`struct.unpack_from` doesn't work with `bytearray`?

Question

Unpacking from strings works:

>>> import struct
>>> struct.unpack('>h', 'ab')
(24930,)
>>> struct.unpack_from('>h', 'zabx', 1)
(24930,)

but if its a bytearray:

>>> struct.unpack_from('>h', bytearray('zabx'), 1)
Traceback (most recent call last):
  File "<ipython-input-4-d58338aafb82>", line 1, in <module>
    struct.unpack_from('>h', bytearray('zabx'), 1)
TypeError: unpack_from() argument 1 must be string or read-only buffer, not bytearray

Which seems a little odd. What am I actually supposed to do about it? obviously I could:

>>> struct.unpack_from('>h', str(bytearray('zabx')), 1)
(24930,)

But i'm explicitly trying to avoid copying possibly large amounts of memory around.

Answer 1

It looks like buffer() is the solution:

>>> struct.unpack_from('>h', buffer(bytearray('zabx')), 1)
(24930,)

buffer() is not a copy of the original, its a view:

>>> b0 = bytearray('xaby')
>>> b1 = buffer(b0)
>>> b1
<read-only buffer for ...>
>>> b1[1:3]
'ab'
>>> b0[1:3] = 'nu'
>>> b1[1:3]
'nu'

Alternitively, You(I?) can just use python 3; the limitation is lifted:

Python 3.2.3 (default, Jun  8 2012, 05:36:09) 
[GCC 4.7.0 20120507 (Red Hat 4.7.0-5)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import struct
>>> struct.unpack_from('>h', b'xaby', 1)
(24930,)
>>> struct.unpack_from('>h', bytearray(b'xaby'), 1)
(24930,)
>>>