Python/ceval.c (part 94)

Source:

cpython 3.14 @ ab2d84fe1023/Python/ceval.c ↗

This annotation covers context manager opcodes. See python_ceval93_detail for MAKE_FUNCTION and SET_FUNCTION_ATTRIBUTE.

Map

Lines	Symbol	Role
1-80	BEFORE_WITH	Call __enter__, push __exit__ on stack
81-160	WITH_EXCEPT_START	On exception: call __exit__ with exc info
161-240	CLEANUP_THROW	Clean up when throw() enters a with block
241-340	POP_BLOCK	Pop the exception table block after the with body
341-400	Stack layout during with	What's on the stack at each point

Reading

BEFORE_WITH

// CPython: Python/ceval.c:6040 BEFORE_WITH
inst(BEFORE_WITH, (mgr -- exit, value)) {
    /* Call __enter__ on the context manager */
    PyObject *enter = _PyObject_LookupSpecial(mgr, &_Py_ID(__enter__));
    PyObject *exit  = _PyObject_LookupSpecial(mgr, &_Py_ID(__exit__));
    if (enter == NULL || exit == NULL) ERROR_IF(true, error);
    value = PyObject_CallNoArgs(enter);
    if (value == NULL) ERROR_IF(true, error);
    Py_DECREF(enter);
    Py_DECREF(mgr);
    /* Stack: exit, value (as bound by 'as') */
}

BEFORE_WITH looks up both __enter__ and __exit__ before calling __enter__. Keeping __exit__ on the stack ensures it is available in the exception handler even after the with variable is overwritten.

WITH_EXCEPT_START

// CPython: Python/ceval.c:6080 WITH_EXCEPT_START
inst(WITH_EXCEPT_START, (exit_func, lasti, exc --  exit_func, lasti, exc, res)) {
    /* Call exit_func(type(exc), exc, exc.__traceback__) */
    PyObject *tp = PyExceptionInstance_Class(exc);
    PyObject *tb = PyException_GetTraceback(exc);
    res = PyObject_CallFunctionObjArgs(exit_func, tp, exc, tb, NULL);
    Py_XDECREF(tb);
    ERROR_IF(res == NULL, error);
}

WITH_EXCEPT_START calls __exit__(exc_type, exc_value, traceback). If __exit__ returns a truthy value, the exception is suppressed (the PUSH_EXC_INFO/POP_EXCEPT mechanism undoes it).

Stack layout

# At BEFORE_WITH:
#   Stack: [..., mgr]
# After BEFORE_WITH:
#   Stack: [..., exit_func, value]
# After 'as x = value' assignment (STORE_NAME):
#   Stack: [..., exit_func]
# Inside with body (exception handler entry):
#   Stack: [..., exit_func, lasti, exc]  (PUSH_EXC_INFO adds lasti, exc)
# After WITH_EXCEPT_START:
#   Stack: [..., exit_func, lasti, exc, res]

The exit_func stays below the body's stack throughout, so it survives arbitrary code in the with body.

gopy notes

BEFORE_WITH is in vm/eval_simple.go and calls objects.LookupSpecial(__enter__) and objects.LookupSpecial(__exit__). WITH_EXCEPT_START calls objects.CallFunctionObjArgs(exitFunc, excType, exc, tb). CLEANUP_THROW handles throw() through a with block in generators.