unpack struct from stream-like object

gistfile1.py

#
# example:
#     f = open('/path/some/file', 'r')
#     age, year, c = unpack('>HHB', f)
#

import struct

def unpack(fmt, f, offset=None):
	"""Unpacks data read from file stream at optionally specified offset"""
	isStruct = isinstance(fmt, struct.Struct)
	size = fmt.size if isStruct else struct.calcsize(fmt)
	pos = f.tell()
	if offset is not None: f.seek(offset)
	s = f.read(size)
	if len(s) < size:
		raise ValueError, "cannot unpack %s - not enough bytes read" % fmt
	if offset is not None: f.seek(pos)
	return fmt.unpack(s) if isStruct else struct.unpack(fmt, s)