[python] fix binary unpacking for python 2.x and 3.x

- unpack always returns a tuple, so take first value
- use bytearray instead of list for buffer
- preallocate bytearray with length of payload for speed

sw/lib/python/pprz_msg/message.py

@@ -184,15 +184,15 @@ def binary_to_payload(self, data):
         for idx, t in enumerate(self.fieldtypes):
             bin_type = self.fieldbintypes(t)
             if '[' in t:
-                array_length = int(struct.unpack('<B', data[msg_offset])[0])
+                array_length = data[msg_offset]
                 msg_offset += 1
                 array_value = []
                 for count in range(0, array_length):
-                    array_value.append(struct.unpack('<' + bin_type[0], "".join(data[msg_offset:msg_offset + bin_type[1]])))
+                    array_value.append(struct.unpack('<' + bin_type[0], data[msg_offset:msg_offset + bin_type[1]])[0])
                     msg_offset = msg_offset + bin_type[1]
                 values.append(array_value)
             else:
-                value = struct.unpack('<' + bin_type[0], "".join(data[msg_offset:msg_offset + bin_type[1]]))
+                value = struct.unpack('<' + bin_type[0], data[msg_offset:msg_offset + bin_type[1]])[0]
                 msg_offset = msg_offset + bin_type[1]
                 values.append(value)
         self.set_values(values)

sw/lib/python/pprz_msg/pprz_transport.py

@@ -40,19 +40,19 @@ def reset_parser(self):
 
     def parse_byte(self, c):
         """parse new byte, return True when a new full message is available"""
-        b = ord(c)
+        b = struct.unpack("<B", c)[0]
         if self.state == PprzParserState.WaitSTX:
             if b == STX:
                 self.state = PprzParserState.GotSTX
         elif self.state == PprzParserState.GotSTX:
             self.length = b - 4
-            self.buf = []
+            self.buf = bytearray(self.length)
             self.ck_a = b % 256
             self.ck_b = b % 256
             self.idx = 0
             self.state = PprzParserState.GotLength
         elif self.state == PprzParserState.GotLength:
-            self.buf.append(c)
+            self.buf[self.idx] = b
             self.ck_a = (self.ck_a + b) % 256
             self.ck_b = (self.ck_b + self.ck_a) % 256
             self.idx += 1
@@ -77,8 +77,8 @@ def get_buffer(self):
 
     def unpack_pprz_msg(self, msg_class, data):
         """Unpack a raw PPRZ message"""
-        sender_id = ord(data[0])
-        msg_id = ord(data[1])
+        sender_id = data[0]
+        msg_id = data[1]
         msg = PprzMessage(msg_class, msg_id)
         msg.binary_to_payload(data[2:])
         return sender_id, msg
@@ -92,7 +92,10 @@ def calculate_checksum(self, msg):
         ck_b = 0
         # start char not included in checksum for pprz protocol
         for c in msg[1:]:
-            ck_a = (ck_a + ord(c)) % 256
+            # try to handle differences between python 2.x and 3.x
+            if isinstance(c, str):
+                c = struct.unpack("<B", c)[0]
+            ck_a = (ck_a + c) % 256
             ck_b = (ck_b + ck_a) % 256
         return ck_a, ck_b