python: use the setdefault method on dict

It allows more concise code and skips some key lookups.

lib/spack/llnl/util/lang.py

@@ -87,10 +87,7 @@ def index_by(objects, *funcs):
     result = {}
     for o in objects:
         key = f(o)
-        if key not in result:
+        result.setdefault(key, []).append(o)
-            result[key] = [o]
-        else:
-            result[key].append(o)
 
     for key, objects in result.items():
         result[key] = index_by(objects, *funcs[1:])

lib/spack/spack/directives.py

@@ -239,12 +239,10 @@ def patch(pkg, url_or_filename, level=1, when=None):
         when = pkg.name
     when_spec = parse_anonymous_spec(when, pkg.name)
 
-    if when_spec not in pkg.patches:
+    cur_patches = pkg.patches.setdefault(when_spec, [])
-        pkg.patches[when_spec] = [Patch(pkg.name, url_or_filename, level)]
-    else:
     # if this spec is identical to some other, then append this
     # patch to the existing list.
-        pkg.patches[when_spec].append(Patch(pkg.name, url_or_filename, level))
+    cur_patches.append(Patch(pkg.name, url_or_filename, level))
 
 
 @directive('variants')

lib/spack/spack/virtual.py

@@ -73,10 +73,8 @@ def update(self, spec):
         for provided_spec, provider_spec in pkg.provided.iteritems():
             if provider_spec.satisfies(spec, deps=False):
                 provided_name = provided_spec.name
-                if provided_name not in self.providers:
-                    self.providers[provided_name] = {}
 
-                provider_map = self.providers[provided_name]
+                provider_map = self.providers.setdefault(provided_name, {})
                 if not provided_spec in provider_map:
                     provider_map[provided_spec] = set()
 
@@ -133,9 +131,7 @@ def _cross_provider_maps(self, lmap, rmap):
                 if lp_spec.name == rp_spec.name:
                     try:
                         const = lp_spec.copy().constrain(rp_spec,deps=False)
-                        if constrained not in result:
+                        result.setdefault(constrained, set()).add(const)
-                            result[constrained] = set()
-                        result[constrained].add(const)
                     except spack.spec.UnsatisfiableSpecError:
                         continue
         return result