signed bitwise arithmetic in Python

Question

I got stuck with the following Python code

>>> a = 0xff
>>> b = 1 << 8
>>> ~a & ~b
-512

Why is it -512? In binary notation it should look like this:

     a   0 1111 1111   -> 255
     b  01 0000 0000   -> 256

    ~a   1 0000 0000   -> -256
    ~b  10 1111 1111   -> -257

  ~a&~b 00 0000 0000   -> 0

I expected 0 as with signed int in C:

signed int a = 0xff;
signed int b = 1 << 8;  
signed int k = ~a & ~b;

Any help?

Tim Pietzcker · Accepted Answer · 2010-10-28 08:59:29Z

4

Assuming 16-bit integers for convenience (the principle doesn't change for 32 or 64 bit):

a  = 0xff = 0000 0000 1111 1111
~a = -256 = 1111 1111 0000 0000

b  = 1<<8 = 0000 0001 0000 0000
~b = -257 = 1111 1110 1111 1111

-256 = 1111 1111 0000 0000
-257 = 1111 1110 1111 1111
--------------------------  &
-512 = 1111 1110 0000 0000

edited Oct 28, 2010 at 8:59

answered Oct 28, 2010 at 8:52

Tim Pietzcker

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