Python/pythonrun.c (part 2)

Source:

cpython 3.14 @ ab2d84fe1023/Python/pythonrun.c ↗

This annotation covers bytecode file execution, the interactive REPL loop, and exception display. See pythonrun_runstring_detail for PyRun_StringFlags and PyRun_FileExFlags.

Map

Lines	Symbol	Role
1-100	run_pyc_file	Read a .pyc file, unmarshal, and exec the code object
101-250	PyRun_InteractiveLoopObject	The interactive REPL loop (reads one statement at a time)
251-400	handle_system_exit	Translate SystemExit into a process exit code
401-580	print_exception	Display Traceback (most recent call last): and exception
581-700	_PyErr_Display	Low-level exception printer used by sys.excepthook

Reading

run_pyc_file

// CPython: Python/pythonrun.c:480 run_pyc_file
static PyObject *
run_pyc_file(FILE *fp, PyObject *globals, PyObject *locals,
             PyCompilerFlags *flags)
{
    /* Read and validate the .pyc header */
    unsigned long magic = PyMarshal_ReadLongFromFile(fp);
    if (magic != PyImport_GetMagicNumber()) {
        PyErr_SetString(PyExc_RuntimeError, "Bad magic number in .pyc file");
        return NULL;
    }
    /* Skip flags, mtime, source size */
    (void)PyMarshal_ReadLongFromFile(fp);  /* bit field */
    (void)PyMarshal_ReadLongFromFile(fp);  /* mtime */
    (void)PyMarshal_ReadLongFromFile(fp);  /* source size */
    /* Unmarshal the code object */
    PyObject *co = PyMarshal_ReadLastObjectFromFile(fp);
    if (!PyCode_Check(co)) {
        PyErr_SetString(PyExc_RuntimeError, ".pyc file does not contain code object");
        return NULL;
    }
    return PyEval_EvalCode(co, globals, locals);
}

The .pyc header is 16 bytes: 4-byte magic number, 4-byte bit-field (hash-based vs timestamp-based), 4-byte mtime or source hash, 4-byte source file size.

PyRun_InteractiveLoopObject

// CPython: Python/pythonrun.c:180 PyRun_InteractiveLoopObject
int
PyRun_InteractiveLoopObject(FILE *fp, PyObject *filename, PyCompilerFlags *flags)
{
    PyObject *v;
    int ret = 0;
    do {
        ret = PyRun_InteractiveOneObjectEx(fp, filename, flags);
        /* ret > 0: error; ret == -1: EOF; ret == 0: success */
        if (ret == E_EOF) { ret = 0; break; }
        if (ret == E_NOMEM) break;
    } while (ret != E_EOF);
    return ret;
}

The loop calls codeop.compile_command (via PyRun_InteractiveOneObjectEx) to collect a complete statement, then executes it. sys.ps1/sys.ps2 are used as prompts.

handle_system_exit

// CPython: Python/pythonrun.c:340 handle_system_exit
static void
handle_system_exit(void)
{
    PyObject *exc = tstate->current_exception;
    PyObject *code = ((PySystemExitObject *)exc)->code;
    if (code == NULL || code == Py_None)
        Py_Exit(0);
    if (PyLong_Check(code))
        Py_Exit((int)PyLong_AsLong(code));
    /* Non-integer: print to stderr, exit with code 1 */
    PyObject_Print(code, stderr, Py_PRINT_RAW);
    Py_Exit(1);
}

SystemExit(0) exits cleanly. SystemExit("message") prints the message to stderr and exits with code 1. SystemExit(42) exits with code 42.

print_exception

// CPython: Python/pythonrun.c:560 print_exception
static void
print_exception(PyObject *f, PyObject *value)
{
    /* Print "ExcType: message" to f.
       If value has __notes__, print each note after. */
    ...
    PyObject *type = PyObject_Type(value);
    const char *dot = strrchr(PyUnicode_AsUTF8(modname), '.');
    ...
    PyFile_WriteObject(type_str, f, Py_PRINT_RAW);
    PyFile_WriteString(": ", f);
    PyFile_WriteObject(value, f, Py_PRINT_RAW);
    ...
    /* Print __notes__ (PEP 678) */
    PyObject *notes = PyObject_GetAttr(value, &_Py_ID(__notes__));
    if (notes != NULL) {
        for each note: PyFile_WriteObject(note, f, Py_PRINT_RAW);
    }
}

__notes__ (added in Python 3.11, PEP 678) are extra strings attached to an exception after raising. e.add_note("hint") appends to e.__notes__.

gopy notes

run_pyc_file is pythonrun.RunPycFile in pythonrun/runpyc.go. PyRun_InteractiveLoopObject is pythonrun.RunInteractiveLoop. handle_system_exit is vm.HandleSystemExit. print_exception is pythonrun.PrintException, which uses objects.WriteObject.