Reading a file line-by-line is a common task in many programming applications. In Python, there are several ways to read a file line-by-line, and in this post, we will explore one of the most common and efficient ways to do it: by using a loop to iterate over each line in the file and appending each line to a list.



Prerequisites

Before we dive into the code, it’s important to have a basic understanding of the following concepts in Python:

  • Variables and data types
  • Loops
  • File I/O operations


Step 1: Open the file

The first step in reading a file line-by-line is to open the file. In Python, you can open a file using the built-in open() function. The open() function takes two arguments: the file name and the mode in which the file will be opened.

Here’s an example of how to open a file in read mode:

file = open('filename.txt', 'r')

In this example, we’re opening a file called “filename.txt” in read mode ('r'). If the file is located in a different directory, you will need to provide the full path to the file.

It’s important to note that when you’re done with the file, you should always close it using the close() method:

file.close()


Step 2: Read the file line-by-line

Once the file is open, you can read the file line-by-line using a loop. In Python, the for loop is the most commonly used loop for iterating over a collection of items.

Here’s an example of how to read a file line-by-line and print each line to the console:

with open('filename.txt', 'r') as file:
    lines = []
    for line in file:
        print(line.strip())

In this example, we’re using a with statement to automatically close the file once the block of code is executed. Inside the with statement, we’re creating an empty list called lines, which we will use to store each line of the file.

We then use a for loop to iterate over each line in the file. The strip() method is used to remove any whitespace characters from the beginning and end of each line before printing it to the console.



Step 3: Append each line to a list

Instead of printing each line to the console, you can append each line to a list so that you can use it later in your code.

Here’s an example of how to read a file line-by-line and append each line to a list:

with open('filename.txt', 'r') as file:
    lines = []
    for line in file:
        lines.append(line.strip())

In this example, we’re using the same with statement and for loop as before. However, instead of printing each line to the console, we’re appending each line to a list called lines using the append() method.

Once the loop is finished, the lines list will contain all of the lines from the file.



Step 4: Close the file

Once you’ve finished reading the file, it’s important to close the file using the close() method. This ensures that any resources used by the file are freed up.

file.close()


Full code example

Here’s the full code example that reads a file line-by-line and stores each line in a list:

with open('filename.txt', 'r') as file:
    lines = []
    for line in file:
        lines.append(line.strip())
    
    # do something with the lines here

file.close()

In this example, we’re opening a file called “filename.txt” in read mode ('r') using the with statement. We’re creating an empty list called lines, which we will use to store each line of the file. We’re then using a for loop to iterate over each line in the file, and appending each line to the lines list using the append() method. Once the loop is finished, the lines list will contain all of the lines from the file. You can then use the lines list to do whatever you need to do with the data.



Conclusion

In this post, we’ve explored how to read a file line-by-line into a list using Python. By following the steps outlined above, you can efficiently and easily read and store the contents of a file in a list, which you can then use to manipulate and analyze the data in your Python code.

I hope you found this post helpful! If you have any questions or comments, please feel free to leave them below.



Leave a Reply