python least significant 6 bits of integer

Question

I'm trying to find the least significant 6 bits of a 16 bit integer. Let's say my integer is 139 / 0x008B.

>>> "{0:b}".format(139)
'10001011'

So the least significant 6 bits are:

'001011'
>>> int('001011', 2)
11

However, I thought I could do this with the >> operator, but that isn't giving me what I expect:

>>> 139 >> 6
2

Can someone explain the difference between these two?

Answer 1

Shifts are for re-positioning bits, not for isolating them. This is called "masking", and in your case you need a bitmask that you "and" with your number. This is done with the &-operator in Python. It's fundamentally different from the logical "and"-operator, so don't confuse them!

>>> bin(139 & 0b111111)
'0b1011'

Answer 2

I think you might want to use the bit-wise 'and' operator ( & ) and not the right-shift operator ( >> ).

Try the following: 139 & 0b111111

That should give you the 6 least-significant bits.