Python/initconfig.c: Interpreter Configuration

Python/initconfig.c owns the full lifecycle of interpreter startup configuration. It defines the PyConfig struct, reads configuration from the environment and command line, and hands a validated config object to Py_InitializeFromConfig.

Map

Lines	Symbol	Role
1-120	PyConfig struct layout	All public config fields: home, pythonpath_env, program_name, argv, sys_path_0, etc.
121-400	PyConfig_InitPythonConfig / PyConfig_InitIsolatedConfig	Zero-initialize then fill safe defaults
401-900	_PyConfig_Read	Priority chain: cmdline overrides env, env overrides defaults
901-1400	config_read_env_vars	Reads PYTHONPATH, PYTHONHOME, PYTHONSTARTUP, PYTHONUTF8, etc.
1401-1900	config_init_argv / config_compute_sys_path	Derives sys.argv and sys.path[0] from program_name and argv
1901-2400	_PyConfig_Write	Pushes finalized config into the interpreter state
2401-2700	Py_InitializeFromConfig	Top-level entry: calls _PyConfig_Read, then _Py_InitializeMain
2701-3000	WASM/WASI guards	Platform-specific overrides added in 3.13-3.14 for emscripten and wasi targets

Reading

PyConfig field priority

_PyConfig_Read applies three passes in fixed order: command-line flags first, then environment variables, then compiled-in defaults. Later passes never overwrite a field that was already set by an earlier pass.

/* Python/initconfig.c ~450 */
static PyStatus
config_read(PyConfig *config, int compute_path_config)
{
    PyStatus status;
    const PyPreConfig *preconfig = &_PyRuntime.preconfig;

    if (config->use_environment) {
        status = config_read_env_vars(config);
        if (_PyStatus_EXCEPTION(status)) return status;
    }
    status = config_read_cmdline(config);
    if (_PyStatus_EXCEPTION(status)) return status;

    status = config_init_defaults(config);
    ...
}

The key invariant: each setter checks if (config->field == NULL) (for strings) or a sentinel integer before writing, so the first writer wins.

program_name and home resolution

program_name is resolved through Py_GetProgramFullPath logic: the runtime searches PATH entries and falls back to the raw argv[0] string. home (equivalent to PYTHONHOME) short-circuits the entire path search and pins both prefix and exec_prefix.

/* Python/initconfig.c ~1050 */
static PyStatus
config_init_home(PyConfig *config)
{
    wchar_t *home = NULL;
    PyStatus status = CONFIG_GET_ENV_DUP(config, &home,
                                         L"PYTHONHOME", "PYTHONHOME");
    if (_PyStatus_EXCEPTION(status)) return status;
    return PyConfig_SetString(config, &config->home, home);
}

3.14 WASM/WASI additions

CPython 3.13 introduced Py_PORTING_WASM guards; 3.14 extends them with WASI reactor-mode support. When __wasi__ is defined and the process has no real argv, config_compute_sys_path skips the usual file-system probing and returns an empty sys.path.

/* Python/initconfig.c ~2730 */
#ifdef __wasi__
    /* WASI reactor mode: no argv[0], no cwd-relative path */
    if (config->argv.length == 0) {
        return _PyWstrList_Append(&config->sys_path_0_calculated,
                                  L"");
    }
#endif

gopy notes

• pythonrun/runstring.go contains a minimal inline config that hardcodes isolated=true and empty argv. A full port of _PyConfig_Read is not yet done; the priority chain is approximated by direct struct assignment.
• The home field is unused in gopy today. PYTHONHOME is never read.
• WASM/WASI guards are out of scope for the current port target (native Go binaries only).
• Task #474 tracks completing Py_InitializeFromConfig parity so that user-supplied PyConfig values propagate correctly into the interpreter state.