[Tutor] Bit-level field extraction

[Terry Carroll]

I want to see if I'm reinventing a wheel here, or maybe doing it 
unpythonically.

I need to extract from a byte (i.e., a one-character string) a field of an
certain number of bits.  For example, a certain byte of an MP3 frame
header has the format xxxVVxxx, where the value of VV (00, 01, 10 or 11)
tels what version of MPEG Audio is being used.  The other bits marked 'x'
are not relevant to the version id, and may be 1s or 0s.

Here's my stab:

def getbits(byte, fieldlength, rightpad):
    '''
    returns an integer with the value derived from a string of bits of
    length fieldlength, right-padded with rightpad number of bits.
    e.g., getbyte(byte, 2, 3) returns a value from bits 3-4
    (counting from the right)
    '''
    bitmask = (2**fieldlength-1) << rightpad
    return (ord(byte) & bitmask) >> rightpad

testbytes = ["\xC0", "\x08", "\xF0", "\x19"]
for byte in testbytes:
    print getbits(byte, 2, 3)
    # should get 0, 1, 2, 3


This works (at least for these 4 test cases).  But is this the best way to 
extract a bit-level field, or am I missing some appropriate module or 
something?