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How to search for a string in text files?


Question

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"
2017/09/27
1
173
9/27/2017 2:01:38 PM

Accepted Answer

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

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")

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')

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')

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

2019/08/22
399
8/22/2019 12:45:01 AM

As Jeffrey Said, you are not checking the value of check(). In addition, your check() function is not returning anything. Note the difference:

def check():
    with open('example.txt') as f:
        datafile = f.readlines()
    found = False  # This isn't really necessary
    for line in datafile:
        if blabla in line:
            # found = True # Not necessary
            return True
    return False  # Because you finished the search without finding

Then you can test the output of check():

if check():
    print('True')
else:
    print('False')
2019/03/30

Here's another way to possibly answer your question using the find function which gives you a literal numerical value of where something truly is

open('file', 'r').read().find('')

in find write the word you want to find and 'file' stands for your file name

2012/11/26

if True:
    print "true"

This always happens because True is always True.

You want something like this:

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

Good luck!

2011/02/09

I made a little function for this purpose. It searches for a word in the input file and then adds it to the output file.

def searcher(outf, inf, string):
    with open(outf, 'a') as f1:
        if string in open(inf).read():
            f1.write(string)
  • outf is the output file
  • inf is the input file
  • string is of course, the desired string that you wish to find and add to outf.
2016/05/11

Your check function should return the found boolean and use that to determine what to print.

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

        return found

found = check()
if found:
    print "true"
else:
    print "false"

the second block could also be condensed to:

if check():
    print "true"
else:
    print "false"
2011/02/09

Source: https://stackoverflow.com/questions/4940032
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