理解First-Class Functions

def logger(msg):

    def log_message():
        print('Log:', msg)

    return log_message # 返回的是函数

log_hi = logger('Hi!') # 此时`log_hi`相当于`log_message`，`Hi`是参数`msg`
log_hi()

def html_tag(tag):

    def wrap_text(msg):
        print('<{0}>{1}</{0}>'.format(tag, msg))

    return wrap_text

print_h1 = html_tag('h1') # 此时`print_h1`相当于`wrap_text`，`h1`相当于参数`tag`
print_h1('Test Headline!') # 此时相当于运行`wrap_text('Test Headline!')，`Test Headline!`相当于参数`msg`
print_h1('Another Headline!')

输出结果：

<h1>Test Headline!</h1>
<h1>Another Headline!</h1>