How Can I Efficiently Check for a String's Presence in Large Text Files in Python?

Inspecting Text Files for String Inclusivity

Consider a scenario where you seek to ascertain the presence of a specific string within text files. Upon its identification, a specific action (X) should be executed; otherwise, an alternate action (Y) should follow. However, a code snippet that aims to achieve this objective consistently returns True, puzzling you about its accuracy.

The culprit responsible for this erroneous behavior is the absence of a condition check within the if statement. The proper implementation should be as follows:

if 'blabla' in line:

However, if your text files are relatively large, it may be more efficient to read the entire file into a string and perform the search using that. Here's an example:

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

For even larger files, you can leverage mmap.mmap() to create a "string-like" object that employs the underlying file instead of loading the entire contents into 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')

In Python 3, it's worth noting that mmaps resemble bytearray objects, necessitating the modification of the search string to a bytes object:

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')

Moreover, you can utilize regular expressions on mmaps for more advanced search capabilities, such as case-insensitive matching:

import mmap
import re

with open('example.txt', 'rb', 0) as file, \
     mmap.mmap(file.fileno(), 0, access=mmap.ACCESS_READ) as s:
    if re.search(br'(?i)blabla', s):
        print('true')

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