What encoding does open() use by default?

The default UTF-8 encoding of Python 3 only extends to conversions between bytes and str types. open() instead chooses an appropriate default encoding based on the environment:

encoding is the name of the encoding used to decode or encode the file. This should only be used in text mode. The default encoding is platform dependent (whatever locale.getpreferredencoding() returns), but any text encoding supported by Python can be used. See the codecs module for the list of supported encodings.

For example, a Windows machine with a Western Europe/North America locale will normally use the 8-bit Windows-1252 character set (Python calls this encoding 'cp1252').