This article shows how to do base conversions in Python with the built-in int, how to write integer literals in other bases, and how to do base conversions in general.
In this article I'll assume you know what a number base is and how there are multiple number bases. My focus will be on showing you the tools that Python provides to work with multiple number bases, an in particular with the binary, octal, decimal, and hexadecimal, number bases.
Before diving right into the tools that Python gives you to work with different number bases I want to stress that a number base doesn't change the underlying number we're talking about, only its representation.
For example, many people around the world have three meals per day: breakfast, lunch, and dinner. It doesn't matter whether I write “three” in English, “três” in Portuguese, “3” in the decimal number base, or “11” in the binary number base. We're always talking about the same number of meals: breakfast, lunch, and dinner.
This is very important!
Python (and most programming languages) let you write integer literals in the decimal number base, and we typically don't even think about it. Python also lets you write integer literals in three other bases:
To write an integer literal in any base other than the decimal base, you always start your integer literal with a 0 followed by the letter that identifies the base.
This is summarised in the table below:
| Base | Prefix |
|---|---|
| binary | 0b |
| octal | 0o |
| hexadecimal | 0x |
Thus, all four assignments below create the same integer literal:
svty_three = 73
svty_three_bin = 0b1001001
svty_three_oct = 0o111
svty_three_hex = 0x49Because the base changes the representation but not the number, printing any of the four variables will print 73:
print(svty_three) # 73
print(svty_three_bin) # 73
print(svty_three_oct) # 73
print(svty_three_hex) # 73In any of these bases, you can use the underscore _ to group digits to make the literals more readable.
For example, in the decimal base you can use the underscore _ as the thousands separator:
huge_number = 17_532_546_253_000_000Python contains 3 built-ins that let you convert integers to string representations in the three other bases:
| Base | Built-in |
|---|---|
| binary | bin |
| octal | oct |
| hexadecimal | hex |
Here's example usages of all three:
print(bin(73)) # 0b1001001
print(oct(73)) # 0o111
print(hex(73)) # 0x49Notice that these converting functions include the base prefix in the converted representation!
If you want to use these string representations to represent the number in its base, you can discard the prefix and then convert each digit.
The code below uses a list comprehension and the built-in bin to convert an integer to a list of its binary digits:
>>> [int(digit) for digit in bin(73)[2:]]
[1, 0, 0, 1, 0, 0, 1]The prefix letters for the integer literals can also be used as formatting specifiers when doing string formatting.
Thus, using b, o, and x, will format a number in binary, octal, and hexadecimal, respectively.
These format specifiers work in f-strings:
>>> f"{73:b}"
'1001001'
>>> f"{73:o}"
'111'
>>> f"{73:x}"
'49'But they also work with the string method format:
>>> "{:b}".format(73)
'1001001'
>>> "{:o}".format(73)
'111'
>>> "{:x}".format(73)
'49'(If you're working with legacy code, %o and %x work but %b doesn't.
If you don't know what these % are for, excellent!
Don't even bother...)
int for base conversionThe built-in int is your one-stop shop for converting numbers in different bases to integers.
By default, it assumes you're trying to convert strings to integers in base 10 and it won't work with integers represented in other bases:
>>> int("73")
73
>>> int("0b1001001")
ValueError: invalid literal for int() with base 10: '0b1001001'However, the built-in int has a parameter base that you can use to specify any base from 2 to 36:
>>> int("0b1001001", base=2)
73
>>> int("0o111", base=8)
73
>>> int("0x49", base=16)
73Notice that the base prefixes are not required:
>>> int("0b1001001", base=2) # With `0b` prefix.
73
>>> int("1001001", base=2) # Without.
73Like I said, the base can be any base from 2 to 36, it doesn't need to be one of 2, 8, 10, or 16:
>>> int("rodrigo", base=36) # Fun(ky)!
60247429944The parameter base also accepts the value 0, which is a special value and that tells int to guess the base from the base prefix:
>>> int("0b1001001", base=0)
73
>>> int("0o111", base=0)
73
>>> int("0x49", base=0)
73The table below shows examples of working with the binary, octal, and hexadecimal bases for the integer 73, summarising everything we've seen so far.
| Base | Base prefix | Literal notation | String formatting | Convert to integer |
|---|---|---|---|---|
| binary | 0b |
0b1001001 |
f"{73:b}" |
int("1001001", base=2)1 |
| octal | 0o |
0o111 |
f"{73:o}" |
int("111", base=8)1 |
| hexadecimal | 0x |
0x49 |
f"{73:x}" |
int("49", base=16)1 |
A number base doesn't have to be base 2, 8, 10, or 16.
A number base can be any integer greater than 1, even if it's greater than 36 (although int only supports up to 36).
The reason why int only supports up to base 36 is because the 10 digits plus the 26 letters of the latin alphabet make up 36 characters, which are typically used to represent digits in bases up to 36.
(For example, colours in hexadecimal are ubiquously represented with the digits 0-9 and the letters A-F.)
We use letters to represent hexadecimal digits just to simplify the writing of numbers in that base and we define that the letters A-F have the values 10-15, respectively.
For hexadecimal, and for any other base, we can represent a number in that base as a list of its digits.
For example, the hexadecimal number 0x89af would be [8, 9, 10, 15].
If we capitalise on this idea, we can write a function that converts any non-negative integer to any base greater than or equal to 2.
That's what the function to_base below does:
def to_base(number, base):
"""Converts a non-negative number to a list of digits in the given base.
The base must be an integer greater than or equal to 2 and the first digit
in the list of digits is the most significant one.
"""
if not number:
return [0]
digits = []
while number:
digits.append(number % base)
number //= base
return list(reversed(digits))
print(to_base(73, 10)) # [7, 3]
print(to_base(73, 2)) # [1, 0, 0, 1, 0, 0, 1]
print(to_base(73, 8)) # [1, 1, 1]
print(to_base(73, 16)) # [4, 9]
# Also works with other bases:
print(to_base(73, 11)) # [6, 7]
print(to_base(753_849, 1000)) # [753, 849]
print(to_base(753_849, 100)) # [75, 38, 49]
print(to_base(60_247_429_944, 36)) # [27, 24, 13, 27, 18, 16, 24]Similarly, we can write a function that does the reverse calculation.
The function from_base, shown below, receives a list of digits and a base and computes the integer that's represented by those digits:
def from_base(digits, base):
"""Converts a list of digits in the given base to an integer.
The first digit is the most significant and the base is assumed to
be an integer greater than or equal to 2.
"""
power = 1
number = 0
for digit in reversed(digits):
number += power * digit
power *= base
return number
from_base([7, 3], 10) # 73
from_base([1, 0, 0, 1, 0, 0, 1], 2) # 73
from_base([1, 1, 1], 8) # 73
from_base([4, 9], 16) # 73
from_base([6, 7], 11) # 73
from_base([75, 38, 49], 100) # 753_849
from_base([753, 849], 1000) # 753_849
from_base([27, 24, 13, 27, 18, 16, 24], 36) # 60247429944+35 chapters. +400 pages. Hundreds of examples. Over 30,000 readers!
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int, https://docs.python.org/3/library/functions.html#int [last accessed 04-01-2024];