String slices
A segment of a string is called a slice. Selecting a slice is similar selecting a character:
The operator [n:m] returns the part of the string from the ‘n-eth’ character to the ‘m-eth’ character, including the first but excluding the last.
If you omit the first index (before the colon), the slice starts at the beginning of the string. If you omit the second index, the slice goes to the end of the string.
If the first index is greater than or equal to the second the result is an empty string. An empty sting contains no characters and has length 0, but other than that, it is the same as any other string.
from Thinking in Python
原文：http://www.cnblogs.com/ryansunyu/p/3799874.html