Fix deprecation warning for flask json_encoder (#1812)

* Fix deprecation warning for flask json_encoder

* binary bitwise or operator not supported in type hints in 3.8

---------

Co-authored-by: Are Schjetne <are.schjetne@basefarm-orange.com>

alerta/app.py

@@ -74,8 +74,9 @@ def create_app(config_override: Dict[str, Any] = None, environment: str = None)
     plugins.register(app)
     custom_webhooks.register(app)
 
-    from alerta.utils.format import CustomJSONEncoder
-    app.json_encoder = CustomJSONEncoder
+    from alerta.utils.format import AlertaJsonProvider
+    app.json_provider_class = AlertaJsonProvider
+    app.json = AlertaJsonProvider(app)
 
     from alerta.views import api
     app.register_blueprint(api)

alerta/utils/format.py

@@ -1,12 +1,29 @@
 import datetime
+import json
 import traceback
-from typing import Any, Optional
+from decimal import Decimal
+from typing import Any, Optional, Union
 
-from flask import json
+from flask.json.provider import JSONProvider
 
 dt = datetime.datetime
 
 
+class AlertaJsonProvider(JSONProvider):
+    """JSON Provider for Flask app to use CustomJSONEncoder."""
+
+    ensure_ascii: bool = True
+    sort_keys: bool = True
+
+    def dumps(self, obj, **kwargs):
+        kwargs.setdefault('ensure_ascii', self.ensure_ascii)
+        kwargs.setdefault('sort_keys', self.sort_keys)
+        return json.dumps(obj, **kwargs, cls=CustomJSONEncoder)
+
+    def loads(self, s: Union[str, bytes], **kwargs):
+        return json.loads(s, **kwargs)
+
+
 class CustomJSONEncoder(json.JSONEncoder):
     def default(self, o: Any) -> Any:  # pylint: disable=method-hidden
         from alerta.models.alert import Alert, History
@@ -23,6 +40,8 @@ class CustomJSONEncoder(json.JSONEncoder):
             return DateTime.iso8601(o)
         elif isinstance(o, datetime.timedelta):
             return int(o.total_seconds())
+        elif isinstance(o, Decimal):
+            return str(o)
         elif isinstance(o, (Alert, History)):
             return o.serialize
         elif isinstance(o, Exception):