bitarray 0.2.1

bitarray: module for efficiently storing bits in a list-like object

This module provides an object type which can efficiently represent a bitarray. Bitarrays are sequence types and behave very much like usual lists. Each bit is represented as an actual bit in memory. For example, this allows the storage of 8Gbits in 1GB of memory. Most of the functionality is implemented in C, for speed.

Requires Python 2.5 or greater, see PEP 353.

Installation

bitarray can be installed from source:

$ tar xzf bitarray-0.2.1.tar.gz
$ cd bitarray-0.2.1
$ python setup.py install

On Unix systems, the latter command may be executed with root privileges. If you have setuptools installed on your system, you can easy_install bitarray. Once you have installed the package, you may want to test it:

$ python -c 'from bitarray import test; test()'
bitarray is installed in: /usr/local/lib/python2.5/site-packages/bitarray
bitarray version: 0.2.1
2.5.2 (r252:60911, Jul 17 2008, 10:38:24)
[GCC 4.2.1 (SUSE Linux)]
...........................................................
----------------------------------------------------------------------
Ran 60 tests in 0.403s

OK

You can always import the function test, and test().wasSuccessful() will return True when the test went OK.

Using the module

Here are a few usage example, pointing out some differences to lists.

>>> from bitarray import bitarray
>>> a = bitarray()            # create empty bitarray
>>> a.append(True)
>>> a.extend([False, True])
>>> a
bitarray('101')
>>> a[-1]
True
>>> del a[1]
>>> len(a)
2

Creating objects:

>>> a = bitarray(1024)        # bitarray of length 1024 (uninitialized)
>>> bitarray('1001011')       # from string
bitarray('1001011')
>>> lst = [True, False, False, True, False, True, True]
>>> bitarray(lst)             # from list, tuple, sequence, iterable
bitarray('1001011')

Bits can be assigned from any Python object. Whenever bits are assigned, the built-in bool() function is used to determine the value of the bit. (Actually the C equivalent of bool() is used.)

>>> a = bitarray([42, '', True, {}, 'foo', None])
>>> a
bitarray('101010')
>>> a.append(a)      # note that bool(a) is True
>>> a
bitarray('1010101')
>>> a.count(42)      # counts occurrences of True
4L
>>> a.remove('')     # removes first occurence of False
>>> a
bitarray('110101')

Like lists, bitarray objects support slice assignment and deletion:

>>> a = bitarray(50)
>>> a.setall(False)
>>> a[11:37:3] = 9*bitarray([True])
>>> a
bitarray('00000000000100100100100100100100100100000000000000')
>>> del a[12::3]
>>> a
bitarray('0000000000010101010101010101000000000')
>>> a[-6:] = bitarray('10011')
>>> a
bitarray('000000000001010101010101010100010011')
>>> a += bitarray('000111')
>>> a
bitarray('000000000001010101010101010100010011000111')

Bit endianness

Since a bitarray allows addressing of individual bits, where the machine represents 8 bits in one byte, there two obvious choices for this mapping; little- and big-endian. When creating a new bitarray object, the endianness can always be specified explicitly:

>>> a = bitarray(endian='little')
>>> a.fromstring('A')
>>> a
bitarray('10000010')
>>> b = bitarray('11000010', endian='little')
>>> b.tostring()
'C'

Here the low-bit comes first because little-endian means that increasing numeric significance corresponds to an increasing address (or index). So a[0] is the lowest and least significant bit, and a[7] is the highest and most significant bit.

>>> a = bitarray(endian='big')
>>> a.fromstring('A')
>>> a
bitarray('01000001')
>>> a[6] = 1
>>> a.tostring()
'C'

Here the high-bit comes first because big-endian means “most-significant first”. So a[0] is now the lowest and most significant bit, and a[7] is the highest and least significant bit.

The bit endianness is a property attached to each bitarray object. When comparing bitarray objects, the endianness (and hence the machine representation) is irrelevant; what matters is the mapping from indices to bits:

>>> bitarray('11001', endian='big') == bitarray('11001', endian='little')
True

When converting to and from machine representation, using the tostring, fromstring, tofile and fromfile methods, the endianness matters:

>>> a = bitarray(endian='little')
>>> a.fromstring('\x01')
>>> a
bitarray('10000000')
>>> b = bitarray(endian='big')
>>> b.fromstring('\x80')
>>> b
bitarray('10000000')
>>> a == b
True
>>> a.tostring() == b.tostring()
False

The endianness can not be changed once an object is created. However, since creating a bitarray from another bitarray just copies the memory representing the data, you can create a new bitarray with different endianness:

>>> a = bitarray('11100000', endian='little')
>>> a
bitarray('11100000')
>>> b = bitarray(a, endian='big')
>>> b
bitarray('00000111')
>>> a == b
False
>>> a.tostring() == b.tostring()
True

The default bit endianness is currently big-endian, however this may change in the future, and when dealing with the machine representation of bitarray objects, it is recommended to always explicitly specify the endianness.

