How to Convert Hex String to Integer in Python

How to Convert Hex String to Integer in Python

Problem Formulation

Given a string in hexadecimal form:

s = '0xff'
# or: s = 'ff'

How to convert the hex string to an integer in Python?

For example, you want to convert the hexadecimal string '0xff' to the decimal integer 255.

Here are a few other examples:

0x0   -->   0
0x4   -->   4
0x8   -->   8
0x12   -->   18
0x16   -->   22
0x20   -->   32
0x24   -->   36
0x28   -->   40

Hex String to Integer using int() with base 16

To convert a hexadecimal string to an integer, pass the string as a first argument into Python’s built-in int() function. Use base=16 as a second argument of the int() function to specify that the given string is a hex number. The int() function will then convert the hex string to an integer with base 10 and return the result.

Here’s a minimal example:

>>> int('0xff', base=16)
255

Examples

And here’s how you can convert the additional examples shown above:

>>> int('0x0', base=16)
0
>>> int('0x4', base=16)
4
>>> int('0x8', base=16)
8
>>> int('0x12', base=16)
18
>>> int('0x16', base=16)
22
>>> int('0x20', base=16)
32
>>> int('0x24', base=16)
36
>>> int('0x28', base=16)
40

You actually don’t need to use the prefix '0x' because your second argument already defines unambiguously that the given string is a hexadecimal number:

>>> int('0', base=16)
0
>>> int('4', base=16)
4
>>> int('8', base=16)
8
>>> int('12', base=16)
18
>>> int('16', base=16)
22
>>> int('20', base=16)
32
>>> int('24', base=16)
36
>>> int('28', base=16)
40

However, skipping the base but leaving the prefix raises a ValueError: invalid literal for int() with base 10: '0x28':

>>> int('0x28')
Traceback (most recent call last):
  File "<pyshell#19>", line 1, in <module>
    int('0x28')
ValueError: invalid literal for int() with base 10: '0x28'

It assumes that the input string is in base 10 when in fact, it isn’t.

💡 Note: Even though passing a prefixed string '0x...' into the int() function is unambiguous, Python’s int() function doesn’t accept it if you don’t also define the base. This may be fixed in future versions!

In fact, you can specify the base argument as 0 to switch on base guessing—which should be the default behavior anyway!

Base Guessing

You can pass a prefixed string '0x...' into the int() function and set the base to 0 to switch on base guessing in Python. This uses the prefix to determine the base automatically—without you needing to set it to 16. Yet, you still have to set it to 0 so the benefit is marginal in practice.

>>> int('0x9', base=16)
9
>>> int('0x9', base=0)
9
>>> int('0x9', 0)
9

Converting Hex Literals to Int

If you don’t have a hex string but a hex number—called a literal—such as 0xff, you don’t even need the int() function because Python will automatically convert it to a decimal number:

>>> 0x10
16
>>> 0xff
255

Background int()

Syntax: int(value [, base])    -->   int
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.

Do you still need more background information about Python’s built-in int() function? No problem, read over the related tutorial.

Related Tutorial: Python’s Built-in int() Function

Hex to Int Table

Just for fun, here are the hex to int conversions of the powers of two:

Hexadecimal LiteralDecimal Literal
11
22
44
88
10hex16dec
20hex32dec
40hex64dec
80hex128dec
100hex256dec
200hex512dec
400hex1024dec
800hex2048dec
1000hex4096dec
2000hex8192dec
4000hex16,384dec
8000hex32,768dec
10000hex65,536dec