Add .writer.json

sdmx/__init__.py

@@ -8,6 +8,7 @@
 from sdmx.rest import Resource
 from sdmx.source import add_source, get_source, list_sources
 from sdmx.writer.csv import to_csv
+from sdmx.writer.json import to_json
 from sdmx.writer.xml import to_xml
 
 __all__ = [
@@ -22,6 +23,7 @@
     "read_sdmx",
     "read_url",
     "to_csv",
+    "to_json",
     "to_pandas",
     "to_xml",
     "to_sdmx",

sdmx/convert/common.py

@@ -1,4 +1,5 @@
 from collections.abc import Callable
+from copy import deepcopy
 from typing import Any
 
 
@@ -46,6 +47,10 @@ class DispatchConverter(Converter):
 
     _registry: dict[type, Callable]
 
+    def __init_subclass__(cls, **kwargs) -> None:
+        super().__init_subclass__(**kwargs)
+        cls._registry = deepcopy(getattr(cls, "_registry", dict()))
+
     def convert(self, obj, **kwargs):
         # Use either type(obj) or a parent type to retrieve a conversion function
         for i, cls in enumerate(type(obj).mro()):
@@ -72,14 +77,7 @@ def register(cls, func: "Callable"):
         `func` must have an argument named `obj` that is annotated with a particular
         type.
         """
-        try:
-            registry = getattr(cls, "_registry")
-        except AttributeError:
-            # First call → registry does not exist → create it
-            registry = dict()
-            setattr(cls, "_registry", registry)
-
         # Register `func` for the class of the `obj` argument
-        registry[getattr(func, "__annotations__")["obj"]] = func
+        cls._registry[getattr(func, "__annotations__")["obj"]] = func
 
         return func

sdmx/format/json.py

@@ -0,0 +1,22 @@
+from dataclasses import dataclass
+
+from sdmx.format.common import Format
+
+
+@dataclass
+class JSONFormat(Format):
+    """Information about an SDMX-JSON format."""
+
+    suffix = "json"
+
+
+class JSON_v10(JSONFormat):
+    version = "1.0"
+
+
+class JSON_v20(JSONFormat):
+    version = "2.0.0"
+
+
+class JSON_v21(JSONFormat):
+    version = "2.1.0"

sdmx/tests/writer/conftest.py

@@ -1,6 +1,9 @@
+from datetime import datetime
+
 import pytest
 
 from sdmx import message
+from sdmx.model import common
 from sdmx.model.common import Agency, Codelist
 from sdmx.model.v21 import Annotation
 
@@ -11,6 +14,21 @@
 ]
 
 
+@pytest.fixture(scope="module")
+def agency() -> common.Agency:
+    return common.Agency(id="TEST")
+
+
+@pytest.fixture(scope="module")
+def header(agency) -> message.Header:
+    return message.Header(
+        id="N_A",
+        prepared=datetime.now(),
+        receiver=common.Agency(id="N_A"),
+        sender=agency,
+    )
+
+
 @pytest.fixture
 def codelist():
     """A Codelist for writer testing."""

sdmx/tests/writer/test_json.py

@@ -0,0 +1,32 @@
+import pytest
+
+import sdmx
+from sdmx import message
+from sdmx.format.json import JSON_v10, JSON_v20, JSON_v21, JSONFormat
+
+
+@pytest.mark.parametrize(
+    "message_class",
+    [
+        message.DataMessage,
+        message.ErrorMessage,
+        message.MetadataMessage,
+        message.StructureMessage,
+    ],
+)
+@pytest.mark.parametrize(
+    "json_format",
+    [
+        pytest.param(JSON_v10, marks=pytest.mark.xfail(raises=NotImplementedError)),
+        JSON_v20,
+        JSON_v21,
+    ],
+)
+def test_empty_message(
+    header: message.Header, message_class, json_format: JSONFormat
+) -> None:
+    msg = message_class(header=header)
+
+    result = sdmx.to_json(msg, format=json_format)
+
+    assert '{"meta": {"test": false, "id": "N_A"}}' == result

sdmx/writer/json/__init__.py

@@ -0,0 +1,18 @@
+from typing import TYPE_CHECKING
+
+from sdmx.format.json import JSON_v20, JSON_v21
+
+if TYPE_CHECKING:
+    from sdmx.message import DataMessage, StructureMessage
+
+
+def to_json(obj: "DataMessage | StructureMessage", **kwargs) -> str:
+    format = kwargs.get("format", JSON_v20)
+    if format in (JSON_v20, JSON_v21):
+        from . import v2
+
+        return v2.JSONWriter(**kwargs).convert(obj)
+    else:
+        from . import v1
+
+        return v1.JSONWriter(**kwargs).convert(obj)

sdmx/writer/json/base.py

@@ -0,0 +1,11 @@
+from typing import TYPE_CHECKING
+
+from sdmx.convert.common import DispatchConverter
+
+if TYPE_CHECKING:
+    from sdmx.format.json import JSONFormat
+
+
+class BaseJSONWriter(DispatchConverter):
+    def __init__(self, format: "JSONFormat") -> None:
+        pass

sdmx/writer/json/v1.py

@@ -0,0 +1,12 @@
+from sdmx import message
+
+from .base import BaseJSONWriter
+
+
+class JSONWriter(BaseJSONWriter):
+    pass
+
+
+@JSONWriter.register
+def _message(w: "JSONWriter", obj: message.Message):
+    raise NotImplementedError("Write SDMX-JSON 1.0")

sdmx/writer/json/v2.py

@@ -0,0 +1,43 @@
+import json
+from typing import Any
+
+from sdmx import message
+
+from .base import BaseJSONWriter
+
+
+class JSONWriter(BaseJSONWriter):
+    pass
+
+
+@JSONWriter.register
+def _data_message(w: "JSONWriter", obj: message.DataMessage):
+    result = {"meta": w.convert(obj.header)}
+    return json.dumps(result)
+
+
+@JSONWriter.register
+def _error_message(w: "JSONWriter", obj: message.ErrorMessage):
+    result = {"meta": w.convert(obj.header)}
+    return json.dumps(result)
+
+
+@JSONWriter.register
+def _metadata_message(w: "JSONWriter", obj: message.MetadataMessage):
+    result = {"meta": w.convert(obj.header)}
+    return json.dumps(result)
+
+
+@JSONWriter.register
+def _structure_message(w: "JSONWriter", obj: message.StructureMessage):
+    result = {"meta": w.convert(obj.header)}
+    return json.dumps(result)
+
+
+@JSONWriter.register
+def _header(w: "JSONWriter", obj: message.Header):
+    result: dict[str, Any] = {"test": obj.test}
+    if obj.id:
+        result.update(id=obj.id)
+
+    return result