Better error reporting

implementations/python/preserves/binary.py

@@ -200,6 +200,8 @@ class Encoder(BinaryCodec):
             try:
                 i = iter(v)
             except TypeError:
+                i = None
+            if i is None:
                 raise TypeError('Cannot preserves-encode: ' + repr(v))
             self.encodevalues(5, i)
 

implementations/python/preserves/text.py

@@ -389,6 +389,8 @@ class Formatter(TextCodec):
             try:
                 i = iter(v)
             except TypeError:
+                i = None
+            if i is None:
                 raise TypeError('Cannot preserves-format: ' + repr(v))
             self.write_seq('[', ']', i)