base.py b/nipype/interfaces/utility/base.py
index 45261c998..03442df20 100644
--- a/nipype/interfaces/utility/base.py
+++ b/nipype/interfaces/utility/base.py
@@ -99,13 +99,29 @@ class IdentityInterface(IOBase):
 class MergeInputSpec(DynamicTraitedSpec, BaseInterfaceInputSpec):
     axis = traits.Enum('vstack', 'hstack', usedefault=True,
                        desc='direction in which to merge, hstack requires same number of elements in each input')
-    no_flatten = traits.Bool(False, usedefault=True, desc='append to outlist instead of extending in vstack mode')
+    no_flatten = traits.Bool(False, usedefault=True,
+                             desc='append to outlist instead of extending in vstack mode')
+    ravel_inputs = traits.Bool(False, usedefault=True,
+                               desc='ravel inputs when no_flatten is False')
 
 
 class MergeOutputSpec(TraitedSpec):
     out = traits.List(desc='Merged output')
 
 
+def _ravel(in_val):
+    if not isinstance(in_val, list):
+        return in_val
+    flat_list = []
+    for val in in_val:
+        raveled_val = _ravel(val)
+        if isinstance(raveled_val, list):
+            flat_list.extend(raveled_val)
+        else:
+            flat_list.append(raveled_val)
+    return flat_list
+
+
 class Merge(IOBase):
     """Basic interface class to merge inputs into a single list
 
@@ -123,23 +139,34 @@ class Merge(IOBase):
     >>> out.outputs.out
     [1, 2, 5, 3]
 
-    >>> merge = Merge()   # Or Merge(1)
-    >>> merge.inputs.in_lists = [1, [2, 5], 3]
+    >>> merge = Merge(1)
+    >>> merge.inputs.in1 = [1, [2, 5], 3]
+    >>> out = merge.run()
+    >>> out.outputs.out
+    [1, [2, 5], 3]
+
+    >>> merge = Merge(1)
+    >>> merge.inputs.in1 = [1, [2, 5], 3]
+    >>> merge.inputs.ravel_inputs = True
     >>> out = merge.run()
     >>> out.outputs.out
     [1, 2, 5, 3]
 
+    >>> merge = Merge(1)
+    >>> merge.inputs.in1 = [1, [2, 5], 3]
+    >>> merge.inputs.no_flatten = True
+    >>> out = merge.run()
+    >>> out.outputs.out
+    [[1, [2, 5], 3]]
     """
     input_spec = MergeInputSpec
     output_spec = MergeOutputSpec
 
-    def __init__(self, numinputs=1, **inputs):
+    def __init__(self, numinputs=0, **inputs):
         super(Merge, self).__init__(**inputs)
         self._numinputs = numinputs
-        if numinputs > 1:
+        if numinputs >= 1:
             input_names = ['in%d' % (i + 1) for i in range(numinputs)]
-        elif numinputs == 1:
-            input_names = ['in_lists']
         else:
             input_names = []
         add_traits(self.inputs, input_names)
@@ -150,8 +177,6 @@ class Merge(IOBase):
 
         if self._numinputs < 1:
             return outputs
-        elif self._numinputs == 1:
-            values = self.inputs.in_lists
         else:
             getval = lambda idx: getattr(self.inputs, 'in%d' % (idx + 1))
             values = [getval(idx) for idx in range(self._numinputs)
@@ -160,7 +185,8 @@ class Merge(IOBase):
         if self.inputs.axis == 'vstack':
             for value in values:
                 if isinstance(value, list) and not self.inputs.no_flatten:
-                    out.extend(value)
+                    out.extend(_ravel(value) if self.inputs.ravel_inputs else
+                               value)
                 else:
                     out.append(value)
         else:

problem_statement: New behaviour of Merge interface with breaks generic cases where numinputs=1 at runtime
### Summary

In the current HEAD, the new behaviour of the Merge interface when `numinputs==1` breaks cases (such as mine) where the `numinputs` is set at runtime depending on parameters of the workflow creation code. A better way to handle this would be to see if `in_lists` is defined or perhaps use `numinputs`==None.

### Actual behavior

`TypeError: '_Undefined' object is not iterable`

### Expected behavior

`in1` is taken to be the only element of the merged list

### How to replicate the behavior

```
import nipype.pipeline.engine as pe
from nipype.interfaces.utility.base import Merge

merge  = pe.Node(Merge(numinputs=1), name='merge')
merge.inputs.in1 = 1

workflow = pe.Workflow(name='workflow')
workflow.add_nodes([merge])
workflow.run()
```

### Platform details:
please paste the output of: `python -c "import nipype; print(nipype.get_info()); print(nipype.__version__)"`

    {'nibabel_version': '2.0.1', 'sys_executable': '/usr/local/opt/python/bin/python2.7', 'networkx_version': '1.11', 'numpy_version': '1.12.0', 'sys_platform': 'darwin', 'sys_version': '2.7.13 (default, Dec 17 2016, 23:03:43) \n[GCC 4.2.1 Compatible Apple LLVM 8.0.0 (clang-800.0.42.1)]', 'commit_source': u'repository', 'commit_hash': '368e184', 'pkg_path': '/Users/tclose/git/nipype/nipype', 'nipype_version': u'0.13.0-g368e184.dev', 'traits_version': '4.6.0', 'scipy_version': '0.14.0'}
    0.13.0-g368e184.dev
repo: nipy/nipype
test_cmd: pytest --no-header -rA --tb=line --color=no -p no:cacheprovider -W ignore::DeprecationWarning
test_patch: diff --git a/nipype/interfaces/utility/tests/test_base.py b/nipype/interfaces/utility/tests/test_base.py
index 68f4bed44..3d2fbd2b5 100644
--- a/nipype/interfaces/utility/tests/test_base.py
+++ b/nipype/interfaces/utility/tests/test_base.py
@@ -56,25 +56,20 @@ def test_split(tmpdir, args, expected):
         ([3], {}, [0, [1, 2], [3, 4, 5]], [0, 1, 2, 3, 4, 5]),
         ([0], {}, None, None),
         ([], {}, [], []),
-        ([], {}, [0, [1, 2], [3, 4, 5]], [0, 1, 2, 3, 4, 5]),
+        ([], {}, [0, [1, 2], [3, 4, 5]], [0, [1, 2], [3, 4, 5]]),
         ([3], {'axis': 'hstack'}, [[0], [1, 2], [3, 4, 5]], [[0, 1, 3]]),
         ([3], {'axis': 'hstack'}, [[0, 1], [2, 3], [4, 5]],
          [[0, 2, 4], [1, 3, 5]]),
         ([3], {'axis': 'hstack'}, [[0, 1], [2, 3], [4, 5]],
          [[0, 2, 4], [1, 3, 5]]),
-        ([1], {'axis': 'hstack'}, [[0], [1, 2], [3, 4, 5]], [[0, 1, 3]]),
-        ([1], {'axis': 'hstack'}, [[0, 1], [2, 3], [4, 5]],
-         [[0, 2, 4], [1, 3, 5]]),
         ])
 def test_merge(tmpdir, args, kwargs, in_lists, expected):
     os.chdir(str(tmpdir))
 
     node = pe.Node(utility.Merge(*args, **kwargs), name='merge')
 
-    numinputs = args[0] if args else 1
-    if numinputs == 1:
-        node.inputs.in_lists = in_lists
-    elif numinputs > 1:
+    numinputs = args[0] if args else 0
+    if numinputs >= 1:
         for i in range(1, numinputs + 1):
             setattr(node.inputs, 'in{:d}'.format(i), in_lists[i - 1])