Bitstring Module

The bitstring module provides four classes, Bits, BitArray, ConstBitStream and BitStream. Bits is the simplest, and represents an immutable sequence of bits, while BitArray adds various methods that modify the contents (these classes are intended to loosely mirror bytes and bytearray). The ‘Stream’ classes have additional methods to treat the bits as a file or stream.

If you need to change the contents of a bitstring after creation then you must use either the BitArray or BitStream classes. If you need to use bitstrings as keys in a dictionary or members of a set then you must use either a Bits or a ConstBitStream. In this section the generic term ‘bitstring’ is used to refer to an object of any of these classes.

Note that for the bitstream classes the bit position within the bitstream (the position from which reads occur) can change without affecting the equality operation. This means that the pos and bytepos properties can change even for a ConstBitStream object.

The public methods, special methods and properties of both classes are detailed in this section.

The auto initialiser

Note that in places where a bitstring can be used as a parameter, any other valid input to the auto initialiser can also be used. This means that the parameter can also be a format string which consists of tokens:

  • Starting with hex=, or simply starting with 0x implies hexadecimal. e.g. 0x013ff, hex=013ff

  • Starting with oct=, or simply starting with 0o implies octal. e.g. 0o755, oct=755

  • Starting with bin=, or simply starting with 0b implies binary. e.g. 0b0011010, bin=0011010

  • Starting with int or uint followed by a length in bits and = gives base-2 integers. e.g. uint8=255, int4=-7

  • To get big, little and native-endian whole-byte integers append be, le or ne respectively to the uint or int identifier. e.g. uintle32=1, intne16=-23

  • For floating point numbers use float: followed by the length in bits and = and the number. The default is big-endian, but you can also append be, le or ne as with integers. e.g. float64=0.2, floatle32=-0.3e12

  • Starting with ue=, uie=, se= or sie= implies an exponential-Golomb coded integer. e.g. ue=12, sie=-4

Multiples tokens can be joined by separating them with commas, so for example se=4, 0b1, se=-1 represents the concatenation of three elements.

Parentheses and multiplicative factors can also be used, for example 2*(0b10, 0xf) is equivalent to 0b10, 0xf, 0b10, 0xf. The multiplying factor must come before the thing it is being used to repeat.

The auto parameter also accepts other types:

  • A list or tuple, whose elements will be evaluated as booleans (imagine calling bool() on each item) and the bits set to 1 for True items and 0 for False items.

  • A positive integer, used to create a bitstring of that many zero bits.

  • A file object, presumably opened in read-binary mode, from which the bitstring will be formed.

  • A bytearray object.

  • An array object. This is used after being converted to it’s constituent byte data via its tostring method.

  • A bytes object.

Compact format strings

For the read, unpack, peek methods and pack function you can use compact format strings similar to those used in the struct and array modules. These start with an endian identifier: > for big-endian, < for little-endian or @ for native-endian. This must be followed by at least one of these codes:




8 bit signed integer


8 bit unsigned integer


16 bit signed integer


16 bit unsigned integer


32 bit signed integer


32 bit unsigned integer


64 bit signed integer


64 bit unsigned integer


16 bit floating point number


32 bit floating point number


64 bit floating point number

For more detail see Compact format strings.

Class properties

Bitstrings use a wide range of properties for getting and setting different interpretations on the binary data, as well as accessing bit lengths and positions. For the mutable BitStream and BitArray objects the properties are all read and write (with the exception of the len), whereas for immutable objects the only write enabled properties are for the position in the bitstream (pos/bitpos and bytepos).