news: add entry for #397

Signed-off-by: Attila Szakacs <attila.szakacs@axoflow.com>

news/fx-feature-397.md

@@ -0,0 +1,36 @@
+`set_fields()`: Added new function to set a dict's fields with overrides and defaults.
+
+A recurring pattern in FilterX is to take a dict and set multiple
+fields in it with overrides or defaults.
+
+`set_fields()` takes a dict as the first argument and `overrides`
+and `defaults` as optional parameters.
+
+`overrides` and `defaults` are also dicts, where the key is
+the field's name, and the value is either an expression, or
+a list of expressions. If a list is provided, each expression
+will be evaluated, and the first successful one will be used
+to set the respective field's value. This is similar to chaining
+null-coalescing (`??`) operators, but is more performant.
+
+`overrides` are always processed for each field. The `defaults`
+for a field are only processed, if the field does not already
+have a value set.
+
+Example usage:
+```
+set_fields(
+  my_dict,
+  overrides={
+    "foo": [invalid_expr, "foo_override"],
+    "baz": "baz_override",
+    "almafa": [invalid_expr_1, invalid_expr_2],
+  },
+  defaults={
+    "foo": [invalid_expr, "foo_default"],
+    "bar": "bar_default",
+    "almafa": "almafa_default",
+    "kortefa": [invalid_expr_1, invalid_expr_2],
+    }
+);
+```