bugfix for python 2.6

reversion_compare/helpers.py

@@ -55,6 +55,23 @@ def highlight_diff(diff_text):
 # Change from ndiff to unified_diff if old/new values are more than X lines:
 LINE_COUNT_4_UNIFIED_DIFF = 4
 
+
+def format_range(start, stop):
+    """
+    Convert range to the "ed" format
+    difflib._format_range_unified() is new in python 2.7
+    see also: https://github.com/jedie/django-reversion-compare/issues/5
+    """
+    # Per the diff spec at http://www.unix.org/single_unix_specification/
+    beginning = start + 1     # lines start numbering with one
+    length = stop - start
+    if length == 1:
+        return '{0}'.format(beginning)
+    if not length:
+        beginning -= 1        # empty ranges begin at line just before the range
+    return '{0},{1}'.format(beginning, length)
+
+
 def unified_diff(a, b, n=3, lineterm='\n'):
     r"""
     simmilar to the original difflib.unified_diff except:
@@ -77,21 +94,14 @@ def unified_diff(a, b, n=3, lineterm='\n'):
     started = False
     for group in difflib.SequenceMatcher(None, a, b).get_grouped_opcodes(n):
         first, last = group[0], group[-1]
-        try:
-            file1_range = difflib._format_range_unified(first[1], last[2])
-            file2_range = difflib._format_range_unified(first[3], last[4])
-        except AttributeError:
-            # difflib._format_range_unified() is new in python 2.7
-            # see also: https://github.com/jedie/django-reversion-compare/issues/5
-            i1, i2, j1, j2 = first[1], last[2], first[3], last[4]
-            file1_range = "%i,%i" % (i1 + 1, i2 - i1)
-            file2_range = "%i,%i" % (j1 + 1, j2 - j1)
+        file1_range = format_range(first[1], last[2])
+        file2_range = format_range(first[3], last[4])
 
         if not started:
             started = True
-            yield '@@ -{} +{} @@'.format(file1_range, file2_range)
+            yield '@@ -{0} +{1} @@'.format(file1_range, file2_range)
         else:
-            yield '{}@@ -{} +{} @@'.format(lineterm, file1_range, file2_range)
+            yield '{0}@@ -{1} +{2} @@'.format(lineterm, file1_range, file2_range)
 
         for tag, i1, i2, j1, j2 in group:
             if tag == 'equal':