config: jinja2 "tojson" template filter to use custom JSONEncoder class

This is similar to renderers.py, unfortunately it isn't really possible to share code here.

This is because the JSON renderer uses adapters through add_adapter, while the jinja2 template tag needs a custom JSONEncoder class instead.

You cannot pass a default= argument when constructing the JSON renderer constructor, as it already does this internally, resulting in an error that default= is listed twice.

Closes #56

src/sambal/__init__.py

@@ -7,6 +7,7 @@
 
 with Configurator(settings=SETTINGS) as config:
     config.include("pyramid_jinja2")
+    config.include("sambal.template")
     config.include("sambal.renderers")
     config.include("sambal.routes")
     config.include("pyramid_session_redis")

src/sambal/template.py

@@ -0,0 +1,57 @@
+import json
+from datetime import datetime
+from decimal import Decimal
+from enum import Enum
+
+from ldb import Dn, MessageElement, Result
+from samba.dcerpc.security import descriptor
+
+
+def includeme(config):
+    """Configure the Jinja2 template environment.
+
+    Register any custom template filters and custom tags here.
+    """
+
+    def setup_jinja2_env():
+        """Configure Jinja2 tojson filter to use the custom JSONEncoder."""
+        env = config.get_jinja2_environment()
+
+        # Jinja2 cannot use adapters so use a custom JSONEncoder instead.
+        env.policies["json.dumps_kwargs"].update(
+            {
+                "cls": JSONEncoder,
+                "indent": 2,
+                "sort_keys": True,
+            }
+        )
+
+    config.action(None, setup_jinja2_env, order=999)
+
+
+class JSONEncoder(json.JSONEncoder):
+    """Custom JSON encoder to deal with special datatypes.
+
+    This feels like repetition from renderers.py, but with pyramid_jinja2
+    you can only replace the JSONEncoder, while with the Pyramid JSON
+    renderer you add adapters instead.
+
+    Trying to get the Pyramid JSON renderer to use a different encoder
+    class leads to nothing but trouble.
+    """
+
+    def default(self, obj):
+        if isinstance(obj, (Decimal, Dn, MessageElement)):
+            return str(obj)
+        if isinstance(obj, Result):
+            return obj.msgs
+        elif isinstance(obj, Enum):
+            return str(obj.value)
+        elif isinstance(obj, datetime):
+            return obj.isoformat()
+        elif isinstance(obj, descriptor):
+            return obj.as_sddl()
+        elif getattr(obj, "__json__", None) and callable(obj.__json__):
+            # The request can be retrieved by thread-local but only if needed.
+            return obj.__json__(request=None)
+        return super().default(obj)