How to read a file line-by-line into a list?

Python String File Readlines

In Python, there are multiple ways to read a file line-by-line and store each line as an element in a list. This can be useful for various tasks such as data processing, text analysis, and more. In this article, we will explore different methods to achieve this goal and provide examples to illustrate each approach.

Method 1: Using a for loop

One simple way to read a file line-by-line into a list is by using a for loop. The basic idea is to iterate over each line of the file and append it to a list. Here's an example:

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

In this code snippet, we first initialize an empty list called "lines". Then, we use the open() function to open the file in read mode. The 'r' argument indicates that we want to open the file for reading. The with statement is used to ensure that the file is properly closed after we finish reading it. Inside the for loop, we iterate over each line of the file using a built-in iteration feature of file objects. We append the stripped line to the "lines" list using the append() method. Finally, we close the file automatically.

This method works well for small to medium-sized files. However, for large files, it may consume a significant amount of memory since the entire file content is loaded into memory before being stored in the list.

Method 2: Using readlines() function

Another way to read a file line-by-line into a list is by using the readlines() function. This function reads all the lines of a file and returns them as a list. Here's an example:

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

In this code snippet, we use the open() function to open the file in read mode, just like in the previous method. Then, we call the readlines() function on the file object, which reads all the lines of the file and returns them as a list. The resulting list is assigned to the "lines" variable. Finally, we close the file automatically.

This method is more memory-efficient than the previous one since it reads the file line-by-line without loading the entire file content into memory. However, it may still consume a large amount of memory for extremely large files.

Method 3: Using list comprehension

A more concise and pythonic way to read a file line-by-line into a list is by using list comprehension. List comprehension is a compact way to create lists based on existing lists or other iterable objects. Here's an example:

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

In this code snippet, we use the aforementioned open() function to open the file in read mode. Inside the list comprehension, we iterate over each line of the file using the same built-in iteration feature of file objects. For each line, we strip any leading or trailing whitespace characters using the strip() method, and the resulting string is added to the new list. Finally, we close the file automatically.

This method has the same memory advantages as Method 2, as it also reads the file line-by-line. Additionally, it offers the conciseness and readability of list comprehension, making the code easier to understand and write.

Conclusion

Reading a file line-by-line into a list is a common task in Python. In this article, we covered three different methods to achieve this: using a for loop, using the readlines() function, and using list comprehension. The method you choose depends on the size of the file and your personal coding style preferences.

Remember to handle exceptions, such as file opening errors, during the file reading process. It's also good practice to use a context manager (as shown in the examples) to ensure proper file handling, even in the case of exceptions.

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