array Initialization

An array is instantiated with an argument describing the type of data to be allowed, and possibly an initialization sequence.

import array
import binascii
s = 'This is the array.'
a = array.array('c', s)
print 'As string:', s
print 'As array :', a
print 'As hex   :', binascii.hexlify(a)

In this example, the array is configured to hold a sequence of bytes and is initialized with a simple string.

$ python array_string.py
As string: This is the array.
As array : array('c', [84, 104, 105, 115, 32, 105, 115,
32, 116, 104, 101, 32, 97, 114, 114, 97, 121, 46])
As hex   : 54686973206973207468652061727261792e

Manipulating Arrays

An array can be extended and otherwise manipulated in the same ways as other Python sequences.

import array
a = array.array('i', xrange(5))
print 'Initial :', a
a.extend(xrange(5))
print 'Extended:', a
print 'Slice   :', a[3:6]
print 'Iterator:', list(enumerate(a))

$ python array_sequence.py
Initial : array('i', [0, 1, 2, 3, 4])
Extended: array('i', [0, 1, 2, 3, 4, 0, 1, 2, 3, 4])
Slice   : array('i', [3, 4, 0])
Iterator: [(0, 0), (1, 1), (2, 2), (3, 3), (4, 4),
(5, 0), (6, 1), (7, 2), (8, 3), (9, 4)]

Arrays and Files

The contents of an array can be written to and read from files using built-in methods coded efficiently for that purpose.

import array
import binascii
import tempfile
a = array.array('i', xrange(5))
print 'A1:', a
# Write the array of numbers to the file
output = tempfile.NamedTemporaryFile()
a.tofile(output.file) # must pass an *actual* file
output.flush()
# Read the raw data
input = open(output.name, 'rb')
raw_data = input.read()
print 'Raw Contents:', binascii.hexlify(raw_data)
# Read the data into an array
input.seek(0)
a2 = array.array('i')
a2.fromfile(input, len(a))
print 'A2:', a2

This example illustrates reading the data “raw”, directly from the binary file, versus reading it into a new array and converting the bytes to the appropriate types.

$ python array_file.py
A1: array('i', [0, 1, 2, 3, 4])
Raw Contents: 0000000001000000020000000300000004000000
A2: array('i', [0, 1, 2, 3, 4])

Alternate Byte Ordering

If the data in the array is not in the native byte order, or needs to be swapped before being written to a file intended for a system with a different byte order, it is easy to convert the entire array without iterating over the elements from Python.

import array
import binascii
def to_hex(a):
    chars_per_item = a.itemsize * 2 # 2 hex digits
    hex_version = binascii.hexlify(a)
    num_chunks = len(hex_version) / chars_per_item
    for i in xrange(num_chunks):
        start = i*chars_per_item
        end = start + chars_per_item
        yield hex_version[start:end]
a1 = array.array('i', xrange(5))
a2 = array.array('i', xrange(5))
a2.byteswap()
fmt = '%10s %10s %10s %10s'
print fmt % ('A1 hex', 'A1', 'A2 hex', 'A2')
print fmt % (('-' * 10,) * 4)
for values in zip(to_hex(a1), a1, to_hex(a2), a2):
    print fmt % values

$ python array_byteswap.py
    A1 hex         A1     A2 hex         A2
---------- ---------- ---------- ----------
  00000000          0   00000000          0
  01000000          1   00000001   16777216
  02000000          2   00000002   33554432
  03000000          3   00000003   50331648
  04000000          4   00000004   67108864

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