Python int() Function

Python’s built-in int(value) function converts the argument value to an integer number. For example, int('42') converts the string value '42' into the integer number 42. The int() function on a float argument rounds down to the closest integer.

Python int() Graphical Explanation
ArgumentvalueA Python object to be converted into an integer number. The value object must have an __int__() method that returns the associated integer number—otherwise a TypeError will be raised.
baseAn optional integer argument base to define the base of the numerical system in the value argument. If you set the base, the value argument must be a string. The base argument determines how the string argument is interpreted.
Return ValueintReturns an integer number after converting the input argument value using its required __int__() method for the conversion.
Input : int('42')
Output : 42

Input : int('-42')
Output : -42

Input : int(0)
Output : 0

Input : int(3.41)
Output : 3

Input : int(3.51)
Output : 3

Input : int('101', base=2)
Output : 5

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Python int() Rounding

The int(x) function on a float value x rounds down to the closest integer.

>>> int(3.41)
3
>>> int(3.51)
3
>>> int(3.9999)
3

Python int() Custom Object

To allow a custom object as an input to the int(object) function, the object must implement the __int__(self) dunder method that returns an integer value. In fact, the int(object) built-in function is semantically equivalent to the object.__int__() function call.

class Car:
    def __int__(self):
        return -1


porsche = Car()
print(int(porsche))
# -1

In the example, you create a class Car and implement the __init__(self) method that always returns the integer -1. Thus, you can pass a Car object porsche into the int() function and Python doesn’t throw an exception.

Speaking of which…

Python int() Exception

If you pass an object into the int() function that doesn’t overwrite the __int__() method—for example, a list, tuple, or set—Python throws a TypeError:

class Car:
    None


porsche = Car()
print(int(porsche))

This leads to the error message:

Traceback (most recent call last):
  File "C:\Users\...\code.py", line 6, in <module>
    print(int(porsche))
TypeError: int() argument must be a string, a bytes-like object or a number, not 'Car'

To fix the error, either pass an object that is convertible to an integer or implement your own __int__(self) method as shown previously:

class Car:
    def __int__(self):
        return -1


porsche = Car()
print(int(porsche))
# -1

Note that the same TypeError appears if you try to convert lists, sets, dictionaries, or tuples to integer values using the int() function.

Lists:

>>> int([1, 2, 3])
Traceback (most recent call last):
  File "<pyshell#10>", line 1, in <module>
    int([1, 2, 3])
TypeError: int() argument must be a string, a bytes-like object or a number, not 'list'

Sets:

>>> int({1, 2, 3})
Traceback (most recent call last):
  File "<pyshell#11>", line 1, in <module>
    int({1, 2, 3})
TypeError: int() argument must be a string, a bytes-like object or a number, not 'set'

Dictionaries:

>>> int({'Alice': 23, 'Bob': 17})
Traceback (most recent call last):
  File "<pyshell#12>", line 1, in <module>
    int({'Alice': 23, 'Bob': 17})
TypeError: int() argument must be a string, a bytes-like object or a number, not 'dict'

Tuples:

>>> int((1, 2, 3))
Traceback (most recent call last):
  File "<pyshell#13>", line 1, in <module>
    int((1, 2, 3))
TypeError: int() argument must be a string, a bytes-like object or a number, not 'tuple'

Python int() base Argument

The int(value, base=10) function has an optional argument base that allows you to define the base of the numerical system in the value argument. If you set the base, the input must be a string. The base argument determines how the string argument is interpreted. For example, int('10', base=2) converts the string '10' —representing a binary number—to the integer 2.

>>> int('10', base=2)
2

Summary

Python’s built-in int(value) function converts the argument value to an integer number.

For example, int('42') converts the string value '42' into the integer number 42.

>>> int('42')
42

The int() function on a float argument rounds down to the closest integer.

>>> int(3.99)
3

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