Python bin() Function: Convert Integers to Binary

Need the bits behind a number? The Python bin() function takes an integer and gives back its binary form as a string that starts with 0b. Negative numbers get a minus sign in front, and floats raise a TypeError:

bin(10)      # '0b1010'
bin(-10)     # '-0b1010'
f"{10:08b}"  # '00001010' (no prefix, padded)

Every output below comes from Command Prompt runs on Python 3.12.5, copied as printed.

In this tutorial, we’ll look at what bin() returns, how to drop the prefix and pad the result, how negatives behave, and which inputs fail.

What does the Python bin() function return?

bin(x) always returns a str. The input can be any integer, including hex or octal literals and True, since a bool is a subclass of int:

for n in [10, 255, 0, -10, True]:
    print(f"bin({n!r:>5}) -> {bin(n)}")

print(type(bin(10)))
print(bin(0x1F), bin(0o17))      # hex and octal literals are just ints

Output:

bin(   10) -> 0b1010
bin(  255) -> 0b11111111
bin(    0) -> 0b0
bin(  -10) -> -0b1010
bin( True) -> 0b1
<class 'str'>
0b11111 0b1111

Because the result is text, you can slice it, count its 1 characters, or print it. Going the other direction, from a number you typed to its bits, is covered step by step in converting decimal numbers to binary in Python.

Remove the 0b prefix and pad bin() output

The 0b prefix is handy for reading but gets in the way when you need fixed-width bits. Here are the usual fixes side by side:

n = 10
print(bin(n)[2:])                 # slice off 0b
print(bin(n).removeprefix("0b"))  # clearer, Python 3.9+
print(format(n, "b"))             # no prefix at all
print(f"{n:08b}")                 # 8 digits, zero-padded
print(f"{n:#010b}")               # keep 0b, 10 chars total
print(bin(n)[2:].zfill(8))

print(bin(-10)[2:])               # slicing breaks on negatives
print(format(-10, "08b"))

Output:

1010
1010
1010
00001010
0b00001010
00001010
b1010
-0001010

I use f"{n:08b}" almost every time because it pads and drops the prefix in one step. On negatives, slicing bin(-10) leaves a stray b, while format() handles the sign. More padding options are in padding numbers with leading zeros in Python.

How does Python bin() handle negative numbers?

Python ints have no fixed width, so bin() doesn’t show two’s complement. It prints a minus sign plus the bits of the absolute value. To get the bit pattern a C program or a hardware register would hold, mask the number to a width first:

n = -10
print("bin(-10):        ", bin(n))
print("8-bit two's comp:", format(n & 0xFF, "08b"))
print("16-bit:          ", format(n & 0xFFFF, "016b"))

print(int("0b1010", 2), int("1010", 2), int("0b1010", 0))
print("bit_length(255):", (255).bit_length())
print("bit_count(255): ", (255).bit_count())   # Python 3.10+

Output:

bin(-10):         -0b1010
8-bit two's comp: 11110110
16-bit:           1111111111110110
10 10 10
bit_length(255): 8
bit_count(255):  8

int(text, 2) reverses bin(), and base 0 reads the prefix for you. See converting a binary string to int in Python for input checks, and checking if a string is binary before parsing user input.

Why does bin() raise a TypeError for floats and strings?

bin() only accepts integers, or objects that define __index__() to say which integer they stand for. A float or a string of digits fails:

for value in [3.5, "10"]:
    try:
        bin(value)
    except TypeError as e:
        print(f"bin({value!r}): TypeError: {e}")

class Seat:
    def __init__(self, row):
        self.row = row
    def __index__(self):
        return self.row

print("bin(Seat(12)):", bin(Seat(12)))
print("bin(int(3.5)):", bin(int(3.5)))

Output:

bin(3.5): TypeError: 'float' object cannot be interpreted as an integer
bin('10'): TypeError: 'str' object cannot be interpreted as an integer
bin(Seat(12)): 0b1100
bin(int(3.5)): 0b11

Convert first: int(3.5) truncates to 3, and int("10") parses text. If you’re adding bit strings together rather than numbers, adding two binary numbers in Python shows the pattern.

Want to keep going with number formats? Try these next:

• Convert binary to decimal in Python
• Format a number with commas in Python
• Pad a string with zeros in Python

Frequently asked questions