rebaseline_server JSON: pass category values as values, not keys

gm/rebaseline_server/utils.py

+#!/usr/bin/python
+
+"""
+Copyright 2014 Google Inc.
+
+Use of this source code is governed by a BSD-style license that can be
+found in the LICENSE file.

[borenet, 2014/05/08 12:40:18]

I've been copy/pasting the same header, which reads:

# Copyright (c) 2013 The Chromium Authors. All rights reserved.
# Use of this source code is governed by a BSD-style license that can be
# found in the LICENSE file.

[epoger, 2014/05/08 15:04:35]

The most recent directive I have on what the copyright headers should look like
is https://b.corp.google.com/issue?id=5072851 ('write/run script to
automatically update license terms in all Skia source files'), which is in turn
based on a 2011 email from Mike.

Is there a more recent directive I should be following?

+
+Misc utilities for use throughout rebaseline_server code.
+"""
+
+def unzip_dict(input_dict):
+  """Splits a dictionary into two lists (keys and values), such that the indexes
+  within the lists can be used to connect key and value.

[borenet, 2014/05/08 12:40:18]

It seems like this should be a built-in function.  I guess there's no guarantee
that the following code would give you correct orderings?

return dict.keys(), dict.values()

If the results don't need to be sorted, this would be faster, since there's no
copy:
for k, v in input_dict.iteritems():
  keys.append(k)
  values.append(v)

[epoger, 2014/05/08 15:04:35]

Right, no guarantee of consistent ordering.  Also vulnerable to multithreading
errors (if dict changes between those two calls).

Actually, what I have already written is already vulnerable to multithreading
errors, in case a key disappears from input_dict while we're in the loop.
At some point before the results are presented to the user they should be
sorted.  This seemed as good a place as any (why wait until the client side?). 
I'll see how I can do that in the list-of-tuples model.

+
+  For this input:
+    {K3:V3, K1:V1, K2:V2}
+
+  It would return:
+    [K1,K2,K3], [V1,V2,V3]
+  """
+  keys = []
+  values = []
+  for key in sorted(input_dict.keys()):
+    value = input_dict[key]
+    keys.append(key)
+    values.append(value)
+  return keys, values