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Creation

You can create bitstrings in a variety of ways. Internally they are stored as byte arrays, which means that no space is wasted, and a bitstring containing 10MB of binary data will only take up 10MB of memory.

The bitstring classes

Four classes are provided by the bitstring module: BitStream and BitArray together with their immutable versions ConstBitStream and Bits:

  • Bits (object): This is the most basic class. It is immutable and so its contents can’t be changed after creation.
  • BitArray (Bits): This adds mutating methods to its base class.
  • ConstBitStream (Bits): This adds methods and properties to allow the bits to be treated as a stream of bits, with a bit position and reading/parsing methods.
  • BitStream (BitArray, ConstBitStream): This is the most versative class, having both the bitstream methods and the mutating methods.

Before verion 3.0 Bits was known as ConstBitArray. The old name is still available for backward compatibility.

The term ‘bitstring’ is used in this manual to refer generically to any of these classes.

Most of the exampes in this manual use the BitArray class, with BitStream used when necessary. For most uses the non-const classes are more versatile and so probably your best choice when starting to use the module.

To summarise when to use each class:

  • If you need to change the contents of the bitstring then you must use BitArray or BitStream. Truncating, replacing, inserting, appending etc. are not available for the const classes.
  • If you need to use a bitstring as the key in a dictionary or as a member of a set then you must use Bits or a ConstBitStream. As BitArray and BitStream objects are mutable they do not support hashing and so cannot be used in these ways.
  • If you are creating directly from a file then a BitArray or BitStream will read the file into memory whereas a Bits or ConstBitStream will not, so using the const classes allows extremely large files to be examined.
  • If you don’t need the extra functionality of a particular class then the simpler ones might be faster and more memory efficient. The fastest and most memory efficient class is Bits.

The Bits class is the base class of the other three class. This means that isinstance(s, Bits) will be true if s is an instance of any of the four classes.

Using the constructor

When initialising a bitstring you need to specify at most one initialiser. These will be explained in full below, but briefly they are:

  • auto : Either a specially formatted string, a list or tuple, a file object, integer, bytearray, bytes or another bitstring.
  • bytes : A bytes object (a str in Python 2.6), for example read from a binary file.
  • hex, oct, bin: Hexadecimal, octal or binary strings.
  • int, uint: Signed or unsigned bit-wise big-endian binary integers.
  • intle, uintle: Signed or unsigned byte-wise little-endian binary integers.
  • intbe, uintbe: Signed or unsigned byte-wise big-endian binary integers.
  • intne, uintne: Signed or unsigned byte-wise native-endian binary integers.
  • float / floatbe, floatle, floatne: Big, little and native endian floating point numbers.
  • se, ue : Signed or unsigned exponential-Golomb coded integers.
  • sie, uie : Signed or unsigned interleaved exponential-Golomb coded integers.
  • bool : A boolean (i.e. True or False).
  • filename : Directly from a file, without reading into memory.

From a hexadecimal string

>>> c = BitArray(hex='0x000001b3')
>>> c.hex
'000001b3'

The initial 0x or 0X is optional. Whitespace is also allowed and is ignored. Note that the leading zeros are significant, so the length of c will be 32.

If you include the initial 0x then you can use the auto initialiser instead. As it is the first parameter in __init__ this will work equally well:

c = BitArray('0x000001b3')

From a binary string

>>> d = BitArray(bin='0011 00')
>>> d.bin
'001100'

An initial 0b or 0B is optional and whitespace will be ignored.

As with hex, the auto initialiser will work if the binary string is prefixed by 0b:

>>> d = BitArray('0b001100')

From an octal string

>>> o = BitArray(oct='34100')
>>> o.oct
'34100'

An initial 0o or 0O is optional, but 0o (a zero and lower-case ‘o’) is preferred as it is slightly more readable.

