Performance: leverage dict comprehension in PEP-0274

PEP-0274 introduced dict comprehensions to replace dict constructor
with a sequence of length-2 sequences, these are benefits copied
from [1]:
  The dictionary constructor approach has two distinct disadvantages
  from the proposed syntax though.  First, it isn't as legible as a
  dict comprehension.  Second, it forces the programmer to create an
  in-core list object first, which could be expensive.
Nova dropped python 2.6 support, we can leverage this now.
There is deep dive about PEP-0274[2] and basic tests about
performance[3].
Note: This commit doesn't handle dict constructor with kwagrs.
This commit also adds a hacking rule.

[1]http://legacy.python.org/dev/peps/pep-0274/
[2]http://doughellmann.com/2012/11/12/the-performance-impact-of-using-dict-instead-of-in-cpython-2-7-2.html
[3]http://paste.openstack.org/show/154798/

Change-Id: Ifb5cb05b9cc2b8758d5a8e34f7792470a73d7c40

HACKING.rst

@@ -47,6 +47,7 @@ Nova Specific Commandments
 - [N334] Change assertTrue/False(A in/not in B, message) to the more specific
   assertIn/NotIn(A, B, message)
 - [N335] Check for usage of deprecated assertRaisesRegexp
+- [N336] Must use a dict comprehension instead of a dict constructor with a sequence of key-value pairs.
 
 Creating Unit Tests
 -------------------

nova/db/sqlalchemy/api.py

@@ -659,8 +659,8 @@ def compute_node_statistics(context):
     fields = ('count', 'vcpus', 'memory_mb', 'local_gb', 'vcpus_used',
               'memory_mb_used', 'local_gb_used', 'free_ram_mb', 'free_disk_gb',
               'current_workload', 'running_vms', 'disk_available_least')
-    return dict((field, int(result[idx] or 0))
-                for idx, field in enumerate(fields))
+    return {field: int(result[idx] or 0)
+            for idx, field in enumerate(fields)}
 
 
 ###################
@@ -3481,8 +3481,8 @@ def quota_reserve(context, resources, project_quotas, user_quotas, deltas,
             usages = project_usages
         else:
             usages = user_usages
-        usages = dict((k, dict(in_use=v['in_use'], reserved=v['reserved']))
-                      for k, v in usages.items())
+        usages = {k: dict(in_use=v['in_use'], reserved=v['reserved'])
+                  for k, v in usages.items()}
         LOG.debug('Raise OverQuota exception because: '
                   'project_quotas: %(project_quotas)s, '
                   'user_quotas: %(user_quotas)s, deltas: %(deltas)s, '
@@ -4503,8 +4503,8 @@ def _dict_with_extra_specs(inst_type_query):
 
     """
     inst_type_dict = dict(inst_type_query)
-    extra_specs = dict([(x['key'], x['value'])
-                        for x in inst_type_query['extra_specs']])
+    extra_specs = {x['key']: x['value']
+                   for x in inst_type_query['extra_specs']}
     inst_type_dict['extra_specs'] = extra_specs
     return inst_type_dict
 
@@ -4711,7 +4711,7 @@ def _flavor_extra_specs_get_query(context, flavor_id, session=None):
 @require_context
 def flavor_extra_specs_get(context, flavor_id):
     rows = _flavor_extra_specs_get_query(context, flavor_id).all()
-    return dict([(row['key'], row['value']) for row in rows])
+    return {row['key']: row['value'] for row in rows}
 
 
 @require_context
@@ -4835,7 +4835,7 @@ def _instance_metadata_get_query(context, instance_uuid, session=None):
 @require_context
 def instance_metadata_get(context, instance_uuid):
     rows = _instance_metadata_get_query(context, instance_uuid).all()
-    return dict((row['key'], row['value']) for row in rows)
+    return {row['key']: row['value'] for row in rows}
 
 
 @require_context
@@ -4901,7 +4901,7 @@ def _instance_system_metadata_get_query(context, instance_uuid, session=None):
 @require_context
 def instance_system_metadata_get(context, instance_uuid):
     rows = _instance_system_metadata_get_query(context, instance_uuid).all()
-    return dict((row['key'], row['value']) for row in rows)
+    return {row['key']: row['value'] for row in rows}
 
 
 @require_context
@@ -5417,7 +5417,7 @@ def aggregate_metadata_get(context, aggregate_id):
                        models.AggregateMetadata).\
                        filter_by(aggregate_id=aggregate_id).all()
 
-    return dict([(r['key'], r['value']) for r in rows])
+    return {r['key']: r['value'] for r in rows}
 
 
 @require_aggregate_exists

nova/db/sqlalchemy/utils.py

@@ -60,8 +60,8 @@ def check_shadow_table(migrate_engine, table_name):
     shadow_table = Table(db._SHADOW_TABLE_PREFIX + table_name, meta,
                          autoload=True)
 
-    columns = dict([(c.name, c) for c in table.columns])
-    shadow_columns = dict([(c.name, c) for c in shadow_table.columns])
+    columns = {c.name: c for c in table.columns}
+    shadow_columns = {c.name: c for c in shadow_table.columns}
 
     for name, column in columns.iteritems():
         if name not in shadow_columns:

nova/tests/unit/policy_fixture.py

@@ -41,8 +41,8 @@ class PolicyFixture(fixtures.Fixture):
 
     def set_rules(self, rules):
         policy = nova.policy._ENFORCER
-        policy.set_rules(dict((k, common_policy.parse_rule(v))
-                               for k, v in rules.items()))
+        policy.set_rules({k: common_policy.parse_rule(v)
+                          for k, v in rules.items()})
 
 
 class RoleBasedPolicyFixture(fixtures.Fixture):