[James Lu]
> > Currently, is <expr1> = <expr2> = <expr3> = <expr4> always equivalent
> > to <expr1> = <expr4>; <expr2> = <expr4>; <expr3> = <expr4>?
[Stephen J. Turnbull[
> No. It's equivalent to
>
> <expr3> = <expr4>
> <expr2> = <expr3>
> <expr1> = <expr2>
>
> and the order matters because the <expr>s may have side effects.
This is tricky stuff. In fact the rightmost expression is evaluated once,
and then the bindings are done left-to-right using the result of evaluating
the rightmost expression. Like so:
>>> def return2():
... print("called return2")
... return 2
>>> xs = [10, 20, 30, 40]
>>> i = xs[i] = return2()
called return2
>>> xs
[10, 20, 2, 40]
>>> i
2
So James's account is closer to what's done, but is missing weasel words to
make clear that <expr4> is evaluated only once.
Sane code doesn't rely on "left to right", but may well rely on "evaluated
only once".
_______________________________________________
Python-ideas mailing list
[email protected]
https://mail.python.org/mailman/listinfo/python-ideas
Code of Conduct: http://python.org/psf/codeofconduct/
