Handle nullable fields properly

The thing description is invalid if we have fields with `allow_none`
enabled.  That's because JSONSchema permits `type` to be a list, which
is the sensible way to represent a field that may be `null` or a value.

I've modified our marshmallow_jsonschema library to
expand this into one duplicated sub-schema for each type in the
list, combined using `oneOf`.  This is allowed by the thing description
(and, as far as I know, is valid JSON Schema as well).

Author: rwb27

src/labthings/json/marshmallow_jsonschema/base.py

@@ -1,6 +1,7 @@
 import datetime
 import decimal
 import uuid
+from copy import deepcopy
 from inspect import isclass
 
 from marshmallow import Schema, fields, missing, validate
@@ -68,6 +69,26 @@
 }
 
 
+def convert_type_list_to_oneof(schema):
+    """Avoid setting `type` to a list, using `oneOf`
+
+    JSONSchema allows datatypes to be specified as lists, which
+    means the type may be any of the specified values.  This is
+    not permitted by W3C Thing Description.  This function will
+    use the oneOf mechanism in JSONSchema to convert schemas
+    that use a list of types into a series of schemas each with
+    one type.  Those schemas are then nested together using
+    OneOf.
+    """
+    if "type" not in schema or type(schema["type"]) != list:
+        return schema
+    subschemas = []
+    for t in schema["type"]:
+        subschemas.append(deepcopy(schema))
+        subschemas[-1]["type"] = t
+    return {"oneOf": subschemas}
+
+
 class JSONSchema(Schema):
     """Converts to JSONSchema as defined by http://json-schema.org/."""
 
@@ -202,6 +223,8 @@ def _get_schema_for_field(self, obj, field):
                 )
                 if base_class is not None and base_class in FIELD_VALIDATORS:
                     schema = FIELD_VALIDATORS[base_class](schema, field, validator, obj)
+        if "type" in schema and type(schema["type"]) == list:
+            schema = convert_type_list_to_oneof(schema)
         return schema
 
     def _from_nested_schema(self, _, field):

tests/test_json_marshmallow_jsonschema.py

@@ -289,18 +289,23 @@ class TestSchema(Schema):
 
 
 def test_allow_none():
-    """A field with allow_none set to True should have type null as additional."""
+    """A field with allow_none set to True should be able to be null.
+
+    Note that this has been modified from JSONSchema behaviour: in
+    JSONSchema, "type" should be set to `["string", "null"]`.  This
+    is not allowed in Thing Descriptions, so instead we need to use
+    `oneOf`.  Our modified JSON schema library does this."""
 
     class TestSchema(Schema):
         id = fields.Integer(required=True)
-        readonly_fld = fields.String(allow_none=True)
+        nullable_fld = fields.String(allow_none=True)
 
     schema = TestSchema()
 
     dumped = validate_and_dump(schema)
 
-    assert dumped["properties"]["readonly_fld"] == {
-        "type": ["string", "null"],
+    assert dumped["properties"]["nullable_fld"] == {
+        "oneOf": [{"type": "string"}, {"type": "null"}],
     }