[1.5.x] PEP 302 source loaders already decode appropriately

Backport of c11aba1775ba0562251e4b2dba78da6a86ff338c from master

django/views/debug.py

@@ -354,15 +354,19 @@ class ExceptionReporter(object):
         if source is None:
             return None, [], None, []
 
-        encoding = 'ascii'
-        for line in source[:2]:
-            # File coding may be specified. Match pattern from PEP-263
-            # (http://www.python.org/dev/peps/pep-0263/)
-            match = re.search(br'coding[:=]\s*([-\w.]+)', line)
-            if match:
-                encoding = match.group(1).decode('ascii')
-                break
-        source = [six.text_type(sline, encoding, 'replace') for sline in source]
+        # If we just read the source from a file, or if the loader did not
+        # apply tokenize.detect_encoding to decode the source into a Unicode
+        # string, then we should do that ourselves.
+        if isinstance(source[0], six.binary_type):
+            encoding = 'ascii'
+            for line in source[:2]:
+                # File coding may be specified. Match pattern from PEP-263
+                # (http://www.python.org/dev/peps/pep-0263/)
+                match = re.search(br'coding[:=]\s*([-\w.]+)', line)
+                if match:
+                    encoding = match.group(1).decode('ascii')
+                    break
+            source = [six.text_type(sline, encoding, 'replace') for sline in source]
 
         lower_bound = max(0, lineno - context_lines)
         upper_bound = lineno + context_lines