Why Does My Python String Search in a Text File Always Return True?

Searching Strings in Text Files

Issue:
An attempt to determine whether a specific string exists within a text file using a particular function always returns True regardless of the string's presence.

Code:

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

Diagnosis and Solution:

The provided code loop through the lines of the text file and checks line-by-line to find the presence of a specific string, "blabla." However, the issue lies in the condition if blabla in line. In Python, in checks for membership, meaning it returns True if the string is anywhere in the line.

To alleviate this issue, different approaches can be considered:

Option 1: Direct String Search in File Contents

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

This approach reads the entire file contents into a string and then checks for the string's presence. If the file is not too large, this method is often faster and more convenient than iterating through each line.

Option 2: Using Memory Mapping for Efficient File Access

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')

This approach utilizes Python's memory mapping technique to create a "string-like" object that manipulates the underlying file directly, allowing for efficient access without fully loading the file into memory.

Option 3: Case-Insensitive Search with Regular Expressions

if re.search(br'(?i)blabla', s):
    print('true')

This approach employs regular expressions to perform a case-insensitive search within the memory-mapped file. The syntax (?i)blabla makes the search ignore case differences.

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