The default program name is hardcoded in Python/initconfig.c:config_init_program_name. It is "python" on Windows, and "python3" elsewhere. The documentation currently suggests that it is "python" everywhere.

Additionally, the documentation currently says:

> The argument should point to a zero-terminated wide character string in static storage whose contents will not change for the duration of the program’s execution.

The code, however, duplicates the string, so I am not sure this is true.