file.readline() reads and returns a single line from an open file, including its trailing newline character, or an empty string at the end of the file.

Syntax

f.readline()
f.readline(size)

Deep Dive Course

Each call to readline() reads from the file's current position up to and including the next newline character, or up to the end of the file if there's no more newline, advancing the position so the next call returns the following line. Unlike iterating over the file directly with a for loop, calling readline() explicitly gives you precise, one-line-at-a-time control — useful for skipping a header line, or reading lines conditionally. At the end of the file, readline() returns an empty string, which is distinct from a blank line, which returns just a newline character.

1Understanding readline()

💡

Reaching the end of the file returns an empty string from readline(), while a genuinely blank line in the file returns just a newline character — checking that the result is falsy correctly distinguishes end-of-file from a blank line, since an empty string is falsy but a lone newline character is truthy.

editor.html

with open("log.txt", "w") as f:
    f.write("First line\nSecond line\n")

with open("log.txt", "r") as f:
    print(repr(f.readline()))
    print(repr(f.readline()))
    print(repr(f.readline()))

localhost:3000

2Practical Example

Here is a real-world application of readline() showing how it is used in production Python code.

editor.html

with open("data.csv", "w") as f:
    f.write("name,age\nAlice,30\nBob,25\n")

with open("data.csv", "r") as f:
    header = f.readline()
    for line in f:
        print(line.strip())

localhost:3000

3Best Practices

Follow these guidelines when working with readline():

1. Prefer iterating over a file object directly (for line in f:) over manually calling readline() in a loop, unless you need precise control over exactly how many lines to read

2. Use readline() to explicitly skip or inspect a header line before processing the rest of a file in a loop

3. Check for a falsy result to detect end-of-file, remembering that a blank line and end-of-file are different

⚠️

Tip: Reaching the end of the file returns an empty string from readline(), while a genuinely blank line in the file returns just a newline character — checking that the result is falsy correctly distinguishes end-of-file from a blank line, since an empty string is falsy but a lone newline character is truthy.

editor.html

with open("log.txt", "w") as f:
    f.write("First line\nSecond line\n")

with open("log.txt", "r") as f:
    print(repr(f.readline()))
    print(repr(f.readline()))
    print(repr(f.readline()))

localhost:3000

Examples

Example 01Basic Usage

with open("log.txt", "w") as f:
    f.write("First line\nSecond line\n")

with open("log.txt", "r") as f:
    print(repr(f.readline()))
    print(repr(f.readline()))
    print(repr(f.readline()))

Example 02Advanced Example

with open("data.csv", "w") as f:
    f.write("name,age\nAlice,30\nBob,25\n")

with open("data.csv", "r") as f:
    header = f.readline()
    for line in f:
        print(line.strip())

Best Practices

Prefer iterating over a file object directly (for line in f:) over manually calling readline() in a loop, unless you need precise control over exactly how many lines to read
Use readline() to explicitly skip or inspect a header line before processing the rest of a file in a loop
Check for a falsy result to detect end-of-file, remembering that a blank line and end-of-file are different

Interview Question

How do you distinguish between a genuinely blank line in a file and reaching the end of the file, when using readline()?▼

Hint: Think about what each situation actually returns.

A blank line in the file still contains a newline character, so readline() returns a string containing just that newline for it, which is truthy. Reaching the actual end of the file, with nothing left to read, makes readline() return a completely empty string, which is falsy. So checking for a falsy result correctly detects end-of-file specifically, without misidentifying a blank line as the end of the file.

Exercises

MediumPractice using readline() in a real scenario.

View Solution

with open("log.txt", "w") as f:
    f.write("First line
Second line
")

with open("log.txt", "r") as f:
    print(repr(f.readline()))
    print(repr(f.readline()))
    print(repr(f.readline()))

Frequently Asked Questions

How do you distinguish between a genuinely blank line in a file and reaching the end of the file, when using readline()?▼

Related Functions

read()with-statementfor-loop