在Python中什么是全局声明？

Question

什么是全球声明？它是如何使用的？我读过Python的官方定义 ;
但是,它对我来说没有多大意义.

Answer 1

python中的每个"变量"都限于某个范围.python"file"的范围是模块范围.考虑以下:

#file test.py
myvariable = 5  # myvariable has module-level scope

def func():
    x = 3       # x has "local" or function level scope.

具有局部作用域的对象在函数退出后立即死亡,并且永远无法检索(除非您return这些对象),但在函数内,您可以访问模块级作用域(或任何包含作用域)中的变量:

myvariable = 5
def func():
    print(myvariable)  # prints 5

def func2():
    x = 3
    def func3():
        print(x)       # will print 3 because it picks it up from `func2`'s scope

    func3()

但是,您不能在该引用上使用赋值,并期望它将传播到外部作用域:

myvariable = 5
def func():
    myvariable = 6     # creates a new "local" variable.  
                       # Doesn't affect the global version
    print(myvariable)  # prints 6

func()
print(myvariable)      # prints 5

现在,我们终于到了global.该global关键字是你告诉你的功能的特定变量是在全局(模块级)范围内定义蟒蛇的方式.

myvariable = 5
def func():
    global myvariable
    myvariable = 6    # changes `myvariable` at the global scope
    print(myvariable) # prints 6

func()
print(myvariable)  # prints 6 now because we were able 
                   # to modify the reference in the function

换句话说,如果使用关键字,则可以myvariable从内部更改模块范围中的值.funcglobal

另外,范围可以任意嵌套:

def func1():
    x = 3
    def func2():
        print("x=",x,"func2")
        y = 4
        def func3():
            nonlocal x  # try it with nonlocal commented out as well.  See the difference.
            print("x=",x,"func3")
            print("y=",y,"func3")
            z = 5
            print("z=",z,"func3")
            x = 10

        func3()

    func2()
    print("x=",x,"func1")

func1()

现在在这种情况下,没有变量在全局范围内声明,而在python2中,没有(简单/干净)方法来改变x范围func1内的值func3.这就是nonlocalpython3.x中引入关键字的原因. nonlocal是一个扩展,global它允许您修改从另一个范围中提取的变量,无论它从哪个范围中提取.