pyln.proto.message: expose array types, add set_field for Message class.

Exposing the array types is required for our dummyrunner in the lnprototest suite, since
it wants to be able to generate fake fields.

The set_field is similarly useful.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

contrib/pyln-proto/pyln/proto/message/__init__.py

@@ -1,3 +1,4 @@
+from .array_types import SizedArrayType, DynamicArrayType, EllipsisArrayType
 from .message import MessageNamespace, MessageType, Message, SubtypeType
 from .fundamental_types import split_field, FieldType
 
@@ -10,6 +11,9 @@ __all__ = [
     "SubtypeType",
     "FieldType",
     "split_field",
+    "SizedArrayType",
+    "DynamicArrayType",
+    "EllipsisArrayType",
 
     # fundamental_types
     'byte',

contrib/pyln-proto/pyln/proto/message/message.py

@@ -545,22 +545,24 @@ class Message(object):
 
         # Convert arguments from strings to values if necessary.
         for field in kwargs:
-            f = self.messagetype.find_field(field)
-            if f is None:
-                raise ValueError("Unknown field {}".format(field))
-
-            v = kwargs[field]
-            if isinstance(v, str):
-                v, remainder = f.fieldtype.val_from_str(v)
-                if remainder != '':
-                    raise ValueError('Unexpected {} at end of initializer for {}'.format(remainder, field))
-            self.fields[field] = v
+            self.set_field(field, kwargs[field])
 
         bad_lens = self.messagetype.len_fields_bad(self.messagetype.name,
                                                    self.fields)
         if bad_lens:
             raise ValueError("Inconsistent length fields: {}".format(bad_lens))
 
+    def set_field(self, field: str, val: Any) -> None:
+        f = self.messagetype.find_field(field)
+        if f is None:
+            raise ValueError("Unknown field {}".format(field))
+
+        if isinstance(val, str):
+            val, remainder = f.fieldtype.val_from_str(val)
+            if remainder != '':
+                raise ValueError('Unexpected {} at end of initializer for {}'.format(remainder, field))
+        self.fields[field] = val
+
     def missing_fields(self) -> List[str]:
         """Are any required fields missing?"""
         missing: List[str] = []