如何在文本文件中搜索字符串? [英] How to search for a string in text files?

问题描述

我想检查一个字符串是否在文本文件中.如果是，则执行 X.如果不是，则执行 Y.但是，由于某种原因，此代码总是返回 True.谁能看出哪里出了问题?

I want to check if a string is in a text file. If it is, do X. If it's not, do Y. However, this code always returns True for some reason. Can anyone see what is wrong?

def check():
    datafile = file('example.txt')
    found = False
    for line in datafile:
        if blabla in line:
            found = True
            break

check()
if True:
    print "true"
else:
    print "false"

推荐答案

你总是得到 True 的原因已经给出，所以我再提供一个建议:

The reason why you always got True has already been given, so I'll just offer another suggestion:

如果您的文件不是太大，您可以将其读入一个字符串，然后直接使用它(比每行读取和检查行更容易且通常更快):

If your file is not too large, you can read it into a string, and just use that (easier and often faster than reading and checking line per line):

with open('example.txt') as f:
    if 'blabla' in f.read():
        print("true")

另一个技巧:您可以通过使用 mmap.mmap() 来缓解可能出现的内存问题 创建一个使用底层文件的类字符串"对象(而不是在内存中读取整个文件):

Another trick: you can alleviate the possible memory problems by using mmap.mmap() to create a "string-like" object that uses the underlying file (instead of reading the whole file in memory):

import mmap

with open('example.txt') as f:
    s = mmap.mmap(f.fileno(), 0, access=mmap.ACCESS_READ)
    if s.find('blabla') != -1:
        print('true')

注意:在 python 3 中，mmap 的行为类似于 bytearray 对象而不是字符串，因此您使用 find() 查找的子序列必须是 bytes 对象而不是字符串，例如.s.find(b'blabla'):

NOTE: in python 3, mmaps behave like bytearray objects rather than strings, so the subsequence you look for with find() has to be a bytes object rather than a string as well, eg. s.find(b'blabla'):

#!/usr/bin/env python3
import mmap

with open('example.txt', 'rb', 0) as file, \
     mmap.mmap(file.fileno(), 0, access=mmap.ACCESS_READ) as s:
    if s.find(b'blabla') != -1:
        print('true')

您也可以在 mmap 上使用正则表达式，例如不区分大小写的搜索:if re.search(br'(?i)blabla', s):

You could also use regular expressions on mmap e.g., case-insensitive search: if re.search(br'(?i)blabla', s):

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