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Introduction
Python File readlines() Method reads until end-of-file (EOF) using readline() and returns a list containing the lines.
This article aims to provide a deep dive into the readlines() method, exploring its syntax, parameters, and various use cases. By the end of this read, you will have a comprehensive understanding of how to leverage readlines() for your file processing tasks.
What is readlines() function in Python?
The readlines() function in Python is used with file objects to read all the lines from a file into a list. Each line in the file becomes an element in the list, with the newline character \n at the end of each element. This function is particularly useful for iterating over each line in a file or when you need to process multiple lines together. For example, after opening a file in read mode, calling file.readlines() will return a list where each item is a line from the file. This allows for easy manipulation of file data within Python scripts.
Syntax
The readlines() method is associated with file objects and is used to read the contents of a file into a list where each line of the file is a separate element in the list.
This is an optional parameter which specifies the number of bytes to be read from the file. By default, it's set to -1, indicating that the whole file should be read.
Basic Usage
The primary use of readlines() is to read all the lines of a file and return them as a list of strings.
# Opening a file
with open('example.txt', 'r') as file:
lines = file.readlines()
print(lines)
Handling Large Files:
For large files, the hint parameter can be utilized to control the number of bytes read from the file, helping in memory management.
with open('large_file.txt', 'r') as file:
partial_lines = file.readlines(100)
print(partial_lines)
In this example, only the first 100 bytes of the file are read and returned as a list of lines.
In the below article, we are going to study Python readlines() line by line from a file.
Examples of readlines() function in Python
Method 1: Read a File Line by Line using readline() :
# Writing to a sample file 'example_readline.txt'
with open('example_readline.txt', 'w') as file:
file.write("Line 1: Hello, World!\n")
file.write("Line 2: This is a test file.\n")
file.write("Line 3: Each line will be read individually.\n")
file.write("Line 4: Using readline() method.")
# Reading the file line by line using readline()
lines_read = []
with open('example_readline.txt', 'r') as file:
while True:
line = file.readline()
if not line:
break
lines_read.append(line.strip()) # Using strip() to remove newline characters
# Printing each line from the file
for line in lines_read:
print(line)
You can also try this code with Online Python Compiler
Method 2: Read a File Line by Line using readlines()
# Writing to a sample file 'example_readlines.txt'
with open('example_readlines.txt', 'w') as file:
file.write("Line 1: Hello, again!\n")
file.write("Line 2: We are using readlines() this time.\n")
file.write("Line 3: Each line is read into a list.\n")
file.write("Line 4: Easy and efficient.")
# Reading the file line by line using readlines()
with open('example_readlines.txt', 'r') as file:
lines = file.readlines()
# Processing each line from the file
for line in lines:
print(line.strip()) # Using strip() to remove newline characters
You can also try this code with Online Python Compiler
# Writing to a sample file 'example_for_loop.txt'
with open('example_for_loop.txt', 'w') as file:
file.write("First Line: Using for loop.\n")
file.write("Second Line: Each line is processed as it's read.\n")
file.write("Third Line: Efficient for large files.\n")
file.write("Fourth Line: No need to load everything into memory.")
# Reading the file line by line using a for loop
lines_read_for_loop = []
with open('example_for_loop.txt', 'r') as file:
for line in file:
lines_read_for_loop.append(line.strip()) # Using strip() to remove newline characters
# Printing each line from the file
for line in lines_read_for_loop:
print(line)
You can also try this code with Online Python Compiler
Method 4: Read a File Line by Line using for loop and list comprehension
# Writing to a sample file 'example_list_comp.txt'
with open('example_list_comp.txt', 'w') as file:
file.write("Line 1: List Comprehension method.\n")
file.write("Line 2: Concise and efficient.\n")
file.write("Line 3: Suitable for compact scripts.\n")
file.write("Line 4: Pythonic way to read files.")
# Reading the file line by line using for loop and list comprehension
with open('example_list_comp.txt', 'r') as file:
lines_read_list_comp = [line.strip() for line in file] # Using list comprehension to process each line
# Printing each line from the file
for line in lines_read_list_comp:
print(line)
You can also try this code with Online Python Compiler
It is more efficient, as it reads one line at a time.
It is less efficient with large files, as it loads all lines into memory.
Use Case
Ideal for large files or when only part of the file is needed.
It's preferred when you need to process the whole file and require line-by-line processing that is not excessively large.
Iteration
Typically used in a loop to read lines sequentially.
Directly provides all lines for immediate iteration or processing.
End of File Handling
Returns an empty string when the end of the file is reached.
It reads until EOF and then stops.
Typical Usage
Used in scenarios where line-by-line processing is required, such as parsing large log files.
Used when the file size is manageable, requiring all lines to be needed simultaneously, like reading configuration files.
Advantages of Python Readiness
Simple and Easy to Use: readlines() provides a straightforward way to read all lines of a file into a list, making it easy to work with.
Effective for Small to Medium Files: It's an efficient method when working with relatively small to medium-sized files.
Disadvantages of Python Readiness
Memory Consumption: For large files, readlines() can consume a lot of memory as it loads the entire file into memory.
Future of readlines()
The readlines() method has been a part of Python's file handling toolkit for a long time and continues to serve its purpose effectively. However, with the advent of more memory-efficient methods and libraries, such as pandas for data processing, the use of readlines() might see a decline for large-scale data processing tasks.
Frequently Asked Questions
What is the readline() method in Python?
Python's readline() method is used to read a single line from a file. It reads the line to the newline character and returns it as a string, making it ideal for sequential file reading.
What does the Readlines ()> method returns?
The readlines() method in Python returns a list containing all the lines in a file, with each line as a separate string element, including newline characters at the end of each line.
How do you use Readlines function?
To use the readlines function in Python, open a file in read mode, call readlines() on the file object, and it will return a list of lines from the file, each line as a separate list item.
What is the difference between readline () and Readlines ()?
readline() reads and returns a single line from a file, while readlines() reads all lines in a file and returns them as a list of strings, with each list item representing one line.
Conclusion
The readlines() method is a fundamental part of Python’s file handling suite, providing a simple and effective way to read text files line by line. Understanding its usage, advantages, and limitations is crucial for anyone looking to master file processing in Python. As we step into an era of big data, exploring memory-efficient alternatives alongside readlines() could be a prudent approach for handling large-scale data processing tasks.
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