Unless, explicity converting to machine representation, using the tostring, fromstring, tofile and fromfile methods, the bit endianness will have no effect on any computation, and you can safely ignore setting the endianness, and other details of this section.

Reference

The bitarray class:

bitarray([initial][endian=string])

Return a new bitarray object whose items are bits initialized from the optional initial, and endianness. If no object is provided, the bitarray is initalized to have length zero. The initial object may be of the following types:

int, long

Create bitarray of length given by the integer. The initial values in the array are random, because only the memory allocated.

string

Create bitarray from a string with ‘0’s and ‘1’s, e.g. bitarray(‘01101001111’).

list, tuple, iterable

Create bitarray from a sequence, each element in the sequence is converted to a bit using the bool function, e.g. bitarray([2, 0, ‘a’, {}]) -> bitarray(‘1010’)

bitarray

Create bitarray from another bitarray. This is done by copying the memory holding the bitarray data, and is hence very fast.

The optional keyword arguments ‘endian’ specifies the bit endianness of the created bitarray object. Allowed values are ‘big’ and ‘little’ (default ‘big’)

Note that setting the bit endianness only has an effect when accessing the machine representation of the bitarray, i.e. when using the methods: tofile, fromfile, tostring, fromstring

A bitarray object supports the following methods:

all()

Returns True when all bits are 1.

any()

Returns True when any bit is 1.

append(x)

Append new value bool(x) to the end of the bitarray.

buffer_info()

Return a tuple (address, length, endianness, unused) giving the current memory address, the length in bytes used to hold the bitarray’s contents, the bit endianness as a string, and the number of unused bits (0..7).

bytereverse()

For all bytes representing the bitarray, reverse the bit order.

count(x)

Return number of occurences of x in the bitarray.

endian()

Return the bit endianness as a string (either ‘little’ or ‘big’).

extend(object)

Append bits to the end of the bitarray. The objects which can be passed to this method are the same which can given to a bitarray object upon upon initialization.

fill()

When the length of the bitarray is not a mutiple of 8, increase the length slightly such that the new length is a mutiple of 8, and set the few new bits to zero. Returns the number of bits added (0..7).

from01(string)

Appends items from the string, containing ‘0’s and ‘1’s, to the bitarray. This method is deprecated, use ‘extend’ method instead.

fromfile(f [, n])

Read n bytes from the file object f and append them to the bitarray interpreted as machine values. When n is omitted, as many bytes are read until EOF is reached.

fromlist(list)

Append bits to bitarray from list. This method is deprecated, use the ‘extend’ method instead.

fromstring(string)

Append from a string, interpreting the string as machine values.

index(x)

Return index of first occurence of x in the bitarray. It is an error when x does not occur in the bitarray

insert(i, x)

Insert a new item x into the bitarray before position i.

invert(x)

Invert all bits in the bitarray IN PLACE, i.e. convert each 1-bit into a 0-bit and vice versa.

length()

Return the length (number of bits) of the bitarray.

pop([i])

Return the i-th element and delete it from the bitarray. i defaults to -1.

remove(x)

Remove the first occurence of x in the bitarray.

reverse()

Reverse the order of bits, IN PLACE.

setall(x)

Set all bits in the bitarray to x.

sort()

sort, IN PLACE.

to01())

Return a string containing ‘0’s and ‘1’s, representing the bits in the bitarray object.

tofile(f)

Write all bits (as machine values) to the file object f. When the length of the bitarray is not a mutiple of 8, the few remaining bits are filled with zeros.

tolist()

Return an ordinary list with the items in the bitarray.

tostring()

Return the string representing (machine values) of the bitarray. When the length of the bitarray is not a mutiple of 8, the few remaining bits are filled with zeros.

Functions defined in the module:

test(verbosity=1)

Selftest the module.

bits2bytes(n)

Return the number of bytes necessary to store n bits.