Liveconfig diff: use pprint to generate dict reprs

The write_live_config script often gives really strange results when
trying to display diffs of changes to dict values, since the ordering of
a dict is not defined. So key/value pairs will sometimes be rearranged
between the old and new versions, creating a confusing diff.

This changes to use the pprint module to generate string representations
for dicts, because it sorts the dict by key before outputting it, so we
should get consistent representations that can be compared more easily.

scripts/write_live_config

@@ -25,6 +25,7 @@
 from difflib import SequenceMatcher
 import itertools
 import os
+from pprint import PrettyPrinter
 import sys
 import json
 import getpass
@@ -81,6 +82,20 @@ def write_config_to_zookeeper(node, username, password, config, live_config):
         client.set(node, compressed_data)
 
 
+def get_comparable_repr(obj):
+    """Return a representation of the object that can be string-compared."""
+
+    # If the object is a dict, we'll use the pprint module, because it
+    # automatically sorts by key when generating its output for dicts
+    if isinstance(obj, dict):
+        # specify a huge width so it never tries to wrap the output
+        printer = PrettyPrinter(width=1000000)
+        return printer.pformat(obj)
+
+    # Otherwise, just use the standard repr for that object type
+    return repr(obj)
+
+
 def print_dict_diff(old, new):
     """Output changes between two dicts."""
 
@@ -105,7 +120,10 @@ def print_dict_diff(old, new):
 
         # otherwise, see how similar the reprs are, and if it's less
         # than 50% similar, just display it as a removal and addition
-        matcher = SequenceMatcher(a=repr(old[key]), b=repr(new[key]))
+        old_repr = get_comparable_repr(old[key])
+        new_repr = get_comparable_repr(new[key])
+        matcher = SequenceMatcher(a=old_repr, b=new_repr)
+
         if matcher.ratio() < 0.5:
             added_keys.add(key)
             removed_keys.add(key)
@@ -131,7 +149,10 @@ def print_dict_diff(old, new):
         if key in changed_keys:
             print "! {key:<{length}s} :".format(key=key, length=max_key_length)
 
-            matcher = SequenceMatcher(a=repr(old[key]), b=repr(new[key]))
+            old_repr = get_comparable_repr(old[key])
+            new_repr = get_comparable_repr(new[key])
+            matcher = SequenceMatcher(a=old_repr, b=new_repr)
+
             for tag, i, j, m, n in matcher.get_opcodes():
                 if tag == "equal":
                     continue