index_by supports compound index keys.

lib/spack/llnl/util/lang.py

@@ -68,6 +68,12 @@ def index_by(objects, *funcs):
 
            index1 = index_by(list_of_specs, 'arch', 'compiler')
            index2 = index_by(list_of_specs, 'compiler')
+
+       You can also index by tuples by passing tuples:
+
+           index1 = index_by(list_of_specs, ('arch', 'compiler'))
+
+       Keys in the resulting dict will look like ('gcc', 'bgqos_0').
     """
     if not funcs:
         return objects
@@ -75,6 +81,8 @@ def index_by(objects, *funcs):
     f = funcs[0]
     if isinstance(f, basestring):
         f = lambda x: getattr(x, funcs[0])
+    elif isinstance(f, tuple):
+        f = lambda x: tuple(getattr(x, p) for p in funcs[0])
 
     result = {}
     for o in objects: