import re
key = "df"
value = "TH"
line = "df,äg"
line = re.sub(ur'%s,' % key, ur'%s,' % value, line.decode('utf-8'), flags=re.UNICODE).encode("utf8")
print(line)
aW1wb3J0IHJlCmtleSA9ICJkZiIKdmFsdWUgPSAiVEgiCmxpbmUgPSAiZGYsw6RnIgpsaW5lID0gcmUuc3ViKHVyJyVzLCcgJSBrZXksIHVyJyVzLCcgJSB2YWx1ZSwgbGluZS5kZWNvZGUoJ3V0Zi04JyksIGZsYWdzPXJlLlVOSUNPREUpLmVuY29kZSgidXRmOCIpCnByaW50KGxpbmUp