Note

The Array class is new in version 4.1 of bitstring, and is considered a ‘beta’ feature for now. There may be some small changes in future point releases and it hasn’t been tested as well as the rest of the library.

This documentation may also be a bit ‘beta’.

Array Class

class Array(fmt: str[, initializer[, trailing_bits]])

Create a new Array whose elements are set by the fmt string. This can be any format which has a fixed length.

The Array class is a way to efficiently store data that has a single type with a set length. The bitstring.Array type is meant as a more flexible version of the standard array.array, and can be used the same way.

import array
import bitstring

x = array.array('f', [1.0, 2.0, 3.14])
y = bitstring.Array('=f', [1.0, 2.0, 3.14])

assert x.tobytes() == y.tobytes()

This example packs three 32-bit floats into objects using both libraries. The only difference is the explicit native endianness for the format string of the bitstring version. The bitstring Array’s advantage lies in the way that any fixed-length bitstring format can be used instead of just the dozen or so typecodes supported by the array module.

For example 'uint4', 'bfloat' or 'hex12' can be used, and the endianness of multi-byte formats can be properly specified.

Each element in the Array must then be something that makes sense for the fmt. Some examples will help illustrate:

from bitstring import Array

# Each unsigned int is stored in 4 bits
a = Array('uint4', [0, 5, 5, 3, 2])

# Convert and store floats in 8 bits each
b = Array('float8_152', [-56.0, 0.123, 99.6])

# Each element is a  7 bit signed integer
c = Array('int7', [-3, 0, 120])

You can then access and modify the Array with the usual notation:

a[1:4]  # Array('uint4', [5, 5, 3])
b[0]    # -56.0
c[-1]   # 120

a[0] = 2
b.extend([0.0, -1.5])

Conversion between Array types can be done by creating a new one with the new format from the elements of the other one. If elements of the old array don’t fit or don’t make sense in the new array then the relevant exceptions will be raised.

>>> x = Array('float64', [89.3, 1e34, -0.00000001, 34])
>>> y = Array('float16', x.tolist())
>>> y
Array('float16', [89.3125, inf, -0.0, 34.0])
>>> y = Array('float8_143', y.tolist())
>>> y
Array('float8_143', [88.0, 240.0, 0.0, 32.0])
>>> Array('uint8', y.tolist())
Array('uint8', [88, 240, 0, 32])
>>> Array('uint7', y.tolist())
bitstring.CreationError: 240 is too large an unsigned integer for a bitstring of length 7. The allowed range is [0, 127].

You can also reinterpret the data by changing the fmt property directly. This will not copy any data but will cause the current data to be shown differently.

>>> x = Array('int16', [-5, 100, -4])
>>> x
Array('int16', [-5, 100, -4])
>>> x.fmt = 'int8'
>>> x
Array('int8', [-1, -5, 0, 100, -1, -4])

The data for the array is stored internally as a BitArray object. It can be directly accessed using the data property. You can freely manipulate the internal data using all of the methods available for the BitArray class.

The Array object also has a trailing_bits read-only data member, which consists of the end bits of the data BitArray that are left over when the Array is interpreted using fmt. Typically trailing_bits will be an empty BitArray but if you change the length of the data or change the fmt specification there may be some bits left over.

Some methods, such as append and extend will raise an exception if used when trailing_bits is not empty, as it not clear how these should behave in this case. You can however still use insert which will always leave the trailing_bits unchanged.

The fmt string can be a type code such as '>H' or '=d' but it can also be a string defining any format which has a fixed-length in bits, for example 'int12', 'bfloat', 'bytes5' or 'bool'.

Note that the typecodes must include an endianness character to give the byte ordering. This is more like the struct module typecodes, and is different to the array.array typecodes which are always native-endian.

The correspondence between the big-endian type codes and bitstring format codes is given in the table below.

Type code	bitstring format
'>b'	'int8'
'>B'	'uint8'
'>h'	'int16'
'>H'	'uint16'
'>l'	'int32'
'>L'	'uint32'
'>q'	'int64'
'>Q'	'uint64'
'>e'	'float16'
'>f'	'float32'
'>d'	'float64'

The endianness character can be '>' for big-endian, '<' for little-endian or '=' for native-endian ('@' can also be used for native-endian). In the bitstring formats the default is big-endian, but you can specify little or native endian using 'le' or 'ne' modifiers, for example:

Type code	bitstring format
'>H'	'uint16' / 'uintbe16'
'=H'	'uintne16'
'<H'	'uintle16'

Note that:

• The array module’s native endianness means that different packed binary data will be created on different types of machines. Users may find that behaviour unexpected which is why endianness must be explicitly given as in the rest of the bitstring module.

