When working with text files in Python, it is common to come across situations where you need to add a character at the beginning of a line that contains a specific string. This can be achieved in different ways, depending on the specific requirements of your task. In this article, we will explore three different approaches to solve this problem.

Approach 1: Using the fileinput module

The fileinput module in Python provides an easy way to modify text files in-place. We can use this module to iterate over each line in the file, check if it contains the specific string, and add the desired character at the beginning of the line.

import fileinput

specific_string = "example"
character_to_add = "*"

for line in fileinput.input("file.txt", inplace=True):
    if specific_string in line:
        line = character_to_add + line
    print(line, end="")

In this code snippet, we define the specific string we are looking for and the character we want to add at the beginning of the line. We then use a for loop to iterate over each line in the file, and if the line contains the specific string, we prepend the character to the line. Finally, we use the print function with the end parameter set to an empty string to avoid adding extra newlines.

Approach 2: Using regular expressions

If you are familiar with regular expressions, you can use the re module in Python to solve this problem. Regular expressions provide a powerful way to search and manipulate text patterns.

import re

specific_string = "example"
character_to_add = "*"

with open("file.txt", "r") as file:
    lines = file.readlines()

with open("file.txt", "w") as file:
    for line in lines:
        if re.search(specific_string, line):
            line = character_to_add + line
        file.write(line)

In this code snippet, we first read all the lines from the file into a list. We then open the file in write mode and iterate over each line. If the line matches the specific string using the re.search function, we prepend the character to the line and write it back to the file.

Approach 3: Using a temporary file

If you prefer to avoid modifying the original file directly, you can create a temporary file to store the modified lines and then replace the original file with the temporary file.

import shutil

specific_string = "example"
character_to_add = "*"

with open("file.txt", "r") as file:
    with open("temp.txt", "w") as temp_file:
        for line in file:
            if specific_string in line:
                line = character_to_add + line
            temp_file.write(line)

shutil.move("temp.txt", "file.txt")

In this code snippet, we open the original file in read mode and the temporary file in write mode. We iterate over each line in the original file, prepend the character if the line contains the specific string, and write the modified line to the temporary file. Finally, we use the shutil.move function to replace the original file with the temporary file.

After exploring these three approaches, it is clear that the best option depends on the specific requirements of your task. If you need to modify the file in-place without creating a temporary file, Approach 1 using the fileinput module is a good choice. If you are comfortable with regular expressions and want more flexibility in pattern matching, Approach 2 using the re module is a suitable option. On the other hand, if you prefer to keep the original file intact and replace it with a modified version, Approach 3 using a temporary file is the way to go.

Ultimately, the best option is the one that meets your specific needs and aligns with your coding style and preferences.