How do I represent and work with n-bit vectors in

Question

In an assignment I am currently working on we need to work with bit vectors, but I am very unsure of how to do this in Python. They should be able to be from 4 bits to 20 bits. I have never worked with bit vector before, but I guess that one would one create arrays of unsigned bytes that you manipulated using the usual AND/OR/XOR operations.

The important restriction here is: I cannot rely on any libraries other than those supplied with standard Python.

I think I know how I would do this in C using arrays of 8 bit unsigned bytes: e.g. to turn the 18th bit of a zeroed array into a one, I would do something like my_bit_array[3] &= 1<<2

But since Python is dynamically typed and does not have a built-in array type, how would I go about doing this in a pythonic way?

And is it possible (how?) to express a bit vector of size 20? I am thinking of perhaps making a 24 bit / 3 byte vector and ignoring the 4 bits.

Answer 1

There is also the pure-Python python-bitstring (with Python 3 support).

Answer 2

A bit dated but I'm going to leave another stdlib option here just for comparison sake. It is also easy to do this using the ctypes module.

For example:

And is it possible (how?) to express a bit vector of size 20 ? I am thinking of perhaps making a 24 bit / 3 byte vector and ignoring the 4 bits.

class Simple(ctypes.LittleEndianStructure):
    _pack_ = 1
    _fields_ = [
                 ('one', ctypes.c_ubyte, 8),
                 ('two', ctypes.c_ubyte, 8),
                 ('three', ctypes.c_ubyte, 8)
               ]

s = Simple(0, 2, 256)
bytearray(s)        # bytearray(b'\x00\x02\x00')
s = Simple(0, 2, 255)
bytearray(s)        # bytearray(b'\x00\x02\xff')

class Simple(ctypes.BigEndianStructure):
    _pack_ = 1
    _fields_ = [
                 ('one', ctypes.c_ubyte, 8),
                 ('two', ctypes.c_ubyte, 8),
                 ('three', ctypes.c_ubyte, 8)
               ]

s = Simple(0, 2, 256)
bytearray(s)        # bytearray(b'\x00\x02\x00')
s = Simple(0, 2, 255)
bytearray(s)        # bytearray(b'\x00\x02\xff')

s.two |= 3
bytearray(s)        # bytearray(b'\x00\x03\xff')

or something more straight forward like this:

class bit_vector(Structure):
    _fields_ = [('bits', c_uint32, 24),
                ('unused', c_uint32, 8),
                ]

bv = bit_vector()
# turn on the 18th bit -- being explicit just to demo it
bv.bits |= int('000000000000000001000000', 2)
bin(bv.bits)   # 0b1000000

Answer 3

It has lists, which you can populate with bools:

[False] * 20

Answer 4

The bitarray module does this efficiently with booleans.

Answer 5

The library BitVector is a pure-Python library for this purpose, and should suit the needs you specified.

Answer 6

Use struct module.