Python `int()` Function Explained

The Python int() function converts a value into an integer.

Beginners often use int() when they want to:

turn text like "42" into the number 42
convert user input into a whole number
remove the decimal part from a float
read numbers written in binary or hexadecimal

It is a built-in function, so you can use it directly without importing anything.

Quick example

print(int("42"))
print(int(3.9))
print(int("101", 2))

Output:

42
3
5

Use int() to convert a number-like value to an integer. Strings must contain a valid whole number unless you also provide a base.

What `int()` does

int() converts a value into an integer.

Key points:

It can turn strings, floats, and booleans into int values.
It returns a new integer value.
It does not change the original object.

Example:

text = "25"
number = int(text)

print(text)
print(number)
print(type(text))
print(type(number))

Output:

25
25
<class 'str'>
<class 'int'>

The original string stays a string. int() creates a new integer from it.

Basic syntax

int(x)
int(x, base)

`int(x)`

Use this form when you want to convert a value like:

a string such as "10"
a float such as 3.9
a boolean such as True

Example:

print(int("10"))
print(int(8.7))
print(int(True))

Output:

10
8
1

`int(x, base)`

Use this form when x is a string that represents a number in another base.

Common bases:

2 for binary
8 for octal
16 for hexadecimal

Example:

print(int("101", 2))
print(int("17", 8))
print(int("1A", 16))

Output:

5
15
26

If you want to compare integer conversion with decimal conversion, see the float() function.

Using `int()` with strings

int() works with strings that contain a valid integer.

Examples that work:

print(int("5"))
print(int("-12"))
print(int("  42  "))

Output:

5
-12
42

This works because each string contains valid whole-number text. Leading and trailing spaces are usually allowed.

Examples that fail:

# print(int("3.14"))
# print(int("12abc"))

These raise ValueError because the strings are not valid integer text.

Valid string examples

print(int("0"))
print(int("+7"))
print(int("-100"))

Output:

0
7
-100

Invalid string examples

values = ["3.14", "12abc", "", "seven"]

for value in values:
    try:
        print(int(value))
    except ValueError as error:
        print(f"{value!r} -> {error}")

Output:

'3.14' -> invalid literal for int() with base 10: '3.14'
'12abc' -> invalid literal for int() with base 10: '12abc'
'' -> invalid literal for int() with base 10: ''
'seven' -> invalid literal for int() with base 10: 'seven'

If you are working with user input, see how to convert user input to numbers in Python and the input() function.

Using `int()` with floats

When you pass a float to int(), Python removes the decimal part.

Important: int() does not round. It truncates toward zero.

print(int(3.9))
print(int(3.1))
print(int(-3.9))
print(int(-3.1))

Output:

3
3
-3
-3

This behavior matters with negative numbers.

int(3.9) becomes 3
int(-3.9) becomes -3

If you want decimal conversion instead, use float(). If you want text conversion, use str().

Using `int()` with booleans

Booleans can also be converted to integers.

print(int(True))
print(int(False))

Output:

1
0

This can be useful in simple counting logic:

passed = True
total = int(passed)

print(total)

Output:

If you want to learn more about boolean values, see the bool() function.

Using the base argument

Use the base argument when a string represents a number in a different number system.

Binary example

print(int("101", 2))

Output:

"101" in binary means:

1 × 4
0 × 2
1 × 1

So the result is 5.

Hexadecimal example

print(int("1A", 16))

Output:

Important rule

The base argument is for string-like input, not normal numeric values.

Good:

print(int("101", 2))

Not correct:

# int(101, 2)

That causes a TypeError because base should not be used with a normal integer value.

What errors can happen

Two common errors with int() are ValueError and TypeError.

`ValueError`

This happens when a string is not a valid integer.

Example:

try:
    print(int("3.14"))
except ValueError as error:
    print(error)

Output:

invalid literal for int() with base 10: '3.14'

A decimal string like "3.14" does not work directly with int(). You need to convert it to a float first:

print(int(float("3.14")))

Output:

If you see this often, read how to convert string to int in Python and how to fix ValueError: invalid literal for int() with base 10.

`TypeError`

This can happen with unsupported input types.

Example:

try:
    print(int(None))
except TypeError as error:
    print(error)

Output:

int() argument must be a string, a bytes-like object or a real number, not 'NoneType'

Another example is using the base argument with a non-string numeric value:

try:
    print(int(101, 2))
except TypeError as error:
    print(error)

Output:

int() can't convert non-string with explicit base

When to use `int()`

Use int() when you need a whole number.

Common uses:

Convert user input from input() into integers
Turn float results into integers when truncation is acceptable
Parse numeric strings from files or APIs
Convert binary or hexadecimal strings with the base argument

Example with user input:

age_text = "18"
age = int(age_text)

print(age + 1)

Output:

Common mistakes

Here are some common problems beginners run into with int().

Passing a string with decimal text

This fails:

# int("4.5")

Why: "4.5" is not an integer string.

Use this instead:

print(int(float("4.5")))

Passing text with letters

This fails:

# int("12abc")

Why: the string contains non-numeric characters.

Trying `int(None)`

This fails because None is not a number or numeric string.

value = None

try:
    print(int(value))
except TypeError as error:
    print(error)

Using the base argument with a non-string

This fails:

# int(101, 2)

Use a string instead:

print(int("101", 2))

Expecting `int()` to round

This is a very common misunderstanding.

print(int(4.9))

Output:

If you expected 5, remember that int() truncates. It does not round.

FAQ

Does `int()` round numbers?

No. int() removes the decimal part. It does not round to the nearest whole number.

Why does `int("3.14")` fail?

Because "3.14" is not an integer string. Convert it with float() first, then use int() if needed.

What does `int(True)` return?

It returns 1. False becomes 0.

When should I use the base argument?

Use it when the input string is written in another base, such as binary or hexadecimal.

Python int() Function Explained

Python `int()` Function Explained