seeking deeper (language theory) reason behind Python design choice

[Chris Angelico]

On Wed, May 9, 2018 at 1:48 PM, Python <python at bladeshadow.org> wrote:
>> since = in a statement on its own is not dangerous. People *almost never*
>> intend to write == for the side-effects only:
>
> Seriously?  I do this--not every day, but more than occasionally, not
> just in Python.
>
>     flag = (spam == arg)
> vs.
>     if spam == arg:
>         flag = True
>     else:
>         flag = False
>     if flag:
>         do_something()
>     else:
>         do_something_else()

That's not "side effects only". You're evaluating a comparison for its
result - its primary effect. You're assigning the result of the
comparison to something.

Using equality comparisons for side effects would look like this:

spam == arg

No assignment. Just that, as a statement all on its own. It's
perfectly legal, but I doubt you've ever actually done this.

ChrisA