字符串操作练习：
>>> ‘\‘No!\‘,Tom didn\‘t want to join them. ‘ #使用斜杠来避免引号冲突 "‘No!‘,Tom didn‘t want to join them. " >>> ‘abc‘*3 ‘abcabcabc‘ >>> words = ‘Let\‘s make the world a better place!‘ >>> len(words) 36 >>> words[35] ‘!‘ >>> words[20:33] ‘ a better pla‘ >>> words[-10:] ‘ter place!‘ >>> words[1]=‘D‘ #String is immutable Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: ‘str‘ object does not support item assignment