In Python, the `nonlocal` keyword is used to declare a non-local variable within a nested function. It allows you to access and modify the variables of the outer function within the inner function.
The following is the specific usage of the `nonlocal` keyword:
1. Declare non-local variables: Use the `nonlocal` keyword in the internal function, and then specify the variable name to be declared as a non-local variable. For example:
def outer_function():
x = 10
def inner_function():
nonlocal x
x = 20
inner_function()
print(x) # 输出: 20
outer_function()
在这个例子中， `nonlocal x` 语句声明了变量 `x` 为非局部变量，并在内部函数 `inner_function` 中对其进行了赋值。
2. Modify non-local variables: Using the `nonlocal` keyword, you can modify the variables of the external function in the internal function. For example:
def outer_function():
x = 10
def inner_function():
nonlocal x
x += 5
inner_function()
print(x) # 输出: 15
outer_function()
在这个例子中， `inner_function` 函数使用 `nonlocal x` 语句来指示 `x` 是一个非局部变量，并将其值增加了5。It should be noted that the `nonlocal` keyword can only be used in nested functions, not in the global scope. It is used to solve the problem that internal functions cannot directly access variables of external functions.
Hope the above explanation is helpful to you! If you have additional questions, please feel free to ask.