Raise a meaningful exception if 'decode_msg' is used improperly (#38)

.github/workflows/main.yml

@@ -9,7 +9,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python: ['3.7']
+        python: ['3.7', '3.8', '3.9']
         os: ['ubuntu-latest']
     steps:
       - uses: actions/checkout@master

CHANGELOG.txt

@@ -1,6 +1,12 @@
 ====================
  pyais CHANGELOG
 ====================
+-------------------------------------------------------------------------------
+ Version 1.6.2  2 May 2021
+-------------------------------------------------------------------------------
+
+* Improves `decode_msg` by adding meaningful error messages
+
 -------------------------------------------------------------------------------
  Version 1.6.0  2 May 2021
 -------------------------------------------------------------------------------

pyais/__init__.py

@@ -4,7 +4,7 @@
 
 
 __license__ = 'MIT'
-__version__ = '1.6.1'
+__version__ = '1.6.2'
 
 __all__ = (
     'decode_msg',

pyais/decode.py

@@ -1,7 +1,7 @@
 from functools import partial
-from typing import Any, Dict, Union
+from typing import Any, Dict, Union, List
 
-import bitarray  # type: ignore
+import bitarray
 
 from pyais import messages
 from pyais.constants import (
@@ -14,7 +14,7 @@
     StationIntervals,
     NavAid
 )
-from pyais.exceptions import UnknownMessageException
+from pyais.exceptions import UnknownMessageException, MissingMultipartMessageException, TooManyMessagesException
 from pyais.util import get_int, encode_bin_as_ascii6, get_mmsi
 
 
@@ -715,8 +715,24 @@ def decode_msg(*args: Union[str, bytes]) -> Dict[str, Any]:
     # Make everything bytes
     message_as_bytes = tuple(msg.encode('utf-8') if isinstance(msg, str) else msg for msg in args)
 
-    # Create temporary messages
-    temp = [messages.NMEAMessage(m) for m in message_as_bytes]
+    # Convert bytes into NMEAMessage and remember fragment_count and fragment_numbers
+    temp: List[messages.NMEAMessage] = []
+    frags: List[int] = []
+    frag_cnt: int = 1
+    for msg in message_as_bytes:
+        nmea = messages.NMEAMessage(msg)
+        temp.append(nmea)
+        frags.append(nmea.fragment_number)
+        frag_cnt = nmea.fragment_count
+
+    # Make sure provided parts assemble a single (multiline message)
+    if len(message_as_bytes) > frag_cnt:
+        raise TooManyMessagesException(f"Got {len(message_as_bytes)} messages, but fragment count is {frag_cnt}")
+
+    # Make sure all parts of a multipart message are provided
+    diff = [x for x in range(1, frag_cnt + 1) if x not in frags]
+    if len(diff):
+        raise MissingMultipartMessageException(f"Missing fragment numbers: {diff}")
 
     # Assemble temporary messages
     final = messages.NMEAMessage.assemble_from_iterable(temp)

pyais/exceptions.py

@@ -6,3 +6,11 @@ class InvalidNMEAMessageException(Exception):
 class UnknownMessageException(Exception):
     """Message not supported yet"""
     pass
+
+
+class MissingMultipartMessageException(Exception):
+    """Multipart message with missing parts provided"""
+
+
+class TooManyMessagesException(Exception):
+    """Too many messages"""

pyais/messages.py

@@ -1,7 +1,7 @@
 import json
 from typing import Any, Dict, Optional, Sequence, Tuple, Type, Union
 
-from bitarray import bitarray  # type: ignore
+from bitarray import bitarray
 
 from pyais.ais_types import AISType
 from pyais.constants import TalkerID
@@ -179,7 +179,7 @@ def __str__(self) -> str:
     def __getitem__(self, item: str) -> Union[int, str, bytes, bitarray]:
         if isinstance(item, str):
             try:
-                return getattr(self, item)
+                return getattr(self, item)  # type: ignore
             except AttributeError:
                 raise KeyError(item)
         else:

pyais/util.py

@@ -4,7 +4,7 @@
 from operator import xor
 from typing import Any, Generator, Hashable, TYPE_CHECKING, Callable
 
