python - 为什么会出现这个输出？

Question

def function(s):
 if len(s) == 1:
    print s[0],
 else:
    function(s[1:])
    print s[0],

function("1234") 结束打印 4 3 2 1

为什么会这样？在函数中，显然不满足第一个条件。在 else 条件下，s[1:] 被放入 s，但它的长度不是 1。我只是看不到 s[0] 之外的任何东西> 将打印到屏幕上。该函数中没有任何内容看起来像打印 s[1:]，更不用说反向打印了。我很困惑。

Answer 1

>>> def function(s):
...     print 's is currently %r' % s
...     if len(s) == 1:
...         print s[0],
...     else:
...         function(s[1:])
...         print s[0],
... 
>>> function("1234")
s is currently '1234'
s is currently '234'
s is currently '34'
s is currently '4'
4 3 2 1

这是一个递归函数，它在打印 s[0] 之前再次调用自身，因此它打印出的项目被反转。

这是一个可能更有帮助的示例。

>>> def function(s):
...     print 's is currently %r' % s
...     if len(s) > 1:
...         print 'calling function again with %r as s' % s[1:]
...         function(s[1:])
...     print s[0],
... 
>>> function('1234')
s is currently '1234'
calling function again with '234' as s
s is currently '234'
calling function again with '34' as s
s is currently '34'
calling function again with '4' as s
s is currently '4'
4 3 2 1