Is there a simpler or more idiomatic way to write this?

```
def jsonToCsvPart1(fields):
    csvFormat = ''
    fields = json.loads(fields)
    for key in fields.keys() :
        tf = str(fields[key])
        mk = ''
        for idx in tf :
            if idx == '{' or idx == '}' or idx == '[' or idx == ']' :
                continue
            else :
               mk += idx
        csvFormat += (mk + ",")
    yield csvFormat

```