目的
把字符串逐字符或逐词反转过来，这个蛮有意思的。
方法
先看逐字符反转吧，第一种设置切片的步长为-1
revchars=astring[::-1]
In [65]: x='abcd'
In [66]: x[::-1]
Out[66]: 'dcba'
In [66]: x[::-1]
Out[66]: 'dcba'
第二种做法是采用reversed()，注意它返回的是一个迭代器，可以用于循环或传递给其它的“累加器”，不是一个已完成的字符串。
revchars=''.join(reversed(astring))
In [56]: y=reversed(x)
In [57]: y
Out[57]: <reversed object at 0x058302F0>
In [58]: ''.join(y)
Out[58]: 'dcba'
In [57]: y
Out[57]: <reversed object at 0x058302F0>
In [58]: ''.join(y)
Out[58]: 'dcba'
接着来看逐词反转。
第一种做法，创建一个列表，将列表反转，用join方法合并
In [38]: s='Today is really a good day'
In [39]: rev=s.split()
In [40]: rev
Out[40]: ['Today', 'is', 'really', 'a', 'good', 'day']
In [41]: rev.reverse()
In [42]: rev
Out[42]: ['day', 'good', 'a', 'really', 'is', 'Today']
In [45]: ' '.join(rev)
Out[45]: 'day good a really is Today'
In [39]: rev=s.split()
In [40]: rev
Out[40]: ['Today', 'is', 'really', 'a', 'good', 'day']
In [41]: rev.reverse()
In [42]: rev
Out[42]: ['day', 'good', 'a', 'really', 'is', 'Today']
In [45]: ' '.join(rev)
Out[45]: 'day good a really is Today'
也有一行解决的代码：
rev=' '.join(s.split()[::-1])
第二种做法，可以不改变原先的空格，采用正则式来做：
In [46]: import re
In [47]: rev=re.split(r'(\s+)',s)
In [48]: rev
Out[48]: ['Today', ' ', 'is', ' ', 'really', ' ', 'a', ' ', 'good', ' ', 'day']
In [49]: rev.reverse()
In [50]: rev
Out[50]: ['day', ' ', 'good', ' ', 'a', ' ', 'really', ' ', 'is', ' ', 'Today']
In [51]: rev=''.join(rev)
In [52]: rev
Out[52]: 'day good a really is Today'
In [47]: rev=re.split(r'(\s+)',s)
In [48]: rev
Out[48]: ['Today', ' ', 'is', ' ', 'really', ' ', 'a', ' ', 'good', ' ', 'day']
In [49]: rev.reverse()
In [50]: rev
Out[50]: ['day', ' ', 'good', ' ', 'a', ' ', 'really', ' ', 'is', ' ', 'Today']
In [51]: rev=''.join(rev)
In [52]: rev
Out[52]: 'day good a really is Today'
可以考虑采用reversed()来代替可读性较差的[::-1]
revwords=' '.join(reversed(s.split()))
revwords=''.join(reversed(re.split(r'(\s+)',s)))
revwords=''.join(reversed(re.split(r'(\s+)',s)))