How to Read a File Line by Line into a List in Python


By squashlabs, Last Updated: November 2, 2023

How to Read a File Line by Line into a List in Python

Reading a file line by line into a list can be a common task in Python when dealing with text files. This process allows you to easily access and manipulate the contents of the file. In this answer, we will explore different approaches to achieve this task.

Method 1: Using a For Loop

One way to read a file line by line into a list is by using a for loop. Here’s an example:

lines = []
with open("file.txt", "r") as file:
    for line in file:

In this example, we open the file using the open() function and specify the mode as “r” for reading. The file is then assigned to the variable file. We use a for loop to iterate over each line in the file, and the strip() method is used to remove any leading or trailing whitespace from each line. Each line is then appended to the lines list.

Related Article: How To Convert a Dictionary To JSON In Python

Method 2: Using List Comprehension

Another approach to read a file line by line into a list is by using list comprehension. Here’s an example:

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

In this example, we use a list comprehension to create a new list called lines. The strip() method is applied to each line to remove any leading or trailing whitespace. This approach is concise and often preferred when working with smaller files.

Best Practices

When reading a file line by line into a list in Python, it’s important to keep a few best practices in mind:

1. Use the with statement: The with statement ensures that the file is properly closed after reading, even if an exception occurs. It is considered best practice when working with files.

2. Handle file encoding: If you are working with files that have non-ASCII characters, specify the appropriate encoding when opening the file. For example, with open("file.txt", "r", encoding="utf-8") as file:.

3. Handle large files: If you are working with large files that may not fit into memory, consider using a generator instead of creating a list. A generator allows you to iterate over the lines of a file without loading the entire file into memory at once.

4. Error handling: Always handle potential errors when reading a file, such as file not found or permission denied. You can use try-except blocks to catch and handle exceptions appropriately.

Alternative Approach

If you need to read a file line by line into a list but want to exclude empty lines or lines containing specific characters, you can modify the above examples accordingly. Here’s an example that excludes empty lines:

with open("file.txt", "r") as file:
    lines = [line.strip() for line in file if line.strip()]

In this example, the if line.strip() condition is added to the list comprehension to exclude empty lines from the resulting list.

Related Article: How to Sort a Dictionary by Key in Python

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