python - Local variable referenced before assignment

最新推荐文章于 2022-02-24 22:26:50 发布

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本文通过具体示例探讨了Python中变量的作用域问题，特别是局部变量在定义前被引用所引发的UnboundLocalError错误，并深入分析了CPython源码层面的错误处理机制。

直接上代码

def foo():
	a = 1
	def bar():
		a = a + 1
	return bar()

运行foo()，报错

import dis
def foo():
	a = 1
	def bar():
		a = a + 1
	dis.dis(bar)
	return bar()

dis打印的内容如下：

很容易明白其意思，,应该是从某个地方加载a，1，执行加法，写回a；

在CPython源码中，搜索“referenced before assignment”

#define NAME_ERROR_MSG \
    "name '%.200s' is not defined"
#define GLOBAL_NAME_ERROR_MSG \
    "global name '%.200s' is not defined"
#define UNBOUNDLOCAL_ERROR_MSG \
    "local variable '%.200s' referenced before assignment"
#define UNBOUNDFREE_ERROR_MSG \
    "free variable '%.200s' referenced before assignment" \
    " in enclosing scope"

接着搜索UNBOUNDLOCAL_ERROR_MSG

TARGET(LOAD_FAST)
    x = GETLOCAL(oparg);
    if (x != NULL) {
        Py_INCREF(x);
        PUSH(x);
        FAST_DISPATCH();
    }
    format_exc_check_arg(PyExc_UnboundLocalError,
        UNBOUNDLOCAL_ERROR_MSG,
        PyTuple_GetItem(co->co_varnames, oparg));
    break;

TARGET(DELETE_FAST)
    x = GETLOCAL(oparg);
    if (x != NULL) {
        SETLOCAL(oparg, NULL);
        DISPATCH();
    }
    format_exc_check_arg(
        PyExc_UnboundLocalError,
        UNBOUNDLOCAL_ERROR_MSG,
        PyTuple_GetItem(co->co_varnames, oparg)
        );
    break;

static void
format_exc_unbound(PyCodeObject *co, int oparg)
{
    PyObject *name;
    /* Don't stomp existing exception */
    if (PyErr_Occurred())
        return;
    if (oparg < PyTuple_GET_SIZE(co->co_cellvars)) {
        name = PyTuple_GET_ITEM(co->co_cellvars,
                                oparg);
        format_exc_check_arg(
            PyExc_UnboundLocalError,
            UNBOUNDLOCAL_ERROR_MSG,
            name);
    } else {
        name = PyTuple_GET_ITEM(co->co_freevars, oparg -
                                PyTuple_GET_SIZE(co->co_cellvars));
        format_exc_check_arg(PyExc_NameError,
                             UNBOUNDFREE_ERROR_MSG, name);
    }
}

初步判断是在执行LOAD_FAST时，报出的错误；

如果能够调试就好了，以后试试吧；

顺便提一下，CpythonDoc目录下有这样的解释

>>> x = 10
>>> def foo():
... print(x)
... x += 1

results in an UnboundLocalError:

>>> foo()
Traceback (most recent call last):
...
UnboundLocalError: local variable 'x' referenced before assignment

This is because when you make an assignment to a variable in a scope, that
variable becomes local to that scope and shadows any similarly named variable
in the outer scope. Since the last statement in foo assigns a new value to
``x``, the compiler recognizes it as a local variable. Consequently when the
earlier ``print(x)`` attempts to print the uninitialized local variable and
an error results.