Does Python have a bitfield type?

I need a compact representation of an array of booleans, does Python have a builtin bitfield type or will I need to find an alternate solution?

If you mainly want to be able to name your bit fields and easily manipulate them, e.g. to work with flags represented as single bits in a communications protocol, then you can use the standard Structure and Union features of ctypes, as described at How Do I Properly Declare a ctype Structure + Union in Python? - Stack Overflow

For example, to work with the 4 least-significant bits of a byte individually, just name them from least to most significant in a LittleEndianStructure. You use a union to provide access to the same data as a byte or int so you can move the data in or out of the communication protocol. In this case that is done via the flags.asbyte field:

import ctypes
c_uint8 = ctypes.c_uint8

class Flags_bits(ctypes.LittleEndianStructure):
    _fields_ = [
            ("logout", c_uint8, 1),
            ("userswitch", c_uint8, 1),
            ("suspend", c_uint8, 1),
            ("idle", c_uint8, 1),
        ]

class Flags(ctypes.Union):
    _fields_ = [("b", Flags_bits),
                ("asbyte", c_uint8)]

flags = Flags()
flags.asbyte = 0xc

print(flags.b.idle)
print(flags.b.suspend)
print(flags.b.userswitch)
print(flags.b.logout)

The four bits (which I've printed here starting with the most significant, which seems more natural when printing) are 1, 1, 0, 0, i.e. 0xc in binary.

Bitarray was the best answer I found, when I recently had a similar need. It's a C extension (so much faster than BitVector, which is pure python) and stores its data in an actual bitfield (so it's eight times more memory efficient than a numpy boolean array, which appears to use a byte per element.)

You should take a look at the bitstring module, which has recently reached version 2.0. The binary data is compactly stored as a byte array and can be easily created, modified and analysed.

You can create BitString objects from binary, octal, hex, integers (big or little endian), strings, bytes, floats, files and more.

a = BitString('0xed44')
b = BitString('0b11010010')
c = BitString(int=100, length=14)
d = BitString('uintle:16=55, 0b110, 0o34')
e = BitString(bytes='hello')
f = pack('<2H, bin:3', 5, 17, '001')

You can then analyse and modify them with simple functions or slice notation - no need to worry about bit masks etc.

a.prepend('0b110')
if '0b11' in b:
    c.reverse()
g = a.join([b, d, e])
g.replace('0b101', '0x3400ee1')
if g[14]:
    del g[14:17]
else:
    g[55:58] = 'uint:11=33, int:9=-1'

There is also a concept of a bit position, so that you can treat it like a file or stream if that's useful to you. Properties are used to give different interpretations of the bit data.

w = g.read(10).uint
x, y, z = g.readlist('int:4, int:4, hex:32')
if g.peek(8) == '0x00':
    g.pos += 10

Plus there's support for the standard bit-wise binary operators, packing, unpacking, endianness and more. The latest version is for Python 2.7 and 3.x, and although it's pure Python it is reasonably well optimised in terms of memory and speed.

Represent each of your values as a power of two:

testA = 2**0
testB = 2**1
testC = 2**3

Then to set a value true:

table = table | testB

To set a value false:

table = table & (~testC)

To test for a value:

bitfield_length = 0xff
if ((table & testB & bitfield_length) != 0):
    print "Field B set"

Dig a little deeper into hexadecimal representation if this doesn't make sense to you. This is basically how you keep track of your boolean flags in an embedded C application as well (if you have limitted memory).

Does Python have a bitfield type?

Related

Recent Posts