《Python Cookbook 3rd》笔记(2.2):字符串开头或结尾匹配

巨輪 2020-11-09 23:53:20
Python def jftp


字符串开头或结尾匹配

问题

你需要通过指定的文本模式去检查字符串的开头或者结尾,比如文件名后缀, URL Scheme 等等。

解法

检查字符串开头或结尾的一个简单方法是使用 str.startswith() 或者是str.endswith() 方法。比如:

>>> filename = 'spam.txt'
>>> filename.endswith('.txt')
True
>>> filename.startswith('file:')
False
>>> url = 'http://www.python.org'
>>> url.startswith('http:')
True
>>>

如果你想检查多种匹配可能,只需要将所有的匹配项放入到一个元组中去,然后传给 startswith() 或者 endswith() 方法:

>>> import os
>>> filenames = os.listdir('.')
>>> filenames
[ 'Makefile', 'foo.c', 'bar.py', 'spam.c', 'spam.h' ]
>>> [name for name in filenames if name.endswith(('.c', '.h')) ]
['foo.c', 'spam.c', 'spam.h'
>>> any(name.endswith('.py') for name in filenames)
True
>>>

另一个例子

from urllib.request import urlopen
def read_data(name):
if name.startswith(('http:', 'https:', 'ftp:')):
return urlopen(name).read()
else:
with open(name) as f:
return f.read()

奇怪的是,这个方法中必须要输入一个元组作为参数。如果你恰巧有一个 list 或者 set 类型的选择项,要确保传递参数前先调用 tuple() 将其转换为元组类型。比如:

>>> choices = ['http:', 'ftp:']
>>> url = 'http://www.python.org'
>>> url.startswith(choices)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: startswith first arg must be str or a tuple of str, not list
>>> url.startswith(tuple(choices))
True
>>>

讨论

startswith() 和 endswith() 方法提供了一个非常方便的方式去做字符串开头和结尾的检查。类似的操作也可以使用切片来实现,但是代码看起来没有那么优雅。比如:

>>> filename = 'spam.txt'
>>> filename[-4:] == '.txt'
True
>>> url = 'http://www.python.org'
>>> url[:5] == 'http:' or url[:6] == 'https:' or url[:4] == 'ftp:'
True
>>>

你可以能还想使用正则表达式去实现,比如:

>>> import re
>>> url = 'http://www.python.org'
>>> re.match('http:jhttps:jftp:', url)
<_sre.SRE_Match object at 0x101253098>
>>>

这种方式也行,但是对于简单的匹配实在是有点杀鸡用牛刀了,本节中的方法更加简洁。

最后提一下,当和其他操作比如普通数据聚合相结合的时候 startswith() 和 endswith() 方法是很不错的。比如,下面这个语句检查某个文件夹中是否存在指定的文件类型:

if any(name.endswith(('.c', '.h')) for name in listdir(dirname)):
...
版权声明
本文为[巨輪]所创,转载请带上原文链接,感谢
https://my.oschina.net/jallenkwong/blog/4710364

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