Maybe expose this publicly?

include/openPMD/auxiliary/JSON.hpp

@@ -61,5 +61,8 @@ namespace json
      */
     std::string
     merge(std::string const &defaultValue, std::string const &overwrite);
+
+    std::string filterByTemplate(
+        std::string const &defaultValue, std::string const &positiveMask);
 } // namespace json
 } // namespace openPMD

src/auxiliary/JSON.cpp

@@ -632,4 +632,26 @@ filterByTemplate(nlohmann::json &defaultVal, nlohmann::json const &positiveMask)
     } // else noop
     return defaultVal;
 }
+
+std::string filterByTemplate(
+    std::string const &defaultValue, std::string const &positiveMask)
+{
+    auto [res, returnFormat] =
+        parseOptions(defaultValue, /* considerFiles = */ false);
+    filterByTemplate(
+        res, parseOptions(positiveMask, /* considerFiles = */ false).config);
+    switch (returnFormat)
+    {
+    case SupportedLanguages::JSON:
+        return res.dump();
+        break;
+    case SupportedLanguages::TOML: {
+        auto asToml = json::jsonToToml(res);
+        std::stringstream sstream;
+        sstream << asToml;
+        return sstream.str();
+    }
+    }
+    throw std::runtime_error("Unreachable!");
+}
 } // namespace openPMD::json

src/binding/python/Series.cpp

@@ -434,4 +434,9 @@ users to overwrite default options, while keeping any other ones.
                  If `defaultValue` was a JSON dataset, then as a JSON string,
                  otherwise as a TOML string.
         )END");
+    m.def(
+        "filter_json_by_template",
+        &json::filterByTemplate,
+        py::arg("default_value") = "{}",
+        py::arg("positive_mask") = "{}");
 }