read bytes little-endian

''' class_ByteReader.py
Little-endian systems store the least significant byte in the smallest address

tested with Python27/34
'''

class ByteReader(object):
    '''
    reads bytes from a file
    '''
    def __init__(self, filename):
        self._buffer = list(bytearray(open(filename, 'rb').read()))
        self.pos = 0
        self._eof = EOFError('unexpected end of file')

    def read_byte(self):
        '''
        read one byte
        advances position
        '''
        try:
            byte = self._buffer[self.pos]
            self.pos += 1
            return byte
        except IndexError:
            raise self._eof

    def peek_byte(self):
        '''
        return the next byte in the file (look-ahead)
        '''
        try:
            return self._buffer[self.pos]
        except IndexError:
            raise self._eof

    def peek_list(self, n):
        '''
        return a list of the next n bytes
        '''
        return self._buffer[self.pos:self.pos+n]

    def read_short(self):
        '''
        read short (2 bytes little endian)
        '''
        a, b = self.read_list(2)
        # same as a * 256 + b
        return a << 8 | b

    def read_long(self):
        '''
        read long (4 bytes little endian)
        '''
        a, b, c, d = self.read_list(4)
        return a << 24 | b << 16 | c << 8 | d

    def read_list(self, n):
        '''
        read n bytes and return as a list
        advances position
        '''
        i = self.pos
        ret = self._buffer[i:i + n]
        if len(ret) < n:
            raise self._eof
        self.pos += n
        return ret

    def __enter__(self):
        return self

    def __exit__(self, type, value, traceback):
        return False


# testing the module ...
if __name__ == '__main__':
    # pick a file you have in the working directory or give full path
    fname = "Farm.gif"
    br = ByteReader(fname)
    print("read first byte = {}".format(br.read_byte()))
    print("peek next 10 bytes = {}".format(br.peek_list(10)))

    print("take 2 bytes:")
    print(br.peek_list(2))
    print("calculate a two byte value (little endian):")
    two_bytes = br.peek_list(2)
    print(two_bytes[0]*256 + two_bytes[1])
    # same result
    print(br.read_short())

    print("take 4 bytes:")
    print(br.peek_list(4))
    print("calculate value (4 bytes little endian):")
    print(br.read_long())

    print("read next 8 bytes:")
    print(br.read_list(8))