Allow registering field subclasses.

This will allow sub-classing fields and allow marshmallow_jsonschema to
render them as the parent class would.

marshmallow_jsonschema/base.py

@@ -80,6 +80,15 @@
 class JSONSchema(Schema):
     """Converts to JSONSchema as defined by http://json-schema.org/."""
 
+    JSONSchemaTypesMapping = {
+        fields.Email: text_type,
+        fields.Dict: dict,
+        fields.List: list,
+        fields.Url: text_type,
+        fields.LocalDateTime: datetime.datetime,
+        fields.Nested: '_from_nested_schema'
+    }
+
     properties = fields.Method('get_properties')
     type = fields.Constant('object')
     required = fields.Method('get_required')
@@ -93,19 +102,16 @@ def __init__(self, *args, **kwargs):
     def _get_default_mapping(self, obj):
         """Return default mapping if there are no special needs."""
         mapping = {v: k for k, v in obj.TYPE_MAPPING.items()}
-        mapping.update({
-            fields.Email: text_type,
-            fields.Dict: dict,
-            fields.Url: text_type,
-            fields.List: list,
-            fields.LocalDateTime: datetime.datetime,
-            fields.Nested: '_from_nested_schema',
-        })
+        for field, field_type in self.JSONSchemaTypesMapping.items():
+            mapping[field] = field_type
         return mapping
 
+    @classmethod
+    def register_jsonschema_field(cls, field, field_type):
+        cls.JSONSchemaTypesMapping[field] = field_type
+
     def get_properties(self, obj):
         """Fill out properties field."""
-        mapping = self._get_default_mapping(obj)
         properties = {}
 
         for field_name, field in sorted(obj.fields.items()):

tests/test_dump.py

@@ -1,4 +1,5 @@
 from marshmallow import Schema, fields, validate
+from marshmallow.compat import text_type
 from marshmallow_jsonschema import JSONSchema
 from jsonschema import Draft4Validator
 import pytest
@@ -266,6 +267,7 @@ class TestSchema(Schema):
         'title'
     ] == 'Brown Cowzz'
 
+
 def test_unknown_typed_field_throws_valueerror():
 
     class Invalid(fields.Field):
@@ -280,6 +282,7 @@ class UserSchema(Schema):
     with pytest.raises(ValueError):
         json_schema.dump(schema).data
 
+
 def test_unknown_typed_field():
 
     class Colour(fields.Field):
@@ -340,3 +343,31 @@ class TestSchema(Schema):
         'type': 'string',
         'description': 'Directly on the field!',
     }
+
+
+def test_custom_field_by_subclassing():
+
+    class Colour(fields.String):
+        def __init__(self, default='red', **kwargs):
+            super(Colour, self).__init__(default=default, **kwargs)
+
+    class UserSchema(Schema):
+        name = fields.String(required=True)
+        favourite_colour = Colour()
+
+    schema = UserSchema()
+    json_schema = JSONSchema()
+
+    with pytest.raises(ValueError):
+        # The custom Color field is not registered
+        dumped = json_schema.dump(schema)
+
+    # Register the field
+    JSONSchema.register_jsonschema_field(Colour, text_type)
+    json_schema = JSONSchema()
+    dumped = json_schema.dump(schema).data
+    try:
+        assert dumped['definitions']['UserSchema']['properties']['favourite_colour'] == {'default': 'red', 'title': 'favourite_colour', 'type': 'string'}
+    finally:
+        # Reset the default mapping dictionary
+        JSONSchema.JSONSchemaTypesMapping.pop(Colour, None)