Python Search for a String in Text Files

Searching for a String in Python Source Code

This Python guide will show you how to perform a text file search using a string. We’ll also look at how to print the line and number where a string was found in a file.

Following this article, you will have gained knowledge in the following areas:

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

This is a step-by-step process in which we will examine each potential

one.

Table of contents

• 
  How to Search for a String in Text File
  
  • 
    Example to search for a string in text file
    
• 
  Search file for a string and Print its line and line number
  
• 
  Efficient way to search string in a large text file
  
• 
  mmap to search for a string in text file
  
• 
  Search string in multiple files
  
• 
  Search file for a list of strings
  

How to Search for a String in Text File

To locate a specific phrase in a text file, you can use the find() function of the string class in conjunction with the read() method of the file. What follows is a

steps.

1. 
   Open file in a read mode
   

   
   Open a file
   
   by setting a file path and access mode to the
   
open()

   function. The access mode specifies the operation you wanted to perform on the file, such as reading or writing. For example, r is for reading.
   
fp= open(r'file_path', 'r')


2. 
   Read content from a file
   

   Once opened,
   
   read all content of a file
   
   using the
   
read()

   method. The
   
read()

   method returns the entire file content in string format.

3. 
   Search for a string in a file
   

   Use the
   
find()

   method of a str class to check the given string or word present in the result returned by the
   
read()

   method. The
   
find()

   method. The find() method will return -1 if the given text is not present in a file

4. 
   Print line and line number
   

   If you need line and line numbers, use the
   
readlines(

   ) method instead of
   
read()

   method. Use the
   
   for loop
   
   and
   
readlines()

   method 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 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')

Produce Results

:

string exists in a file

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

Follow these instructions if you want to print the line number and line where you found the text or word you were searching for in a file.

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 Here we’ll show how to search a file for the string “laptop,” print out the line containing that string along with the line number where it was found, and so on.

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)

Produce Results

:

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

Efficient way to search string in a large text file

These methods all work by loading the complete file into memory. For huge files, reading them entirely into memory is inefficient.

See how to search for a string in a large text quickly and with minimal memory usage.

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.

Keep in mind that this is a space-saving method because enumerate(file_pointer) doesn’t have to read the entire file. 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

Conduct a Sample

:

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

mmap to search for a string in text file

Here we’ll examine the most time- and memory-efficient strategy for searching for a string in a huge text file.

As an alternative, the mmap module can be used to search for a string within a very large file. Instead of reading the entire file into memory, the mmap.mmap() method generates a bytearray object that checks the underlying file. 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')

Produce Results

:

string exist in a file

Search string in multiple files

It’s helpful to be able to perform a string search on a number of files within a directory at once. Follow the instructions below to do a text search across all of a

directory.

As an Illustration,

• 
  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.

:

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

Produce Results

:

string found

Search file for a list of strings

It’s helpful to be able to search a file for more than one string at a time. The code snippet below demonstrates how to perform a generic word search in a text file. 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')

Produce Results

:

string exist in a file