The Python source code is utf-8 by default in Python 3. But it says nothing about a character encoding used to save to a file. It is locale-dependent by default.

    # string literals create str objects using utf-8 by default
    Path("filenames use their own encoding").write_text("file content encoding uses yet another encoding")

The corresponding encodings are:

- utf-8 [tokenize.open] - sys.getfilesystemencoding() [os.fsencode] - locale.getpreferredencoding() [open]