I’d guess because CPython is assuming the input to -c is ISO-8859-1 (Latin-1) when it decodes it using Py_DecodeLocale(): main() … setlocale(LC

I’d guess because CPython is assuming the input to -c is ISO-8859-1 (Latin-1) when it decodes it using Py_DecodeLocale(): main() … setlocale(LC_ALL, "") … argv_copy[i] = Py_DecodeLocale(argv[i], NULL) … mbstowcs() or mbrtowc() … setlocale(LC_ALL, oldloc) … Py_Main(argc, argv_copy) While the REPL’s encoding (sys.stdin.encoding) is set to UTF-8 due to LANG/LC_CTYPE settings. You can get the same error when invoking the REPL as: LANG="en_US.iso8859-1" python2.7 So the shell isn’t doing anything to the text—it’s providing UTF-8 bytes in both cases, it’s just that Python is interpreting them differently.