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 thefmt
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. Thebitstring.Array
type is meant as a more flexible version of the standardarray.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 thefmt
. 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 thedata
property. You can freely manipulate the internal data using all of the methods available for theBitArray
class.The
Array
object also has atrailing_bits
read-only data member, which consists of the end bits of thedata
BitArray
that are left over when theArray
is interpreted usingfmt
. Typicallytrailing_bits
will be an emptyBitArray
but if you change the length of thedata
or change thefmt
specification there may be some bits left over.Some methods, such as
append
andextend
will raise an exception if used whentrailing_bits
is not empty, as it not clear how these should behave in this case. You can however still useinsert
which will always leave thetrailing_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 thearray.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 thearray
module isn’t supported as its length is platform dependent.The
'e'
type code isn’t one of thearray
supported types, but it is used in thestruct
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 (seetrailing_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 anarray.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¶
Properties¶
- Array.data¶
The bit data of the
Array
, as aBitArray
. Read and write, and can be freely manipulated with all ofBitArray
methods.Note that some
Array
methods such asappend
andextend
require thedata
to have a length that is a multiple of theArray
’sitemsize
.
- 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 theArray
. However, changing the format won’t change the underlying bit data in any way.Note that some
Array
methods such asappend
andextend
require the bit data to have a length that is a multiple of theArray
’sitemsize
.
- 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 thedata
that is not a multiple of theitemsize
. Read only.This will typically be an empty
BitArray
, but if an thefmt
or thedata
of anArray
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 aValueError
if there are any trailing bits.