读 Python behind the scenes #5: how variables are implemented in CPython

a = b

// "2 STORE_NAME               1 (a)"
case TARGET(STORE_NAME): {
    // 1. 通过字节码的参数 oparg（index 位置），获取变量名（string 对象）
    PyObject *name = GETITEM(names, oparg);
    // 2. 从 value stack 中 pop 一个值（python 对象的引用）
    PyObject *v = POP();
    PyObject *ns = f->f_locals;
    int err;
    if (ns == NULL) {
        _PyErr_Format(tstate, PyExc_SystemError,
                      "no locals found when storing %R", name);
        Py_DECREF(v);
        goto error;
    }
    // 3. 关联 `*name` & `*v`
    // P.S. 通过 frame object 中的 f_locals 变量维护 mapping 关系 
    if (PyDict_CheckExact(ns))
        err = PyDict_SetItem(ns, name, v);
    else
        err = PyObject_SetItem(ns, name, v);
    Py_DECREF(v);
    if (err != 0)
        goto error;
    DISPATCH();
}

case TARGET(LOAD_NAME): {
    // 1. 同上获取变量名
    PyObject *name = GETITEM(names, oparg);
    PyObject *locals = f->f_locals;
    PyObject *v;

    if (locals == NULL) {
        _PyErr_Format(tstate, PyExc_SystemError,
                        "no locals when loading %R", name);
        goto error;
    }

    // 2. 首先在 f -> f_locals 中查找变量值
    // p.s. f 是一个 frame
    // f = _PyFrame_New_NoTrack(tstate, co, globals, locals);
    if (PyDict_CheckExact(locals)) {
        v = PyDict_GetItemWithError(locals, name);
        if (v != NULL) {
            Py_INCREF(v);
        }
        else if (_PyErr_Occurred(tstate)) {
            goto error;
        }
    }
    else {
        v = PyObject_GetItem(locals, name);
        if (v == NULL) {
            if (!_PyErr_ExceptionMatches(tstate, PyExc_KeyError))
                goto error;
            _PyErr_Clear(tstate);
        }
    }

    // 3. 如果不存在，继续查找：
    // 3.1 `f->f_globals`：全局变量
    // 3.2 `f->f_builtins`：内置类型/方法/异常与常量，例如 int, next, ValueError and None
    if (v == NULL) {
        v = PyDict_GetItemWithError(f->f_globals, name);
        if (v != NULL) {
            Py_INCREF(v);
        }
        else if (_PyErr_Occurred(tstate)) {
            goto error;
        }
        else {
            if (PyDict_CheckExact(f->f_builtins)) {
                v = PyDict_GetItemWithError(f->f_builtins, name);
                if (v == NULL) {
                    // 4. 如果找不到，抛出 `NameError` 异常
                    if (!_PyErr_Occurred(tstate)) {
                        format_exc_check_arg(
                                tstate, PyExc_NameError,
                                NAME_ERROR_MSG, name);
                    }
                    goto error;
                }
                Py_INCREF(v);
            }
            else {
                v = PyObject_GetItem(f->f_builtins, name);
                if (v == NULL) {
                    if (_PyErr_ExceptionMatches(tstate, PyExc_KeyError)) {
                        format_exc_check_arg(
                                    tstate, PyExc_NameError,
                                    NAME_ERROR_MSG, name);
                    }
                    goto error;
                }
            }
        }
    }
    // 4. 如果找到了，将值 push 到 stack 中
    PUSH(v);
    DISPATCH();
}

x = 1

def f(y, z):
    def _():
        return z

    return x + y + z

def f():
    x = 1      # STORE_FAST x
    return x   # LOAD_FAST x

# 存储位置：
# frame.f_localsplus[i] -> x

def outer():
    x = 1

    def inner():
        nonlocal x
        x = x + 1     # LOAD_DEREF + STORE_DEREF
        return x      # LOAD_DEREF

    return inner

# 存储位置：同一个 cell object（x）：
# - outer frame object -> f_localsplus -> cell
# - inner function object -> __closure__ -> same cell

x = 1

def f():
    global x
    x = 2      # STORE_GLOBAL
    return x   # LOAD_GLOBAL

# 存储位置：
# frame.f_globals["x"] -> frame.f_builtins

x = 10

class A:
    y = x          # LOAD_NAME x
    z = len([])    # LOAD_NAME len

# 存储位置：
# frame.f_locals["len"] -> f_globals["len"] -> f_builtins["len"]

class D:
    x = 1             # STORE_NAME x（存进 class body 的 f_locals）
    def method(self):
        print(x)      # LOAD_GLOBAL x -> NameError: name 'x' is not defined

D().method()

def outer():
    x = 1             # STORE_DEREF

    class C:
        print(x)      # LOAD_CLASSDEREF

# 查找位置：
# class frame.f_locals["x"] 
#   -> C.__closure__
#     -> outter.cell -> value