python函数如何指定默认值

>>> def student(name, age=18):

...     print('Hello, My name is ' + name + ', I am ' + str(age) + ' years old')

...

>>> student('bob')

Hello, My name is bob, I am 18 years old

>>> student('nicole')

Hello, My name is nicole, I am 18 years old

>>> student('bob', 20)

Hello, My name is bob, I am 20 years old