Pythonic bit twiddling

[Darrell]

Working with unsigned is a pain.

>>> a=0x80000000
>>> hex(a>>32)
'0xffffffff'

>>> int('0x80000000',16)
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
ValueError: int() literal too large: 0x80000000
>>> eval('0x80000000')
-2147483648
>>>

And eval doesn't need to be told the number base in advance.
Other than the 0x... that is.

Longs help.

>>> b=0x80000000L
>>> b
2147483648L
>>> hex(b)
'0x80000000L'
>>> hex(b>>32)
'0x0L'
>>> hex(b>>31)
'0x1L'
>>>

But there are problems with this also.

>>> struct.pack("i",b)
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
OverflowError: long int too large to convert to int
>>> struct.pack("l",b)
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
OverflowError: long int too large to convert to int
>>> struct.pack("l",int(b))
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
OverflowError: long int too large to convert to int

You can play games to get around this.

>>> bs=b>>1
>>> bs=bs<<1
>>> struct.pack("i",bs)
'\x00\x00\x00\x80'
>>>

--Darrell


Paul Rubin wrote:
> 
> Am I missing something?  Python has the usual bit operations that work
> on ints.
> 
> a = do_some_stuff(byte >> 4)        # upper nibble
> b = do_similar_stuff(byte & 0x0f)   # lower nibble
> new_byte = (a << 4) + b     # assume a and b are nibble results