• The 'u' type code from the array module isn’t supported as its length is platform dependent.

• The 'e' type code isn’t one of the array supported types, but it is used in the struct module and we support it here.

• The 'b' and 'B' type codes need to be preceded by an endianness character even though it makes no difference which one you use as they are only 1 byte long.

Methods

Note

Some methods that are available for array.array objects are deliberately omitted in this interface as they don’t really add much. In particular, some omissions and their suggested replacements are:

a.fromlist(alist) → a.extend(alist)

a.frombytes(s) → a.data.extend(s)

Array.append(x: float | int | str | bytes) → None

Add a new element with value x to the end of the Array. The type of x should be appropriate for the type of the Array.

Raises a ValueError if the Array’s bit length is not a multiple of its format length (see trailing_bits).

Array.byteswap() → None

Change the byte endianness of each element.

Raises a ValueError if the format is not an integer number of bytes long.

Array.count(value: float | int | str | bytes) → int

Returns the number of elements set to value.

Array.extend(iterable: Iterable | Array) → None

Extend the Array by constructing new elements from the values in a list or other iterable.

The iterable can be another Array or an array.array, but only if the format (or typecode) is the same.

Array.fromfile(f: BinaryIO, n: int | None) → None

Append items read from a file object.

Array.insert(i: int, x: float | int | str | bytes) → None

Insert an item at a given position.

Array.pop(i: int | None) → float | int | str | bytes

Remove and return an item.

Array.pp(fmt: str | None, width: int, sep: str, show_offset: bool, stream: TextIO) → None

Pretty print the Array.

Array.reverse() → None

Reverse the order of all items in the Array.

Array.tobytes() → bytes

Return Array data as bytes object, padding with zero bits at the end if needed.

Array.tofile(f: BinaryIO) → None

Write Array data to a file, padding with zero bits at the end if needed.

Array.tolist() → List[float | int | str | bytes]

Return Array items as a list.

Special Methods

Array.__add__(other: Array | Iterable | int | float)

Array.__radd__(other: Iterable)

Array.__len__(self) → int

Array.__getitem__(self, key: int | slice) → float | int | str | bytes

Array.__setitem__(self, key: int | slice, value) → None

Array.__delitem__(self, key: int | slice) → None

Array.__eq__(self, other) → bool

Array.__ne__(self, other) → bool

Array.__truediv__(self, other: int | float) → Array

Array.__floordiv__(self, other: int | float) → Array

Array.__sub__(self, other: int | float) → Array

Array.__mul__(self, other: int | float) → Array

Array.__rshift__(self, other: int) → Array

Array.__lshift__(self, other: int) → Array

Array.__and__(self, other: Bits) → Array

Array.__or__(self, other: Bits) → Array

Array.__xor__(self, other: Bits) → Array

Array.__itruediv__(self, other: int | float) → Array

Array.__ifloordiv__(self, other: int | float) → Array

Array.__iadd__(self, other: int | float) → Array

Array.__isub__(self, other: int | float) → Array

Array.__imul__(self, other: int | float) → Array

Array.__irshift__(self, other: int) → Array

Array.__ilshift__(self, other: int) → Array

Array.__iand__(self, other: Bits) → Array

Array.__ior__(self, other: Bits) → Array

Array.__ixor__(self, other: Bits) → Array

Properties

Array.data

The bit data of the Array, as a BitArray. Read and write, and can be freely manipulated with all of BitArray methods.

Note that some Array methods such as append and extend require the data to have a length that is a multiple of the Array’s itemsize.

Array.fmt

The format string used to initialise the Array type. Read and write.

Changing the format for an already formed Array will cause all of the bit data to be reinterpreted and can change the length of the Array. However, changing the format won’t change the underlying bit data in any way.

Note that some Array methods such as append and extend require the bit data to have a length that is a multiple of the Array’s itemsize.

Array.itemsize

The size in bits of each item in the Array. Read-only.

Note that this gives a value in bits, unlike the equivalent in the array module which gives a value in bytes.

>>> a = Array('>h')
>>> b = Array('bool')
>>> a.itemsize
16
>>> b.itemsize
1

Array.trailing_bits

A BitArray object equal to the end of the data that is not a multiple of the itemsize. Read only.

This will typically be an empty BitArray, but if an the fmt or the data of an Array object has been altered after its creation then there may be left-over bits at the end of the data.

Note that any methods that append items to the Array will fail with a ValueError if there are any trailing bits.