python - Writing bits to a binary file

Question

I have 23 bits represented as a string, and I need to write this string to a binary file as 4 bytes. The last byte is always 0. The following code works (Python 3.3), but it doesn't feel very elegant (I'm rather new to Python and programming). Do you have any tips of making it better? It seems a for-loop might be useful, but how do I do the slicing within the loop without getting a IndexError? Note that that when I extract the bits into a byte, I reverse the bit-order.

from array import array

bin_array = array("B")
bits = "10111111111111111011110"    #Example string. It's always 23 bits
byte1 = bits[:8][::-1]
byte2 = bits[8:16][::-1]
byte3 = bits[16:][::-1]
bin_array.append(int(byte1, 2))
bin_array.append(int(byte2, 2))
bin_array.append(int(byte3, 2))
bin_array.append(0)

with open("test.bnr", "wb") as f:
    f.write(bytes(bin_array))

# Writes [253, 255, 61, 0] to the file

Answer 1

You can treat it as an int, then create the 4 bytes as follows:

>>> bits = "10111111111111111011110"
>>> int(bits[::-1], 2).to_bytes(4, 'little')
b'xfdxff=x00'