Python readlines() method explained

Python readlines() Method Detailed Explanation

1. Overview

In Python programming, we often need to read the contents of files. The readlines() method is a built-in method in Python for reading file contents. It reads an entire file and returns a list containing the contents of each line of the file.

readlines() has the following syntax:

fileObject.readlines(size)

Here, fileObject is the file object, and size is an optional parameter that specifies the number of bytes to read.

2. Example

Let’s look at a simple example using the readlines() method to read the contents of a text file:

# Open the file
file = open("example.txt", "r")

# Read the file contents
lines = file.readlines()

# Print the file contents
for line in lines:
print(line)

# Close the file
file.close()

Suppose our file “example.txt” contains the following content:

Hello
World
Running the above code example produces the following output:

<pre><code class="language-python line-numbers">Hello
World
3. Usage Details</h2>
<h3>3.1 Reading an Entire File</h3>
<p><code>readlines() reads an entire file at once and stores each line in a list. We can use a loop to iterate over the list and process the file line by line. 

3.2 Specifying the number of bytes to read
readlines()’s optional parameter size specifies the number of bytes to read. If the size parameter is not specified, the entire file is read by default.
Let’s take a look at an example to read the first 10 bytes of a binary file:
# Open a binary file
file = open("example.bin", "rb")

# Read the first 10 bytes
data = file.readlines(10)

# Print the read result
print(data)

# Close the file
file.close()

Running the above example code, the output is:
[b'Hello World']

Here we read the first line of the file, which contains 10 bytes. 
3.3 Notes on Reading Large Files
For large files, reading the entire file at once may cause memory overflow. Therefore, when reading large files, we should use appropriate methods to read in chunks.
Let's look at an example to read the contents of a large file:
# Open a large file
file = open("large_file.txt", "r")

# Read the file contents in chunks
while True:
lines = file.readlines(100000)
if not lines:
break
for line in lines:
process_line(line)

# Close the file
file.close()

In the above example, we use a while loop and the readlines() method to read the file in chunks and then process each chunk line by line. 
3.4 Exception Handling
When reading files, some exceptions may occur, such as file non-existence or insufficient permissions. To prevent these exceptions from causing program crashes, we should use exception handling. 
Let's look at an example that handles the case where a file doesn't exist:
try:
# Open the file
file = open("nonexistent.txt", "r")

# Read the file contents
lines = file.readlines()

# Close the file
file.close()
except FileNotFoundError:
print("File not found.")

In the above example, we attempt to open a non-existent file. If the file doesn't exist, a FileNotFoundError exception is thrown. We use the try-except structure to catch this exception and output a corresponding message. 
4. Summary
This article introduced Python's readlines() method, which reads each line of a file and returns a list containing all the lines. We learned the basic usage of the readlines() method and some precautions, including specifying the number of bytes to read, handling large files, and exception handling.