Implement messages handling within interchange

Fix #19

pydifact/segmentcollection.py

@@ -215,6 +215,10 @@ class Interchange(AbstractSegmentsContainer):
 
     Functional groups are not yet supported
 
+    Messages are supported, see get_message() and get_message(), but are
+    optional: interchange segments can be accessed without going through
+    messages.
+
     https://www.stylusstudio.com/edifact/40100/UNB_.htm
     https://www.stylusstudio.com/edifact/40100/UNZ_.htm
     """
@@ -252,6 +256,38 @@ def get_footer_segment(self) -> Segment:
             self.control_reference,
         )
 
+    def get_messages(self) -> List[Message]:
+       message = None
+       for segment in self.segments:
+           if segment.tag == 'UNH':
+               if not message:
+                   message = Message(segment.elements[0], segment.elements[1])
+               else:
+                   raise SyntaxError(
+                       "Missing UNT segment before new UNH: {}".format(segment)
+                   )
+           elif segment.tag == 'UNT':
+               if message:
+                   yield message
+               else:
+                   raise SyntaxError(
+                       'UNT segment without matching UNH: "{}"'.format(segment)
+                   )
+           else:
+               if message:
+                   message.add_segment(segment)
+
+
+    def add_message(self, message: Message) -> "Interchange":
+        segments = (
+            [message.get_header_segment()]
+            + message.segments
+            + [message.get_footer_segment()]
+        )
+        self.add_segments(i for i in segments if i is not None)
+        return self
+
+
     @classmethod
     def from_segments(
         cls, segments: list or collections.Iterable

tests/test_segmentcollection.py

@@ -125,6 +125,70 @@ def test_empty_interchange_from_str():
     )
 
 
+def test_interchange_messages():
+    i = Interchange(
+        sender='1234',
+        recipient='3333',
+        timestamp=datetime.datetime(2020,1,2,22,12),
+        control_reference='42',
+        syntax_identifier=('UNOC', 1),
+    )
+
+    m = Message(
+        reference_number='42z42',
+        identifier=('PAORES', 93, 1, 'IA'),
+    )
+
+    assert(len(list(i.get_messages())) == 0)
+
+    i.add_message(m)
+
+    assert(len(list(i.get_messages())) == 1)
+
+    assert str(i) == (
+        "UNB+UNOC:1+1234+3333+200102:2212+42'"
+        "UNH+42z42+PAORES:93:1:IA'"
+        "UNT+42z42+0'"
+        "UNZ+2+42'"
+    )
+
+
+def test_interchange_messages_from_str():
+    i = Interchange.from_str(
+        "UNB+UNOC:1+1234+3333+200102:2212+42'"
+        "UNH+42z42+PAORES:93:1:IA'"
+        "UNT+42z42+0'"
+        "UNZ+2+42'"
+    )
+    assert str(i) == (
+        "UNB+UNOC:1+1234+3333+200102:2212+42'"
+        "UNH+42z42+PAORES:93:1:IA'"
+        "UNT+42z42+0'"
+        "UNZ+2+42'"
+    )
+
+
+def test_faulty_interchange_messages():
+    i = Interchange.from_str(
+        "UNB+UNOC:1+1234+3333+200102:2212+42'"
+        "UNH+42z42+PAORES:93:1:IA'"
+        "UNH+42z42+PAORES:93:1:IA'"
+        "UNZ+2+42'"
+    )
+
+    with pytest.raises(SyntaxError):
+        list(i.get_messages())
+
+    i = Interchange.from_str(
+        "UNB+UNOC:1+1234+3333+200102:2212+42'"
+        "UNT+42z42+0'"
+        "UNZ+2+42'"
+    )
+
+    with pytest.raises(SyntaxError):
+        list(i.get_messages())
+
+
 def test_empty_message():
     m = Message(
         reference_number='42z42',