Convert binary string to bytearray in Python 3

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Here’s an example of doing it the first way that Patrick mentioned: convert the bitstring to an int and take 8 bits at a time. The natural way to do that generates the bytes in reverse order. To get the bytes back into the proper order I use extended slice notation on the bytearray with a step of -1: b[::-1].

def bitstring_to_bytes(s):
    v = int(s, 2)
    b = bytearray()
    while v:
        b.append(v & 0xff)
        v >>= 8
    return bytes(b[::-1])

s = "0110100001101001"
print(bitstring_to_bytes(s))

Clearly, Patrick’s second way is more compact. 🙂

However, there’s a better way to do this in Python 3: use the int.to_bytes method:

def bitstring_to_bytes(s):
    return int(s, 2).to_bytes((len(s) + 7) // 8, byteorder="big")

If len(s) is guaranteed to be a multiple of 8, then the first arg of .to_bytes can be simplified:

return int(s, 2).to_bytes(len(s) // 8, byteorder="big")

This will raise OverflowError if len(s) is not a multiple of 8, which may be desirable in some circumstances.

Another option is to use double negation to perform ceiling division. For integers a & b, floor division using //

n = a // b

gives the integer n such that
n <= a/b < n + 1
Eg,
47 // 10 gives 4, and

-47 // 10 gives -5. So

-(-47 // 10) gives 5, effectively performing ceiling division.

Thus in bitstring_to_bytes we could do:

return int(s, 2).to_bytes(-(-len(s) // 8), byteorder="big")

However, not many people are familiar with this efficient & compact idiom, so it’s generally considered to be less readable than

return int(s, 2).to_bytes((len(s) + 7) // 8, byteorder="big")

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