python中不会更改sub函数的全局变量

# 定义一个全局变量
global_var = 10

def sub():
    # 使用global关键字声明要修改的是全局变量
    global global_var
    # 修改全局变量的值
    global_var -= 5
    print("Inside function, global_var is:", global_var)

print("Before function call, global_var is:", global_var)
sub()
print("After function call, global_var is:", global_var)

Before function call, global_var is: 10
Inside function, global_var is: 5
After function call, global_var is: 5

global_var = 10

def sub():
    # 没有使用global关键字，Python会认为global_var是局部变量
    global_var -= 5  # UnboundLocalError: local variable 'global_var' referenced before assignment
    print("Inside function, global_var is:", global_var)

sub()