What is the difference between non local variable and global variable?

回答1

"nonlocal" means that a variable is "neither local or global", i.e, the variable is from an enclosing namespace (typically from an outer function of a nested function).

An important difference between nonlocal and global is that the a nonlocal variable must have been already bound in the enclosing namespace (otherwise an syntaxError will be raised) while a global declaration in a local scope does not require the variable is pre-bound (it will create a new binding in the global namespace if the variable is not pre-bound).

回答2

The nonlocal variables are present in a nested block. A keyword nonlocal is used and the value from the nearest enclosing block is taken. For example:

def outer():
    x = "local"
    
    def inner():
        nonlocal x
        x = "nonlocal"
        print("inner:", x)
    
    inner()
    print("outer:", x)

The output will be "nonlocal" both the times as the value of x has been changed by the inner function.