How to Convert a Python String to int

Representing Integers in Python

An integer can be stored using different types. Two possible Python data types for representing an integer are:

1. str
2. int

For example, you can represent an integer using a string literal:

>>> s = "110"

Here, Python understands you to mean that you want to store the integer 110 as a string. You can do the same with the integer data type:

>>> i = 110

It’s important to consider what you specifically mean by "110" and 110 in the examples above. As a human who has used the decimal number system for your whole life, it may be obvious that you mean the number one hundred and ten. However, there are several other number systems, such as binary and hexadecimal, which use different bases to represent an integer.

For example, you can represent the number one hundred and ten in binary and hexadecimal as 1101110 and 6e respectively.

You can also represent your integers with other number systems in Python using the str and int data types:

>>> binary = 0b1010
>>> hexadecimal = "0xa"

Notice that binary and hexadecimal use prefixes to identify the number system. All integer prefixes are in the form 0?, in which you replace ? with a character that refers to the number system:

• b: binary (base 2)
• o: octal (base 8)
• d: decimal (base 10)
• x: hexadecimal (base 16)

>>> decimal = 303
>>> hexadecimal_with_prefix = 0x12F
>>> hexadecimal_no_prefix = 12F
  File "<stdin>", line 1
    hexadecimal_no_prefix = 12F
                              ^
SyntaxError: invalid syntax

>>> decimal = "303"
>>> hexadecimal_with_prefix = "0x12F"
>>> hexadecimal_no_prefix = "12F"

Now that you have some foundational knowledge about how to represent integers using str and int, you’ll learn how to convert a Python string to an int.

Converting a Python String to an int

If you have a decimal integer represented as a string and you want to convert the Python string to an int, then you just pass the string to int(), which returns a decimal integer:

>>> int("10")
10
>>> type(int("10"))
<class 'int'>

By default, int() assumes that the string argument represents a decimal integer. If, however, you pass a hexadecimal string to int(), then you’ll see a ValueError:

>>> int("0x12F")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: invalid literal for int() with base 10: '0x12F'

The error message says that the string is not a valid decimal integer.

>>> binary = "11010010"
>>> int(binary)  # Using the default base of 10, instead of 2
11010010

>>> int("0b11010010")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: invalid literal for int() with base 10: '0b11010010'

When you pass a string to int(), you can specify the number system that you’re using to represent the integer. The way to specify the number system is to use base:

>>> int("0x12F", base=16)
303

Now, int() understands you are passing a hexadecimal string and expecting a decimal integer.

>>> int("10", base=3)
3

Great! Now that you’re comfortable with the ins and outs of converting a Python string to an int, you’ll learn how to do the inverse operation.

Converting a Python int to a String

In Python, you can convert a Python int to a string using str():

>>> str(10)
'10'
>>> type(str(10))
<class 'str'>

By default, str() behaves like int() in that it results in a decimal representation:

>>> str(0b11010010)
'210'

In this example, str() is smart enough to interpret the binary literal and convert it to a decimal string.

If you want a string to represent an integer in another number system, then you use a formatted string, such as an f-string (in Python 3.6+), and an option that specifies the base:

>>> octal = 0o1073
>>> f"{octal}"  # Decimal
'571'
>>> f"{octal:x}"  # Hexadecimal
'23b'
>>> f"{octal:b}"  # Binary
'1000111011'

str is a flexible way to represent an integer in a variety of different number systems.

Conclusion

Congratulations! You’ve learned so much about integers and how to represent and convert them between Python string and int data types.

In this tutorial, you learned:

• How to use str and int to store integers
• How to specify an explicit number system for an integer representation
• How to convert a Python string to an int
• How to convert a Python int to a string

Now that you know so much about str and int, you can learn more about representing numerical types using float(), hex(), oct(), and bin()!