为什么在全局范围内打印变量有效，但是修改它不起作用？

x = 1
def fn():
    print x
fn()

This prints "1":

这打印“1”：

x = 1
def fn():
    x += 1
    print x
fn()

This raises "UnboundLocalError: local variable 'x' referenced before assignment"

这会引发“UnboundLocalError：赋值前引用的局部变量'x'”

What is going on here?

这里发生了什么？

4 个解决方案

#1

In Python, assigning to a variable is also an implicit local declaration, that is resolved during the bytecode compilation. So

在Python中，分配给变量也是一个隐式本地声明，在字节码编译期间解析。所以

x += 1

will create a local variable x and compile to this byte code:

将创建一个局部变量x并编译为此字节代码：

0 LOAD_FAST                0 (x)
3 LOAD_CONST               1 (1)
6 INPLACE_ADD
7 STORE_FAST               0 (x)

The command LOAD_FAST will try to load a local variable x which is not yet defined, that's why it fails.

命令LOAD_FAST将尝试加载尚未定义的局部变量x，这就是失败的原因。

However, if you define x as global explicitly, then it will use LOAD_GLOBAL/STORE_GLOBAL instead.

但是，如果将x定义为全局显式，则它将使用LOAD_GLOBAL / STORE_GLOBAL。

In the case of print in your first function, the compiler assumes that since no local variable is declared (assigned) ever in the function body, you should mean a global variable.

在第一个函数中打印的情况下，编译器假定由于函数体中没有声明（赋值）局部变量，因此您应该表示全局变量。

#2

The act of assigning to a name, anywhere inside a function, makes that name local only.

在函数内的任何位置分配名称的行为仅使该名称为本地名称。

#3

You're using a local variable , a different binding, not the global one.

您使用的是局部变量，不同的绑定，而不是全局变量。

A scope defines the visibility of a name within a block. If a local variable is defined in a block, its scope includes that block. If the definition occurs in a function block, the scope extends to any blocks contained within the defining one, unless a contained block introduces a different binding for the name

范围定义块中名称的可见性。如果在块中定义了局部变量，则其范围包括该块。如果定义发生在功能块中，则范围将扩展到定义块中包含的任何块，除非包含的块为名称引入了不同的绑定

#4

Because you have to add global x to the function:

因为您必须向函数添加全局x：

def fn():
    global x
    x += 1
    print x

This tells the interpreter that you intend to modify a global variable. This is not necessary for certain objects like mutable sequences (e.g. list).

这告诉解释器您打算修改全局变量。对于像可变序列（例如列表）这样的某些对象，这不是必需的。

#1