bitarray 0.2.0

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 functionallity 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-0.2.0.tar.gz
$ cd bitarray-0.2.0
$ 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 insalled in: /usr/local/lib/python2.5/site-packages/bitarray
bitarray version: 0.2.0
2.5.2 (r252:60911, Jul 17 2008, 10:38:24)
[GCC 4.2.1 (SUSE Linux)]
....................................
----------------------------------------------------------------------
Ran 36 tests in 0.294s

OK

In fact, one can always import the function test which in addition to printing the test, also returns an instance of unittest._TextTestResult; for example 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 attachet to each bitarray object. When comparing bitarray objects, the endianness (and hence the machine representation) is irrelevant; what matters is the mapping from indicies 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')
>>> 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')
>>> 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 represention of bitarray objects, it is recommanded to always explicitly specifiy the endianness.

Unless, excplicity converting to machine representation, using the tostring, fromstring, tofile and fromfile methods, the bit endianness will have no effect on any computation, and you don’t have to worry about setting the endianness, and other (possibly confusing) details of this section.

Reference

A bitarray object has the following methods:

append(x)

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

buffer_info()

Return a tuple (address, length, endianness) giving the current memory address, and the length in bytes used to hold the bitarray’s contents, and the bit endianness as a string (either ‘little’ or ‘big’). The length in bytes multiplied by 8 is slightly larger than the number of bits the bitarray holds.

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(bitarray or iterable)

Append bits to the end of the bitarray. When you want to extend from a list, it’s faster to use fromlist.

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.

from01(string)

Appends items from the string (containing ‘0’s and ‘1’s) to the bitarray.

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.

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, 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 the bitarray.

setall(x)

Set all bits in the bitarray to x.

to01(string)

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.