tlv: Don't require encoder/decoder methods for TLV without value

There are instances where a TLV IE is used as just a flag, i.e.
length zero and no value part.  In those situations, it would require
a lot of boilerplate code to require the TLV_IE class definitions to
have _to_bytes/_from_bytes methods that do nothing.

So instead, add a shortcut: If we want to encode 'None', then return
b'', and if we want to decode b'' return None.

Change-Id: Ie8eb2830e8eefa81e94b8b8b157062c085aeb777

1 files changed, 6 insertions, 2 deletions

diff --git a/pySim/tlv.py b/pySim/tlv.py
index 05f824e..0dc746b 100644
--- a/pySim/tlv.py
+++ b/pySim/tlv.py
@@ -80,7 +80,9 @@ class Transcodable(abc.ABC):
     def to_bytes(self) -> bytes:
         """Convert from internal representation to binary bytes.  Store the binary result
         in the internal state and return it."""
-        if self._construct:
+        if not self.decoded:
+            do = b''
+        elif self._construct:
             do = self._construct.build(self.decoded, total_len=None)
         elif self.__class__._construct:
             do = self.__class__._construct.build(self.decoded, total_len=None)
@@ -97,7 +99,9 @@ class Transcodable(abc.ABC):
         """Convert from binary bytes to internal representation. Store the decoded result
         in the internal state and return it."""
         self.encoded = do
-        if self._construct:
+        if self.encoded == b'':
+            self.decoded = None
+        elif self._construct:
             self.decoded = parse_construct(self._construct, do)
         elif self.__class__._construct:
             self.decoded = parse_construct(self.__class__._construct, do)