


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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