Python Search for a String in Text Files

In this Python tutorial, you’ll learn to search a string in a text file. Also, we’ll see how to search a string in a file and print its line and line number.

After reading this article, you’ll learn the following cases.

• If a file is small, read it into a string and use the find() method to check if a string or word is present in a file. (easier and faster than reading and checking line per line)
• If a file is large, use the mmap to search a string in a file. We don’t need to read the whole file in memory, which will make our solution memory efficient.
• Search a string in multiple files
• Search file for a list of strings

We will see each solution one by one.

How to Search for a String in Text File

Example to search for a string in text file

def search_str(file_path, word):
    with open(file_path, 'r') as file:
        # read all content of a file
        content = file.read()
        # check if string present in a file
        if word in content:
            print('string exist in a file')
        else:
            print('string does not exist in a file')

search_str(r'E:\demos\files_demos\account\sales.txt', 'laptop')

Output:

string exists in a file

Search file for a string and Print its line and line number

Use the following steps if you are searching a particular text or a word in a file, and you want to print a line number and line in which it is present.

• Open a file in a read mode.
• Next, use the readlines() method to get all lines from a file in the form of a list object.
• Next, use a loop to iterate each line from a file.
• Next, In each iteration of a loop, use the if condition to check if a string is present in a current line and print the current line and line number.

Example: In this example, we’ll search the string ‘laptop’ in a file, print its line along with the line number.

# string to search in file
word = 'laptop'
with open(r'E:\demos\files_demos\account\sales.txt', 'r') as fp:
    # read all lines in a list
    lines = fp.readlines()
    for line in lines:
        # check if string present on a current line
        if line.find(word) != -1:
            print(word, 'string exists in file')
            print('Line Number:', lines.index(line))
            print('Line:', line)

Output:

laptop string exists in a file
line: laptop 10 15000
line number: 1

Efficient way to search string in a large text file

All above way read the entire file in memory. If the file is large, reading the whole file in memory is not ideal.

In this section, we’ll see the fastest and most memory-efficient way to search a string in a large text file.

• Open a file in read mode
• Use for loop with enumerate() function to get a line and its number. The enumerate() function adds a counter to an iterable and returns it in enumerate object. Pass the file pointer returned by the open() function to the enumerate().
• We can use this enumerate object with a for loop to access the each line and line number.

Note: The enumerate(file_pointer) doesn’t load the entire file in memory, so this is an efficient solution.

Example:

with open(r"E:\demos\files_demos\account\sales.txt", 'r') as fp:
    for l_no, line in enumerate(fp):
        # search string
        if 'laptop' in line:
            print('string found in a file')
            print('Line Number:', l_no)
            print('Line:', line)
            # don't look for next lines
            break

Example:

string found in a file
Line Number: 1
Line: laptop 10 15000

mmap to search for a string in text file

In this section, we’ll see the fastest and most memory-efficient way to search a string in a large text file.

Also, you can use the mmap module to find a string in a huge file. The mmap.mmap() method creates a bytearray object that checks the underlying file instead of reading the whole file in memory.

Example:

import mmap

with open(r'E:\demos\files_demos\account\sales.txt', 'rb', 0) as file:
    s = mmap.mmap(file.fileno(), 0, access=mmap.ACCESS_READ)
    if s.find(b'laptop') != -1:
        print('string exist in a file')

Output:

string exist in a file

Search string in multiple files

Sometimes you want to search a string in multiple files present in a directory. Use the below steps to search a text in all files of a directory.

• List all files of a directory
• Read each file one by one
• Next, search for a word in the given file. If found, stop reading the files.

Example:

import os

dir_path = r'E:\demos\files_demos\account'
# iterate each file in a directory
for file in os.listdir(dir_path):
    cur_path = os.path.join(dir_path, file)
    # check if it is a file
    if os.path.isfile(cur_path):
        with open(cur_path, 'r') as file:
            # read all content of a file and search string
            if 'laptop' in file.read():
                print('string found')
                break

Output:

string found

Search file for a list of strings

Sometimes you want to search a file for multiple strings. The below example shows how to search a text file for any words in a list.

Example:

words = ['laptop', 'phone']
with open(r'E:\demos\files_demos\account\sales.txt', 'r') as f:
    content = f.read()
# Iterate list to find each word
for word in words:
    if word in content:
        print('string exist in a file')

Output:

string exist in a file