Uses None instead of mutable lists for function param defaults

Addressing bug 1174809, changes use of mutable lists as as default
arguments and defaults them within the function. Otherwise, those
defaults can be unexpectedly persisted with the function between
invocations and erupt into mass hysteria on the streets.

To my knowledge there aren't known cases of the current use causing
specific issues, but even stylistically needs addressing to avoid
problems in the future. I couldn't find any comments or related
historical commit messages indicating the current use is meant to take
advantage of this behavior in Python.

Change-Id: I4a89afada08b2ce220724f585631a9e2072bf1bd
Fixes: bug 1174809

swift/obj/replicator.py

@@ -158,7 +158,7 @@ def invalidate_hash(suffix_dir):
         write_pickle(hashes, hashes_file, partition_dir, PICKLE_PROTOCOL)
 
 
-def get_hashes(partition_dir, recalculate=[], do_listdir=False,
+def get_hashes(partition_dir, recalculate=None, do_listdir=False,
                reclaim_age=ONE_WEEK):
     """
     Get a list of hashes for the suffix dir.  do_listdir causes it to mistrust
@@ -179,6 +179,10 @@ def get_hashes(partition_dir, recalculate=[], do_listdir=False,
     force_rewrite = False
     hashes = {}
     mtime = -1
+
+    if recalculate is None:
+        recalculate = []
+
     try:
         with open(hashes_file, 'rb') as fp:
             hashes = pickle.load(fp)
@@ -595,7 +599,7 @@ class ObjectReplicator(Daemon):
         self.job_count = len(jobs)
         return jobs
 
-    def replicate(self, override_devices=[], override_partitions=[]):
+    def replicate(self, override_devices=None, override_partitions=None):
         """Run a replication pass"""
         self.start = time.time()
         self.suffix_count = 0
@@ -604,9 +608,16 @@ class ObjectReplicator(Daemon):
         self.replication_count = 0
         self.last_replication_count = -1
         self.partition_times = []
+
+        if override_devices is None:
+            override_devices = []
+        if override_partitions is None:
+            override_partitions = []
+
         stats = eventlet.spawn(self.heartbeat)
         lockup_detector = eventlet.spawn(self.detect_lockups)
         eventlet.sleep()  # Give spawns a cycle
+
         try:
             self.run_pool = GreenPool(size=self.concurrency)
             jobs = self.collect_jobs()