As with hex and bin, the auto initialiser will work if the octal string is prefixed by 0o:

>>> o = BitArray('0o34100')

From an integer

>>> e = BitArray(uint=45, length=12)
>>> f = BitArray(int=-1, length=7)
>>> e.bin
'000000101101'
>>> f.bin
'1111111'

For initialisation with signed and unsigned binary integers (int and uint respectively) the length parameter is mandatory, and must be large enough to contain the integer. So for example if length is 8 then uint can be in the range 0 to 255, while int can range from -128 to 127. Two’s complement is used to represent negative numbers.

The auto initialise can be used by giving a colon and the length in bits immediately after the int or uint token, followed by an equals sign then the value:

>>> e = BitArray('uint:12=45')
>>> f = BitArray('int:7=-1')

The plain int and uint initialisers are bit-wise big-endian. That is to say that the most significant bit comes first and the least significant bit comes last, so the unsigned number one will have a 1 as its final bit with all other bits set to 0. These can be any number of bits long. For whole-byte bitstring objects there are more options available with different endiannesses.

Big and little-endian integers

>>> big_endian = BitArray(uintbe=1, length=16)
>>> little_endian = BitArray(uintle=1, length=16)
>>> native_endian = BitArray(uintne=1, length=16)

There are unsigned and signed versions of three additional ‘endian’ types. The unsigned versions are used above to create three bitstrings.

The first of these, big_endian, is equivalent to just using the plain bit-wise big-endian uint initialiser, except that all intbe or uintbe interpretations must be of whole-byte bitstrings, otherwise a ValueError is raised.

The second, little_endian, is interpreted as least significant byte first, i.e. it is a byte reversal of big_endian. So we have:

>>> big_endian.hex
'0001'
>>> little_endian.hex
'0100'

Finally we have native_endian, which will equal either big_endian or little_endian, depending on whether you are running on a big or little-endian machine (if you really need to check then use import sys; sys.byteorder).

From a floating point number

>>> f1 = BitArray(float=10.3, length=32)
>>> f2 = BitArray('float:64=5.4e31')

Floating point numbers can be used for initialisation provided that the bitstring is 32 or 64 bits long. Standard Python floating point numbers are 64 bits long, so if you use 32 bits then some accuracy could be lost.

Note that the exact bits used to represent the floating point number could be platform dependent. Most PCs will conform to the IEEE 754 standard, and presently other floating point representations are not supported (although they should work on a single platform - it just might get confusing if you try to interpret a generated bitstring on another platform).

Similar to the situation with integers there are big and little endian versions. The plain float is big endian and so floatbe is just an alias.

As with other initialisers you can also auto initialise, as demonstrated with the second example below:

>>> little_endian = BitArray(floatle=0.0, length=64)
>>> native_endian = BitArray('floatne:32=-6.3')

Exponential-Golomb codes

Initialisation with integers represented by exponential-Golomb codes is also possible. ue is an unsigned code while se is a signed code. Interleaved exponential-Golomb codes are also supported via uie and sie:

>>> g = BitArray(ue=12)
>>> h = BitArray(se=-402)
>>> g.bin
'0001101'
>>> h.bin
'0000000001100100101'

For these initialisers the length of the bitstring is fixed by the value it is initialised with, so the length parameter must not be supplied and it is an error to do so. If you don’t know what exponential-Golomb codes are then you are in good company, but they are quite interesting, so I’ve included a section on them (see Exponential-Golomb Codes).

The auto initialiser may also be used by giving an equals sign and the value immediately after a ue or se token:

>>> g = BitArray('ue=12')
>>> h = BitArray('se=-402')

You may wonder why you would bother with auto in this case as the syntax is slightly longer. Hopefully all will become clear in the next section.

From raw byte data

Using the length and offset parameters to specify the length in bits and an offset at the start to be ignored is particularly useful when initialising from raw data or from a file.

a = BitArray(bytes=b'\x00\x01\x02\xff', length=28, offset=1)
b = BitArray(bytes=open("somefile", 'rb').read())

The length parameter is optional; it defaults to the length of the data in bits (and so will be a multiple of 8). You can use it to truncate some bits from the end of the bitstring. The offset parameter is also optional and is used to truncate bits at the start of the data.

