How to Read Files Line by Line in Python Using readline() and readlines()

Python offers two convenient methods, `readline()` and `readlines()`, for reading text files line by line. These methods are particularly useful when dealing with large files, as they allow you to process the contents without loading the entire file into memory. This article will provide a comprehensive guide on using these functions, accompanied by illustrative examples.

1. Using readline() to Read Lines Individually.

1. The `readline()` method reads a single line from a text file and returns it as a string.
2. It takes an optional parameter, `size`, which specifies the maximum number of bytes to read.
3. If the `size` parameter is omitted, the entire line is read. Here’s an example of using `readline()`:

   def readline_function():    
       with open('example.txt', 'r') as file:
           line = file.readline()
           print(line)
   
   if __name__ == "__main__":
       readline_function()

4. Output.

   Hello, Python.
   

5. The above code will print the first line of the file `example.txt`.
6. To read subsequent lines, you can call `readline()` again in a loop:
   

   def read_all_lines_function():    
       with open('example.txt', 'r') as file:
           line = file.readline()
           print(line)
           while line:
               line = file.readline()
               print(line)
   
   
   if __name__ == "__main__":
       read_all_lines_function()

7. This loop will iterate through the file, printing each line until the end of the file is reached, indicated by an empty string returned by `readline()`.
8. Output.

   Hello, Python.
   
   This text is appended to the file.
   

2. Using readlines() to Read All Lines at Once.

1. The `readlines()` method reads all lines from a text file and returns them as a list of strings.
2. Each line in the file becomes a separate element in the list.
3. Here’s an example of using `readlines()`:

   def readlines_function():
       with open('example.txt', 'r') as file:
           lines = file.readlines()
           line_number = 1
           for line in lines:
               print(line_number, ":", line)
               line_number += 1
   
   
   if __name__ == "__main__":
       readlines_function()

4. This code will read all lines from the file `example.txt` and print each line individually.
5. Output.

   1 : Hello, Python.
   
   2 : This text is appended to the file.

6. The `readlines()` method is particularly efficient when dealing with small files, as it reads the entire file contents at once.

3. Handling Newline Characters.

1. Both `readline()` and `readlines()` return strings with the newline character (`\n`) at the end of each line.
2. If you want to remove the newline characters, you can use the `strip()` method:

   def remove_new_line_character():
           with open('example.txt', 'r') as file:
               line = file.readline()
               print(line.strip())
               while line:
                   line = file.readline()
                   print(line.strip())
   
   if __name__ == "__main__":
       remove_new_line_character()

3. This will remove the newline character from each line before printing it.
   

   Hello, Python.
   This text is appended to the file.

4. Practical Applications.

1. The ability to read files line by line is essential for various tasks in Python programming. Here are some practical applications:
2. Processing large log files: Analyzing large log files to identify patterns or errors.
3. Parsing configuration files: Reading and interpreting configuration settings from text files.
4. Importing data from CSV files: Loading data from CSV files into structured data formats.
5. Performing text analysis: Analyzing text files for sentiment, keywords, or other linguistic features.
6. Iterating over lines of poetry or code: Displaying lines of poetry or code one at a time.

5. Conclusion.

1. The `readline()` and `readlines()` methods are valuable tools for reading text files line by line in Python.
2. They provide efficient and convenient ways to process file contents, making them essential for various programming tasks.