Python自定义函数定义默认值与给定全局变量用哪个

#自定义函数参数需将未给定值的变量放在前面，给定变量的值放在后面
def func(b,a='red'):
    c = a + b
    print (c)
    print("a:",a,
                "b:",b,
                "c:",c)

a = 'yell'
b = 'ow'
func(a,b)#3

zbx@zbxpc:~$ /usr/bin/python3 /home/zbx/桌面/mofan_python.py
owyell
a: ow b: yell c: owyell