You can also use a bytearray object, either explicitly with a bytes=some_bytearray keyword or via the auto initialiser:

c = BitArray(a_bytearray_object)

If you are using Python 3.x you can use this trick with bytes objects too. This should be used with caution as in Python 2 it will instead be interpreted as a string (it’s not possible to distinguish between str and bytes in Python 2) and so your code won’t work the same between Python versions.

d = BitArray(b'\x23g$5')   # Use with caution! Only works correctly in Python 3.

From a file

Using the filename initialiser allows a file to be analysed without the need to read it all into memory. The way to create a file-based bitstring is:

p = Bits(filename="my2GBfile")

This will open the file in binary read-only mode. The file will only be read as and when other operations require it, and the contents of the file will not be changed by any operations. If only a portion of the file is needed then the offset and length parameters (specified in bits) can be used.

Note that we created a Bits here rather than a BitArray, as they have quite different behaviour in this case. The immutable Bits will never read the file into memory (except as needed by other operations), whereas if we had created a BitArray then the whole of the file would immediately have been read into memory. This is because in creating a BitArray you are implicitly saying that you want to modify it, and so it needs to be in memory.

It’s also possible to use the auto initialiser for file objects. It’s as simple as:

f = open('my2GBfile', 'rb')
p = Bits(f)

The auto initialiser

The auto parameter is the first parameter in the __init__ function and so the auto= can be omitted when using it. It accepts either a string, an iterable, another bitstring, an integer, a bytearray or a file object.

Strings starting with 0x or hex: are interpreted as hexadecimal, 0o or oct: implies octal, and strings starting with 0b or bin: are interpreted as binary. You can also initialise with the various integer initialisers as described above. If given another bitstring it will create a copy of it, (non string) iterables are interpreted as boolean arrays and file objects acts a source of binary data. Finally you can use an integer to create a zeroed bitstring of that number of bits.

>>> fromhex = BitArray('0x01ffc9')
>>> frombin = BitArray('0b01')
>>> fromoct = BitArray('0o7550')
>>> fromint = BitArray('int:32=10')
>>> fromfloat = BitArray('float:64=0.2')
>>> acopy = BitArray(fromoct)
>>> fromlist = BitArray([1, 0, 0])
>>> f = open('somefile', 'rb')
>>> fromfile = BitArray(f)
>>> zeroed = BitArray(1000)
>>> frombytes = BitArray(bytearray(b'xyz'))

It can also be used to convert between the BitArray and Bits classes:

>>> immutable = Bits('0xabc')
>>> mutable = BitArray(immutable)
>>> mutable += '0xdef'
>>> immutable = Bits(mutable)

As always the bitstring doesn’t know how it was created; initialising with octal or hex might be more convenient or natural for a particular example but it is exactly equivalent to initialising with the corresponding binary string.

>>> fromoct.oct
'7550'
>>> fromoct.hex
'f68'
>>> fromoct.bin
'111101101000'
>>> fromoct.uint
3994
>>> fromoct.int
-152

>>> BitArray('0o7777') == '0xfff'
True
>>> BitArray('0xf') == '0b1111'
True
>>> frombin[::-1] + '0b0' == fromlist
True

Note how in the final examples above only one half of the == needs to be a bitstring, the other half gets auto initialised before the comparison is made. This is in common with many other functions and operators.

You can also chain together string initialisers with commas, which causes the individual bitstrings to be concatenated.

>>> s = BitArray('0x12, 0b1, uint:5=2, ue=5, se=-1, se=4')
>>> s.find('uint:5=2, ue=5')
True
>>> s.insert('0o332, 0b11, int:23=300', 4)

Again, note how the format used in the auto initialiser can be used in many other places where a bitstring is needed.