python 使用嵌套函數報local variable xxx referenced before assignment或者 local variable XXX defined in enclosing scope

情況一： a 直接引用外部的，正常運行

def toplevel():
    a = 5
    def nested():
        print(a + 2) # theres no local variable a so it prints the nonlocal one
    nested()
    return a

情況二：創建local 變量a，直接打印，正常運行

def toplevel():
    a = 5 
    def nested():
        a = 7 # create a local variable called a which is different than the nonlocal one
        print(a) # prints 7
    nested()
    print(a) # prints 5
    return a

情況三：由於存在 a = 7，此時a代表嵌套函數中的local a , 但在使用a + 2 時，a還未有定義出來，所以報錯

def toplevel():
    a = 5
    def nested():
        print(a + 2) # tries to print local variable a but its created after this line so exception is raised
        a = 7
    nested()
    return a
toplevel()

針對情況三的解決方法, 在嵌套函數中增加nonlocal a ,代表a專指外部變量即可

def toplevel():
    a = 5
    def nested():
        nonlocal a
        print(a + 2)
        a = 7
    nested()
    return a