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py3学习笔记2（字符串）
除了数字计算，python还可以通过多种方式操作字符串
他们可以被封装在单引号（‘...‘）或者（"..."）双引号中，使用这两种方式都可以得到相同的结果。（在单引号中，‘\n‘等将不像在双引号中和C语言等其他语言的字符串定义中显示回车，而是显示其本身，只有显示单引号时使用‘\‘‘）
使用‘\‘可以摆脱引号的引用关系与字符意义。
>>> ‘spam eggs‘ # single quotes‘spam eggs‘>>> ‘doesn\‘t‘ # use \‘ to escape the single quote..."doesn‘t">>> "doesn‘t" # ...or use double quotes instead"doesn‘t">>> ‘"Yes," he said.‘‘"Yes," he said.‘>>> "\"Yes,\" he said."‘"Yes," he said.‘>>> ‘"Isn\‘t," she said.‘‘"Isn\‘t," she said.‘
使用print()函数将获得一个可读性更好的字符串
>>> ‘"Isn\‘t," she said.‘‘"Isn\‘t," she said.‘>>> print(‘"Isn\‘t," she said.‘)"Isn‘t," she said.>>> s = ‘First line.\nSecond line.‘ # \n means newline>>> s # without print(), \n is included in the output‘First line.\nSecond line.‘>>> print(s) # with print(), \n produces a new lineFirst line.Second line.
如果你需要将‘\‘以及其后的字符显示字符意义，可以在字符串前加‘r‘转义成行字符串
>>> print(‘C:\some\name‘) # here \n means newline!C:\someame>>> print(r‘C:\some\name‘) # note the r before the quoteC:\some\name
有些字符串需要占用许多行，这时可以使用"""..."""或者‘‘‘...‘‘‘行尾将自动地包含在字符串中，也可以加入额外的‘\‘来确保这一点。
print("""Usage: thingy [OPTIONS] -h Display this usage message -H hostname Hostname to connect to""")
以上代码将打印
Usage: thingy [OPTIONS] -h Display this usage message -H hostname Hostname to connect to
另外，字符串可以使用‘+‘与‘*‘进行操作
>>> # 3 times ‘un‘, followed by ‘ium‘>>> 3*‘un‘ + ‘ium‘‘unununium‘
两个或多个相邻的字符串将自动组合在一起
>>> ‘Py‘ ‘thon‘‘Python‘
但无法使用变量对其进行以上操作
>>> prefix = ‘Py‘>>> prefix ‘thon‘ # can‘t concatenate a variable and a string literal ...SyntaxError: invalid syntax>>> (‘un‘*3) ‘ium‘ ...SyntaxError: invalid syntax
如果你渴望使用变量对字符串进行连接，可以使用‘+‘
>>> prefix + ‘thon‘‘Python‘
这个特性可以用于当你想分割一个非常长的句子时
>>> text = (‘Put several strings within parentheses ‘... ‘to have them joined together.‘)>>> text‘Put several strings within parentheses to have them joined together.‘
你也可以使用类似于其他语言中队数组的索引(index)操作来操作字符串
>>> word = ‘Python‘>>> word[0] # character in position 0‘P‘>>> word[5] # character in position 5‘n‘
负数将从最后开始倒着索引
>>> word[-1] # last character‘n‘>>> word[-2] # second-last character‘o‘>>> word[-6]‘P‘
当然，-0和0是一样的，负数索引从-1开始。
除了索引，python字符串还支持切片（slice），截取字符串中的一部分
>>> word[0:2] # characters from position 0 (included) to 2 (excluded)‘Py‘>>> word[2:5] # characters from position 2 (included) to 5 (excluded)‘tho‘
如下形式的两个切片形式将组成一个完整的字符串
>>> word[:2] + word[2:]‘Python‘>>> word[:4] + word[4:]‘Python‘
当冒号左侧没有数字时默认将从头开始，右侧没有数字时将读到末尾
>>> word[:2] # character from the beginning to position 2 (excluded)‘Py‘>>> word[4:] # characters from position 4 (included) to the end‘on‘>>> word[-2:] # characters from the second-last (included) to the end‘on‘
你可以用以下这种方法简介地记住切片的位置
使用index方法访问超过数组长度的元素将返回错误
>>> word[42] # the word only has 6 charactersTraceback (most recent call last):File "<stdin>", line 1, in <module>IndexError: string index out of range
而使用slice方法将不受限制，超出时将默认为字符串末尾，没有内容时将返回空字符串
>>> word[4:42]‘on‘>>> word[42:]‘‘
注意，在python中，字符串一旦被定义将无法被改变，以下试图改变字符串内容的行为都将报错
>>> word[0] = ‘J‘ ...TypeError: ‘str‘ object does not support item assignment>>> word[2:] = ‘py‘ ...TypeError: ‘str‘ object does not support item assignment
如果你需要一个不一样的字符串，只能创建一个新的
>>> ‘J‘ + word[1:]‘Jython‘>>> word[:2] + ‘py‘‘Pypy‘
另外，你也可以使用len()函数来获取一个字符串的长度
>>> s = ‘supercalifragilisticexpialidocious‘>>> len(s)34
py3学习笔记2（字符串）