AVRO-1741: Py3: Fix error when codec is not in the header. Contribute…

…d by Matthew Hayes.

This happens when the compression codec is None, meaning the
"null"/uncompressed codec.

CHANGES.txt

@@ -12,6 +12,9 @@ Trunk (not yet released)
 
   BUG FIXES
 
+    AVRO-1741: Python3: Fix error when codec is not in the header.
+    (Matthew Hayes via blue)
+
 Avro 1.8.1 (14 May 2016)
 
   INCOMPATIBLE CHANGES

lang/py3/avro/datafile.py

@@ -349,9 +349,11 @@ def __init__(self, reader, datum_reader):
     self._read_header()
 
     # ensure codec is valid
-    self.codec = self.GetMeta('avro.codec').decode('utf-8')
-    if self.codec is None:
+    avro_codec_raw = self.GetMeta('avro.codec')
+    if avro_codec_raw is None:
       self.codec = "null"
+    else:
+      self.codec = avro_codec_raw.decode('utf-8')
     if self.codec not in VALID_CODECS:
       raise DataFileException('Unknown codec: %s.' % self.codec)