int()

int(x=0) / int(x, base=10)

Returns: int · Added in v3.0 · Updated March 13, 2026 · Built-in Functions

built-in conversion numbers typecasting

The int() function converts a number or string to an integer. It is one of Python’s most frequently used built-in functions, essential for type conversion, parsing user input, and working with numerical data. When given a float, int() truncates toward zero (it does not round).

Syntax

int(x=0)
int(x, base=10)

Parameters

Parameter	Type	Default	Description
`x`	`int` \| `float` \| `str` \| `bytes` \| `bytearray`	`0`	A number, string, or byte-like object to convert to an integer. Defaults to 0 if omitted.
`base`	`int`	`10`	The base (radix) for string conversion. Must be between 2 and 36. Only valid when `x` is a string.

Returns: An integer object. If x is already an int, returns x unchanged (not a copy).

Examples

Converting from float

When converting from a float, int() truncates toward zero rather than rounding:

# Float truncation (toward zero)
print(int(3.7))
# 3

print(int(-3.7))
# -3

# Note: this is NOT rounding
print(int(3.5))
# 3
print(int(3.999))
# 3

Converting from string

Strings can be converted to integers. Leading and trailing whitespace is ignored:

# Basic string to int
print(int("42"))
# 42

# With whitespace
print(int("  100  "))
# 100

# Negative numbers in strings
print(int("-273"))
# -273

Converting with different bases

Use the base parameter to parse strings in different numeral systems:

# Binary (base 2)
print(int("1010", 2))
# 10

# Octal (base 8)
print(int("755", 8))
# 493

# Hexadecimal (base 16)
print(int("FF", 16))
# 255

# Case-insensitive for bases > 10
print(int("ff", 16))
# 255

# Base 36 (uses 0-9 and a-z)
print(int("z", 36))
# 35

print(int("hello", 36))
# 1836316

Converting from bytes

Bytes and bytearray objects can be converted using a specific encoding:

# Bytes to int
print(int(b"42"))
# 42

Using the default value

When called with no arguments, int() returns 0:

# No argument defaults to 0
print(int())
# 0

Boolean conversion

Booleans are a subclass of int, but int() explicitly converts them:

# True becomes 1, False becomes 0
print(int(True))
# 1
print(int(False))
# 0

Common Patterns

Parsing user input

User input always comes as a string—use int() to convert it:

# Simple age parser
user_input = "25"
age = int(user_input)
print(age + 1)
# 26

# Handling potential errors
def safe_int_parse(value):
    try:
        return int(value)
    except ValueError:
        return None

print(safe_int_parse("42"))
# 42
print(safe_int_parse("not a number"))
# None

Working with file data

When reading numerical data from files, convert strings to integers:

# Simulated file content
lines = ["100", "200", "300"]
numbers = [int(line) for line in lines]
print(sum(numbers))
# 600

Base conversion utilities

Create your own base conversion helpers:

def from_base(encoded, base):
    return int(encoded, base)

def to_base(number, base):
    if number == 0:
        return "0"
    digits = "0123456789abcdefghijklmnopqrstuvwxyz"
    result = []
    while number > 0:
        result.append(digits[number % base])
        number //= base
    return "".join(reversed(result))

print(from_base("FF", 16))
# 255
print(to_base(255, 16))
# ff

Rounding vs truncation

Remember that int() truncates, not rounds. For rounding, use round():

# int() truncates
print(int(3.9))
# 3

# round() rounds to nearest
print(round(3.9))
# 4
print(round(3.5))
# 4
print(round(3.499))
# 3

Errors

ValueError: invalid literal for int()

# Non-numeric string
int("3.14")
# ValueError: invalid literal for int() with base 10: '3.14'

# Use float() first
int(float("3.14"))
# 3

ValueError: int() base must be >= 2

# Invalid base
int("42", 1)
# ValueError: int() base must be >= 2

# Valid bases are 2-36
int("42", 2)   # OK
int("42", 36)  # OK

TypeError: int() argument must be a string

# Lists cannot be converted directly
int([42])
# TypeError: int() argument must be a string, a bytes-like object or a number, not 'list'

# For containers, extract the value first
int(["42"][0])
# 42