I created the function fill_default packaged in jsonschema-fill-default which fills all missing defaults in an existing instance with its schema:

pip install jsonschema-fill-default

from jsonschema_fill_default import fill_default

schema = {
    "type": "object",
    "properties": {
        "key_1": {},
        "key_2": {
            "type": "string",
            "default": "do_not_overwrite_if_key_exists",
        },
        "key_3": {
            "type": "string",
            "default": "use_it_if_key_does_not_exist",
        },
    },
    "required": ["key_1"],
}

json_dict = {"key_1": "key_1_value", "key_2": "key_2_value"}

fill_default(json_dict, schema)  # Mutates instance!

>>> json_dict

{"key_1": "key_1_value", "key_2": "key_2_value", "key_3": "use_it_if_key_does_not_exist"}

The filling is done recursively for all missing and nested defaults. It works with all combinations of "properties", "allOf", "anyOf", "oneOf", "dependentSchemas", "if-then(-else)", "prefixItems", and "items" keywords.

Note:

• The instance must already be valid to its schema.
• The schema itself must be a valid Draft 2020-12 JSON Schema.
• The filled instance is not automatically validated.

See Examples on the project's GitHub for details.

I created this because the need shows up in discussions online, along with there being a gap in a solution that works with more than just the "properties" keyword.