python: how to check if a str is available in a text file with in an if statement

Question

what I am trying to do is add a string only if not available in a text file The problem is I can't read the content of the text file within an if statement

import random
import time

def random_srting(length=1):
    digits ='abcdefghijklmnopqrstuvwxyz1234567890._'
    return ''.join(random.choice(digits) for i in range(length))

Input = str(input())

with open('4LWordList.txt','a+') as f:
    while Input != "s":               

      time.sleep(1)
      word = (random_srting(1)+".abc")

      if (word in f.read()):           #here is the problem 'word' isn't being checked in 'f' because it's not reading the content of 'f'
          print("skip  :  "+word)    

      elif (word not in f.read()):     #same problem here
         f.write(word+"\n") 
         print("add  :  "+word)

elsholz · Answer 1 · 2022-07-09T20:56:52.917

The problem here is, that the read function has a pointer to mark how far it read trough the file. If you don't reset the pointer to the beginning, it will continue reading at the end of the file in every subsequent call, which will result in an empty string. So, instead of reading the file contents in each if statement, read the contents either once and save it to a variable, or reset the pointer before reading using f.seek(0).

See also this related question: Re-read an open file Python

Btw theres some other issues with your code: The input function isn't placed within the while loop, hence it is only asking for input once, then looping forever or not at all. Also, the condition in the first if statement can either be True or False. Hence, you should replace the elif by else instead, as the code only enters this branch, if the condition of the if evaluates to False.

thank you for explaining the read function I am a beginner so this stuff confuses me . I know about my input problem but I've pushed it aside because its a relatively easy to fix and focuses more on the read function problem anyway thanks a lot — zanix, Jul 10 '22 at 16:45

score 0 · Accepted Answer · answered Jul 09 '22 at 21:01

Store the file contents in a set, and then you don't need to re-read the file each time you add a word:

import random

def random_string(length):
    return ''.join(random.choices(
        'abcdefghijklmnopqrstuvwxyz1234567890._',
        k=length
    ))

with open('4LWordList.txt') as f:
    words = set(f.readlines())

with open('4LWordList.txt','a') as f:
    while input() != "s":               
        word = random_string(1) + ".abc"

        if word in words:
            print("skip  : ", word)    
        else:
            words.add(word)
            f.write(word+"\n") 
            print("add  : ", word)

When you open a file, you create an iterator that starts at the start of the file, and read() consumes the iterator, advancing it to the end of the file; to re-read it you need to either seek back to the beginning or re-open the file. The simpler (and faster) solution for a small file is to just keep the contents in memory.

python: how to check if a str is available in a text file with in an if statement

2 Answers2