MNT: add isiterable utility function

Also fix deprecation warning in Python 3.7 on `Iterable`
move to `collections.abc`.

src/diffpy/structure/structure.py

@@ -16,7 +16,6 @@
 """This module defines class Structure.
 """
 
-import collections
 import copy
 import numpy
 import codecs
@@ -25,6 +24,7 @@
 from diffpy.structure.lattice import Lattice
 from diffpy.structure.atom import Atom
 from diffpy.structure.utils import _linkAtomAttribute, atomBareSymbol
+from diffpy.structure.utils import isiterable
 
 # ----------------------------------------------------------------------------
 
@@ -388,7 +388,7 @@ def __getitem__(self, idx):
         # check if there is any string label that should be resolved
         scalarstringlabel = isinstance(idx, six.string_types)
         hasstringlabel = scalarstringlabel or (
-            isinstance(idx, collections.Iterable) and
+            isiterable(idx) and
             any(isinstance(ii, six.string_types) for ii in idx))
         # if not, use numpy indexing to resolve idx
         if not hasstringlabel:

src/diffpy/structure/utils.py

@@ -16,8 +16,20 @@
 """Small shared functions.
 """
 
+import six
 import numpy
 
+if six.PY2:
+    from collections import Iterable as _Iterable
+else:
+    from collections.abc import Iterable as _Iterable
+
+
+def isiterable(obj):
+    """True if argument is iterable."""
+    rv = isinstance(obj, _Iterable)
+    return rv
+
 
 def isfloat(s):
     """True if argument can be converted to float"""