checking if two things do not equal None

[MRAB]

On 2014-03-30 13:21, Roy Smith wrote:
> In article <5337b4e4$0$29994$c3e8da3$5496439d at news.astraweb.com>,
>   Steven D'Aprano <steve+comp.lang.python at pearwood.info> wrote:
>
>> I think Johannes got it right: boolean logic is easier to reason about
>> when there is a minimum of "not"s.
>
> I used to do a lot of digital logic design.  In certain logic families,
> it's easier to build a NAND gate than an AND gate (and similarly, NOR is
> easier than OR).  This leads to lots of inverted logic.  Adding to the
> confusion, many designs would use "active low" logic, which means a 1
> was represented by a low voltage, and a 0 by a high voltage.  So, you
> quickly end up with gibberish like, "not active low clear nand not
> active low enable clock".  I'm glad I don't do that stuff any more.
>
When you're building with discrete logic chips, NAND gates are useful
because you can use them as inverters too, which helps to keep the chip
count down.