Mastering Python’s readline() – Efficient Line‑by‑Line File Reading

What is Python readline()?

readline() is a built‑in file method that returns the next full line from a file object. The returned string ends with a newline character (\n). When the end of the file is reached, it returns an empty string.

You can optionally pass a size argument to limit the number of bytes read. A negative or omitted value causes the entire line to be returned.

Key Characteristics

• Reads one complete line per call.
• Appends a trailing newline unless the line is the last one without one.
• Returns a string in text mode; a bytes object in binary mode.
• Supports an optional size parameter (default is -1).

Syntax

file_object.readline([size])

Parameters

• size (int, optional): Number of bytes to read. Default is -1 (read whole line).

Return Value

A string (or bytes) containing the next line, or an empty string when the file is exhausted.

Example 1: Read the First Line

Assume demo.txt contains:

Testing - FirstLine
Testing - SecondLine
Testing - Third Line
Testing - Fourth Line
Testing - Fifth Line

Python code:

with open("demo.txt", "r") as f:
    first_line = f.readline()
    print(first_line)

Output:

Testing - FirstLine

Example 2: Use the size Argument

Read only the first 10 characters of the first line:

with open("demo.txt", "r") as f:
    partial = f.readline(10)
    print(partial)

Output:

Testing -

Basic File I/O in Python

Opening files is handled by open(path, mode). Common modes:

Mode	Description
r	Read (default)
w	Write (truncate)
a	Append
rb	Read binary
wb	Write binary

Reading a File Line‑by‑Line

Using a while loop with readline():

with open("demo.txt", "r") as f:
    line = f.readline()
    while line:
        print(line)
        line = f.readline()

Using a for loop (preferred for readability):

with open("demo.txt", "r") as f:
    for line in f:
        print(line)

Read All Lines at Once

Python’s readlines() returns a list of all lines:

with open("test.txt", "r") as f:
    lines = f.readlines()
    print(lines)

Example output:

["Line No 1\n", "Line No 2\n", "Line No 3\n", "Line No 4\n", "Line No 5"]

Summary

• Use readline() for controlled, line‑by‑line reading.
• Pass size to limit bytes read; default reads the entire line.
• For simple iteration, prefer for line in file over a manual while loop.
• When you need all lines in memory, readlines() is convenient.
• Always close files; the with statement handles this automatically.