-from bitarray import bitarray  # type: ignore
+from bitarray import bitarray
 
 if TYPE_CHECKING:
     BaseDict = OrderedDict[Hashable, Any]
@@ -43,7 +43,7 @@ def decode_into_bit_array(data: bytes) -> bitarray:
         # Convert 8 bit binary to 6 bit binary
         c -= 0x30 if (c < 0x60) else 0x38
         c &= 0x3F
-        bit_arr += f'{c:06b}'
+        bit_arr += bitarray(f'{c:06b}')
 
     return bit_arr
 

tests/test_decode_raw.py

@@ -1,7 +1,7 @@
 import unittest
 
 from pyais import decode_msg
-from pyais.exceptions import InvalidNMEAMessageException
+from pyais.exceptions import InvalidNMEAMessageException, MissingMultipartMessageException, TooManyMessagesException
 
 
 class TestDecode(unittest.TestCase):
@@ -64,9 +64,36 @@ def test_decode_multiline_message(self):
         self.assertEqual(decoded["shipname"], "NORDIC HAMBURG")
         self.assertEqual(decoded["destination"], "CTT-LAYBY")
 
-        decoded = decode_msg(
-            b'!AIVDM,2,1,1,A,538CQ>02A;h?D9QC800pu8@T>0P4l9E8L0000017Ah:;;5r50Ahm5;C0,0*07',
-            b'!AIVDM,2,2,1,A,F@V@00000000000,2*35',
-            b'!AIVDM,2,1,9,A,538CQ>02A;h?D9QC800pu8@T>0P4l9E8L0000017Ah:;;5r50Ahm5;C0,0*0F',
-            b'!AIVDM,2,2,9,A,F@V@00000000000,2*3D',
+    def test_too_many_messages(self):
+        with self.assertRaises(TooManyMessagesException) as err:
+            decode_msg(
+                b'!AIVDM,2,1,1,A,538CQ>02A;h?D9QC800pu8@T>0P4l9E8L0000017Ah:;;5r50Ahm5;C0,0*07',
+                b'!AIVDM,2,2,1,A,F@V@00000000000,2*35',
+                b'!AIVDM,2,1,9,A,538CQ>02A;h?D9QC800pu8@T>0P4l9E8L0000017Ah:;;5r50Ahm5;C0,0*0F',
+                b'!AIVDM,2,2,9,A,F@V@00000000000,2*3D',
+            )
+        self.assertEqual(str(err.exception), "Got 4 messages, but fragment count is 2")
+
+    def test_multipart_error_message(self):
+        """Refer to issue #37"""
+        msg_1 = "!AIVDM,2,1,0,A,539p4OT00000@7W3K@08ThiLE8@E:0000000001S0h9135Pl?0R0C@UDQp00,0*68"
+        msg_2 = "!AIVDM,2,2,0,A,00000000000,2*24"
+
+        with self.assertRaises(MissingMultipartMessageException) as err:
+            decode_msg(msg_1)
+        self.assertEqual(str(err.exception), "Missing fragment numbers: [2]")
+
+        with self.assertRaises(MissingMultipartMessageException) as err:
+            decode_msg(msg_2)
+        self.assertEqual(str(err.exception), "Missing fragment numbers: [1]")
+
+        with self.assertRaises(MissingMultipartMessageException) as err:
+            decode_msg(
+                "!AIVDM,3,2,0,A,539p4OT00000@7W3K@08ThiLE8@E:0000000001S0h9135Pl?0R0C@UDQp00,0*68",
+            )
+        self.assertEqual(str(err.exception), "Missing fragment numbers: [1, 3]")
+
+        decode_msg(
+            msg_1,
+            msg_2
         )