Serialize object with umlauts to json file with Marshmallow

I want to serialize a pyhthon object to json. The object has some strings with umlauts in it. I use Marshmallow for the Schema. Below a code snippet.

from marshmallow import Schema, fields, post_load, post_dump
import logging as log
import os.path
log.basicConfig(level=log.DEBUG)

class Umlaut():
    def __init__(self, name): self.name = name

class UmlautSchema(Schema):
    name = fields.Str()
    
    @post_dump
    def post_dump(self, data, many=False):
        log.debug(data) # Umlauts are fine
        return data

class Filehandling():
    def write(self, u):
        pathToFile = os.path.abspath("/tmp/")
        schema = UmlautSchema()
        res = schema.dumps(u)
        log.debug(res)  # Here we have 'Umlaut-\u00f6-\u00e4-\u00fc-\u00c4-\u00d6-\u00dc'
        file = os.path.join(pathToFile, "umlauts.json")
        with open(file, mode="w", encoding="UTF-8") as outfile:
            outfile.write(res)
            outfile.close()

def run():
    filehandling = Filehandling()
    u = Umlaut("Umlaut-ö-ä-ü-Ä-Ö-Ü")
    log.debug(u.name) # Umlauts are fine
    filehandling.write(u)

When I run my code the umlauts in the json file looks like \u00f6. What can I do to get the umlauts properly encoded like 'ä, ö, ü'?

When you look at the log statements in the post_dump method the umlauts are ok but when

Thanks for your help Stephan

According to the docs additional kwargs are passed to the configured JSON renderer, by default json from Python itself is used (ref).

Using your classes with ensure_ascii=False:

>>> schema.dumps(u, ensure_ascii=False)
'{"name": "Umlaut-ö-ä-ü-Ä-Ö-Ü"}'

For other renderers you have to check their documentation.