Tuesday, May 1, 2012

bytearray

I came across a post by Python guru David Beazley introducing the bytearray data type. Here is one of his examples, modified slightly:

>>> s = "Hello World"
>>> b = bytearray(s)
>>> b
bytearray(b'Hello World')
>>> b[:5] = "Cruel"
>>> b
bytearray(b'Cruel World')
>>> for i in b:
...     print i
... 
67
114
117
101
108
32
87
111
114
108
100
>>> b.append(33)
>>> b
bytearray(b'Cruel World!')
>>> print b
Cruel World!

The usage b"mystring" was new to me as well. According to this SO answer:

the b prefix for string literals is ineffective in 2.6, but it is a useful marker in the program, which flags explicitly the intent of the programmer to have the string as a data string rather than a text string. This info can then be used by the 2to3 converter or similar utilities when the program is ported to Py3k.
More info here (thanks, Ned).

A bytearray has the methods of lists including __getitem__ and __setitem__. Unusually for Python, one can use arguments of both int and str types. The actual type of a bytearray item is < type 'int'>.

>>> b = bytearray('Hello World!')
>>> b[-1] = '*'
>>> b
bytearray(b'Hello World*')
>>> ord('!')
33
>>> b[-1] = 33
>>> b
bytearray(b'Hello World!')

We can use a bytearray to do a small job of encryption in a very convenient way:

>>> import numpy as np
>>> msg = bytearray("Hello World")
>>> n = len(msg)
>>> L = np.random.randint(0,256,n)
>>> key = bytearray(list(L))
>>> len(key)
11
>>> key
bytearray(b'N\x963\x8a\x06\xf2\xe9\x92\xb9\xf4L')
>>>
>>>
>>> ctext = bytearray()
>>> for c,k in zip(msg,key):
...     ctext.append(c ^ k)
... 
>>> ctext
bytearray(b'\x06\xf3_\xe6i\xd2\xbe\xfd\xcb\x98(')
>>> ptext = bytearray()
>>> for c,k in zip(ctext,key):
...     ptext.append(c ^ k)
... 
>>> ptext
bytearray(b'Hello World')

No comments: