[experiments][api] Fix sorting of experiments for Python 3

Python 3 has brought several modifications regarding sorting.
This patch refactors the code so it can be properly used with
both versions of Python.

beat/web/experiments/api.py

@@ -29,6 +29,7 @@ import re
 import uuid
 
 import simplejson
+import functools
 
 from django.conf import settings
 from django.shortcuts import get_object_or_404
@@ -104,6 +105,15 @@ class ListCreateExperimentsView(ListCreateContributionView):
         def _getStatusLabel(status):
             return [s for s in Experiment.STATUS if s[0] == status][0][1]
 
+        def _cmp(a, b):
+            """
+            cmp is not available anymore in Python 3. This method is implemetend
+            as recommended in the documentation for this kind of use case.
+            Based on:
+              https://docs.python.org/3.0/whatsnew/3.0.html#ordering-comparisons
+            """
+            return (a > b) - (a < b)
+
         def _custom_compare(exp1, exp2):
             PENDING = _getStatusLabel(Experiment.PENDING)
 
@@ -113,7 +123,7 @@ class ListCreateExperimentsView(ListCreateContributionView):
                 if exp2['status'] != PENDING:
                     return -1
                 elif exp1['creation_date'] != exp2['creation_date']:
-                    return cmp(exp2['creation_date'], exp1['creation_date'])
+                    return _cmp(exp1['creation_date'], exp2['creation_date'])
                 else:
                     return 1
             elif exp2['status'] == PENDING:
@@ -123,13 +133,13 @@ class ListCreateExperimentsView(ListCreateContributionView):
             tier3_states = [_getStatusLabel(Experiment.DONE), _getStatusLabel(Experiment.FAILED)]
 
             if (exp1['status'] in tier3_states) and (exp2['status'] in tier3_states):
-                return cmp(exp2['end_date'], exp1['end_date'])
+                return _cmp(exp2['end_date'], exp1['end_date'])
             elif (exp1['status'] in tier3_states) and (exp2['status'] not in tier3_states):
                 return -1
             elif (exp1['status'] not in tier3_states) and (exp2['status'] in tier3_states):
                 return 1
 
-            return cmp(exp1['start_date'], exp2['start_date'])
+            return _cmp(exp1['start_date'], exp2['start_date'])
 
         # Retrieve the experiments
         fields_to_return_original = self.get_serializer_fields(request)
@@ -140,8 +150,10 @@ class ListCreateExperimentsView(ListCreateContributionView):
         experiments = self.model.objects.from_author_and_public(request.user, author_name).select_related()
 
         serializer = self.get_serializer(experiments, many=True, fields=fields_to_return)
-        result = serializer.data
-        result.sort(_custom_compare)
+        if six.PY2:
+            result = sorted(serializer.data, cmp=_custom_compare)
+        else:
+            result = sorted(serializer.data, key=functools.cmp_to_key(_custom_compare))
 
         processed_result = []
         for experiment in result: